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Electrolung: The First Mixed Gas Rebreather Was Available to Sport Divers in 1968

While today mixed gas rebreathers are a standard tool for tech divers, it’s easy to forget that this enabling, underwater life support technology has been more than 50 years in the making. In fact, the first commercially available unit was available to sport divers in the late sixties! Here inventor and marine biologist Walter Starck dives into the deets of his Electrolung’s innovative design.

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By Walter Starck

Header image: W. Starck diving with first production run of the Electrolung on a drop-off in San Miguel, Cozumel, September 1968. The 180º Fisheye image used an optical dome port also developed by Starck in 1964.  Photos courtesy of W.Starck

This article was first published in serial form on the Rebreather Email List in July 1998, at the suggestion of Dr. Peter Heseltine, a researcher in diving and hyperbaric medicine.

Development of the Electrolung came about through my chance meeting with John Kanwisher aboard Ed Link’s diving research vessel in the Bahamas in early 1968. Ed was trying out his new diver lock-out submarine Deep Diver and had invited along several researchers with relevant interests. I was there to do some deep biological collecting, and John was there to do heart rate/respiration measurements on divers using some new acoustical telemetry equipment he had developed.

Lock-out dives from Deep Diver were done using hose-fed OC Kirby Morgan helmets. Gas for this purpose and to pressurize the lock-out chamber was supplied from a large, high-pressure sphere carried by the sub. The large amount of gas required for a single dive severely limited the number of dives which could be made and involved substantial cost and logistic considerations. The need for more efficient utilization of gas was clearly apparent.

It turned out that John and I had both been considering the feasibility of a mixed gas CCR using electronic sensors to control PPO2. We both knew in general terms what was needed, but John wasn’t a diver or a machinist, and I didn’t know that much about electronics. However, I had been diving for 15 years and had built a wide range of underwater equipment. John, in addition to being a physiologist, had invented the first polarographic oxygen sensor and held a dual appointment at Woods Hole Oceanographic Institute and MIT where he lectured on electronic instrument design.

The author in 1998 explaining the operation of the Electrolung

When we returned to our homes John started putting together the sensors and control circuit, and I started getting together the hardware and machining all of the necessary bits. Six weeks later we both had our respective parts together. John sent me the board and sensors; I installed them, and it worked. The overall configuration and design was basically as described, but there were, of course, numerous details to clean up. The electronics for example were wire connected on a breadboard, and the solenoid valve I had hand made and actuated with a solenoid scavenged from a battery operated coo-coo clock.

Although the prototype was put together quite quickly, it was far from a “first thing which comes to mind” effort. Quite a few years’ experience and thought had led up to it, so that when actual construction began we both knew pretty clearly what needed to be done and how to do it. Later, at Beckman, I had the opportunity of working with a whole group of specialists on improving the same device. The outcome was some tidying up of details but no fundamental improvement. 

A young Walter Starck about to dive in the Coral Sea with his Electrolung in 1971

The first production units with printed circuit boards and commercial pneumatic control valves were produced in August 1968 at my company, Oceanic Equipment Company in Miami. The price was $1995 and it was the first commercially available mixed gas electronically controlled rebreather. Production and sales were continued by Oceanic Equipment for two years and was then continued with further refinement of details by Beckman Instruments in Los Angeles.  

Buyers included commercial diving companies, the National Aeronautics and Space Administration (NASA), for use in their weightless simulation facility), the central Intelligence Agency (CIA) for unknown purposes, the Israeli Defense Forces, and the US Navy.  At Beckman they heard that the Navy had used the Electrolung at 1000-foot depth/306 m in a lockout dive from a submarine in the Arctic, but no other details. Then, last year declassified information revealed that the Navy had developed a nuclear sub with deep lockout capabilities at that time and had used it to tap into the Russian military communication network on a submarine cable running from Vladivostok across to the Kamchatka Peninsula in the Arctic. It appears that this must have been where the Electrolung was used; but what role it may have played is unknown.

At Beckman, the biggest problem was to prevent the creation of problems which didn’t previously exist but could be introduced through changes made by specialists who were unaware of consequences outside of the narrow area of their expertise. The experience gave me a real appreciation of both the power and the limitations of specialist expertise and the importance of systems analysis in coordinating and integrating the input of specialists.

Polarographic Oxygen Sensors

The Electrolung used three polarographic oxygen sensors. The sensors were robust hand-made ones we constructed ourselves. They had a central platinum cathode about 1/4″ in diameter surrounded by a concentric silver anode about 3/8″ Diam. In between was an annular groove for the KOH electrolyte. A .001″ teflon membrane held in place by a thick silicone rubber boot retained the electrolyte. Although the sensors would run for weeks before desiccation of the electrolyte became limiting, our SOP was to make them up fresh and calibrate them for each day’s diving. At the end of the day’s diving, the membranes would be removed and the sensors washed with distilled water. Making up and washing off the sensors only took a few minutes and assured that we always had fresh sensors.


Polarographic oxygen sensors. Left, bare sensor.  Middle, sensor with silicone rubber boot pushed half-way down over teflon membrane.  Right, sensor and boot in sensor holder.   Boot O.D. and hole in holder are tapered for snug fit.

Sensors of this type don’t wear out, so they are hardwired into the circuit. Unlike galvanic sensors, they don’t use oxygen but rather just respond to its presence. They work equally well submerged, so the effect of any condensation is negligible. A drop of water fully covering the end of the sensor would only slow the response time. In practice, we never had any condensation in the sensor area ,as this came immediately after the canister, so the gas was at its warmest and driest point in the circuit. Thick plastic walls probably also helped in avoiding condensation on cold surfaces.

The chief advantages of the sensors were that they were always fresh, and condensation wasn’t a worry. The disadvantage is that in making them up with fresh electrolyte we could screw up by contaminating the sensor via sloppy technique. Any significant change in calibration after a fresh makeup would be an indicator, and determining why should be mandatory before proceeding further. Still, there are the black box mentalities who will simply crank the trim pots until they get the reading they want and then assume all is well.

Cathode (center) is platinum. Anode (outer) is silver.  Electrolyte groove is between.  Body is epoxy.

There were two trim pots for calibrating each sensor. One for zero. The other for gain. Zero was checked each time before the sensors were made up. Gain was calibrated initially with air, and then the unit was put together, and a check with pure O2 was done. The permeability of teflon to O2 varies with temperature. The sensors were of potted epoxy construction with the electrodes embedded in the epoxy. A thermistor in contact with the underside of the cathode was also embedded. This thermistor had a similar response curve to the teflon and compensated for the temperature effect keeping output linear over the desired range.

Our chosen set point was 0.5 atm partial pressure of oxygen (PPO2).

The bottom line was that with proper care the sensors were very reliable. Enough so, that they could be hard wired in, and I know of no case where one ever had to be replaced.

The Electronics: Control, Readouts, Alarm

Unlike galvanic sensors, polarographic electrodes don’t generate electricity. Rather, the conductivity of the cell varies in the presence of oxygen. A bias potential from an external source is applied between anode and cathode, and the resulting flow of current is a function of the molecular concentration of oxygen present.  The current involved is very small, so an Op Amp is used with each sensor to boost power to a level useful for control and monitoring. Hermetically sealed trim pots which incorporate an O-ring seal around the adjustment screw provide for zero and gain adjustment of each Op Amp, thus enabling calibration.

Electronics section.  Solenoid and sensors at left, wrist display above.  Leads to wrist display are inside hose covered by SS braid connecting to the electronics section at ambient pressure.

The amplified signal is read out to a wrist display consisting of a stack of three edgewise panel meters. We used 100 microamp meters in conjunction with high resistance to prevent a possible short in this circuit from affecting the solenoid control. Mil Spec, so-called “shock resistant” meters were used. These resist minor bumps but still they won’t stand up if you drop the display on a steel deck or concrete. In practice it wasn’t a significant problem, but occasionally a meter did require replacement. This was quick and easy to do.

The big advantage of this type of analog display is that you can tell at a glance everything you need to know. In use, all you need to verify is that all of the readouts are in line with one another and at or a bit above the set point which was exactly mid-scale. This kind of meter is also precise enough for calibration purposes. If I were doing it today, I would look at bar type LCD or LED readouts for monitoring and perhaps a separate switchable numeric display for calibration. I would also seriously consider a miniature heads-up display in the mask instead of one on the wrist. I don’t like numeric displays for monitoring, as they entail reading and mentally comparing numbers which requires much more attention than just noticing if position and alignment are where they should be.  Possibly some of the commercial RBs have already done all this.

Main circuit board.  Front modules contain amplifiers for each sensor including zero and gain trim pots on front.  Module behind controls solenoid.   Silica gel canister and double throw power switch is at top.  Push button, momentary-on switches at right were a later addition enabling pre-dive battery check under load using the wrist display meters to read.  Previously this was done using an external meter.

The amplified signals from all three sensors were fed into a fourth Op Amp which in effect averaged them and used the resulting value to control the solenoid set point via a switching transistor. We used a fixed set point of 0.5 atm PPO2, but it would be simple to add a trim pot to provide an adjustable set point. Clipping circuits limited the input to the control Op Amp from each sensor to values corresponding to 0.25 and 0.75 atm PPO2. If any one sensor began to read drastically different from the others, its effect on automatic solenoid control was thus limited. Clipping came after the meter display, thus they would continue to read true output even if the input to the control Op Amp was clipped. Clipping also activated an audible alarm. If the alarm sounded, a glance at the meters would tell you what the situation was. If only one was off, the other two would continue to exercise control. If all were high, low, or different from one another, you could use manual control while aborting the dive.


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The Op Amps require a + and a – voltage power supply. This was supplied by a pair of 9V Manganese Alkaline transistor radio batteries. Bias to the sensors was provided from the same source via a voltage dividing resistor circuit. A second pair of the same batteries provided switchable backup power. A third pair used in parallel provided separate power for the solenoid. The solenoid did not have backup, as this is non-critical because manual control of O2 is easily affected.  The snap terminals used for this type of battery were securely attached to a bulkhead. A screw-adjusted base plate held the batteries firmly in place, and against the terminals, avoiding any possibility of a loose battery connection.

Battery holder and base plate (sponge rubber pad has come loose from base plate in photo).  Audible alarm is at right.  This is on the opposite side of the longitudinal bulkhead from the electronics.  Connection for O2 to solenoid is at lower left.

All the electronics were incorporated on a single circuit board about 4”x5″/10-13 cm. This was mounted on one side of a longitudinal bulkhead in the electronics housing with the batteries and audible alarm on the other. This longitudinal bulkhead was itself mounted on a transverse bulkhead which separated the electronics compartment from a plenum above the absorbent canister. The solenoid and sensors were mounted on the opposite side of this transverse bulkhead, thus everything electrical other than the wrist display was immediately adjacent to each other.

In the units I made, all of the electronic components were on a printed circuit board. After assembly, the boards were coated with a spray-on waterproofing compound as is widely used for marine electronics. At Beckman, the components were assembled into 4 micro-welded, epoxy potted modules which plugged into gold plated sockets on the circuit board. In theory, this is a better way to go, but in practice, it didn’t make any noticeable difference.

With respect to the reliability of electronics in this kind of application, recently someone posed the question, “When was the last time your TV failed?” to which Robert made the wonderful reply, “The last time I took the bastard underwater.” Both comments reflect important points. Electronics in themselves can be extremely reliable. In terms of MTBF, they are far more reliable than most mechanical devices. Enough so that they can be trusted for things like passenger aircraft control systems where thousands of systems are in everyday use and a single failure means the loss of hundreds of lives. But Robert is right too. If you flood them with water, they fail.

The problem then is really a mechanical one. Can electronics be reliably enclosed so as to prevent flooding in underwater use? If it were solely a matter of constructing a watertight pressure-proof housing for the electronics, that alone wouldn’t be too hard. Unfortunately there is also the matter of connections for sensors, displays, a solenoid, and a switch, plus keeping all these external devices themselves dry.  The possibilities for leaks begin to multiply. With a great deal of care in construction and use, high reliability is achievable, but I think there is a much easier way to reliably keep out the water.

Wrist display of three edgewise panel meters.  Construction is teflon coated cast aluminum with o-ring sealed 1/4″ acrylic face plate held in place by a large circlip.

The key to the solution is pressure. Keeping things watertight under 100-200 psi is difficult. Doing it under 0.5-1 psi is easy. In the Electrolung everything was at ambient pressure. The electronics compartment was vented via a small canister of silica gel with the rest of the system. A standpipe for the vent orifice prevented any accumulated moisture in the canister plenum from being pushed into the electronics compartment. In anything but a head-down position, the electronics were above the counterlung; thus, any leak would normally result in gas escaping rather than water coming in. In practice, with the kinds of seals involved and the very low pressure differentials, leakage anywhere in the electronics section was never a problem.

Humidity and condensation were also not problematic. Plastic construction probably helped in avoiding the latter, and the waterproof coating seemed to be quite sufficient for the former, as is well attested by a wide array of complex devices and vast usage experience in the marine electronics industry.

The only practical way to get your TV underwater with this type of system is to flood the entire system. This is inherently no more likely nor any more or less disastrous than it would be with any other rebreather, regardless of type.

Gas Supply

Two, 9 cf at 2100 psi (1.75 ltr @ 145 bar) steel gas cylinders were used for O2 and inert gas. These were lightweight models FAA certified for use in aircraft. Initially we used chrome plating to protect them; later we went to teflon coating. Beckman liked the more military look, and there was some concern over possible hydrogen embrittlement from the chroming process.

Standard, old style, “K” valves were used as cylinder valves. On the inert side, a SCUBA regulator-type yoke was used to mount a high pressure 1/8″ NPT needle valve operated by rotary action of a T-shaped handle. Inert gas was valved in manually directly from the tank as needed using this valve. In use, it had a very smooth precise feel. Inert gas was valved into the plenum at the bottom of the absorbent canister so that some mixing would take place before it got to the sensors.

Jon Kanwisher wearing the Electrolung
 

On the O2 side, a piston-type first stage of a U.S. Diver’s single hose regulator was used to reduce tank pressure to about 60 psi. This is somewhat lower than such first stages normally delivered and was achieved by using a weaker piston spring. The normal hose to the second stage was used to connect the O2 supply to the solenoid valve. The octopus port of the first stage was used to attach an O2 bypass valve. This was a spring action, lever activated low pressure valve and it was protected by an enclosure which required opening a spring closed cover to get at the valve. The manual bypass valved O2 directly into the sensor compartment, so the result was immediately readable.

I will digress briefly on O2. In addition to the physiological risks recently discussed in some detail on the list, there is also the danger of fire and explosion. Valves, regulators, fittings, and any other equipment used for O2 have to be thoroughly degreased of any petroleum based lubricants. If lubrication is required, as for example with the o-ring seal of a regulator piston, non-combustible silicone based lubricants must be used. Be aware that even a fingerprint oily with suntan lotion can start an explosive fire with O2. Once an O2 fire starts, all sorts of things you might not ordinarily think of as combustible burn ferociously. I have heard stories of chamber fires in which everything inside, including the occupants, was reduced to ash.

My partner Kanwisher was on one of the advisory panels to National Aeronautics and Space Administration (NASA) in connection with the Apollo program. Although he recommended using a mixed gas atmosphere in the Apollo capsule, he was overridden by the engineers who felt that monitoring the PPO2 was too difficult. John knew better, as he had been doing it for several years in conjunction with his work on respiration, but the engineers prevailed. The result was the fire that killed three astronauts.

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The solenoid valve we used was a miniature 12-volt one made for pneumatic control. We equipped it with a miniature screw-adjusted needle valve outlet. When the setpoint is reached and the solenoid is triggered, it takes perhaps three or four seconds for the sensors to respond and rise enough to cut it off again. The solenoid needle valve was adjusted so that the O2 injected raised the PPO2 to a peak pulse of about 0.75 atm and would usually trigger a couple of beeps from the audible alarm. Within a couple of breaths, mixing brought the level back to perhaps 0.65, after which it dropped more slowly, as it was consumed by metabolism until the setpoint was reached again after about a minute or so. That would be for moderate activity such as easy swimming. At complete rest, it would of course take longer to drop back to the set point and less time if you were actively swimming.

If the needle valve was adjusted to a lower flow rate, solenoid activation would be more frequent and of longer duration, placing an unnecessary drain on the solenoid batteries. If much higher flow was adjusted for the O2 spikes would be too high, and the alarm would be sounding much of the time. I think there are now smaller, more power-efficient solenoid valves available.

The solenoid and manual bypass valves were of the downstream type so that if high pressure leakage from the regulator occurred, it would release when it reached the level where it overcame the spring tension which normally closed the valve. This is important to prevent either valve lockup or blowing out the supply hose in the event of a high-3wqaz¸pressure leak. In the event of O2 leakage from either valve, the cylinder valve could be used to cut it off.

High pressure needle valve for control of inert gas supply.

Oxygen regulator and bypass valve.  Spring loaded cover over bypass valve actuator is being held up.

The gas cylinders were mounted on either side of the central larger cylinder containing the absorbent canister and electronics section. This assembly was worn as a backpack with the valves at the bottom at hip level. Inert gas had to be added several times on descent and at other times if you lost any from nasal exhalation. Manual O2 was normally only used in decompression. The inert gas valve was therefore on the diver’s left side leaving the right hand free for more complex tasks. Swapping sides for southpaws would have been easy, but I don’t recall anyone ever raising the question. It was no big thing either way.

The Breathing Circuit

The Electrolung mouthpiece was originally constructed of PVC. The main body of the mouthpiece assembly was made in three parts glued with PVC solvent cement. At Beckman, this part was manufactured as a teflon-coated aluminum investment casting which made a nicer looking part. A rotating drum-type valve operated by a small lever permitted closure of the mouthpiece if you wanted to take it out of the mouth underwater. Two check valves directed gas flow to the proper hoses for inhale and exhale cycles.

Mouthpiece valve assembly viewed from front.  Mouthpiece mounts at top back.  Close-off is actuated by lifting the lever at right.  Bottom hose goes to counterlung.  Right and left hoses go to and from the absorbent canister.

Three breathing hoses were attached to the mouthpiece. One went straight down to the counterlung which was worn on the chest. The other two were routed to either side over the shoulders to the inlet and outlet of the absorbent canister/electronics section which was worn on the back. Initially, hoses from the old style twin hose scuba regulators were used. At Beckman we found a manufacturer in L.A. who made high quality fiber reinforced hoses for the O2 breathing systems in military aircraft. The price was only a little more than the scuba hose, and they could be made to order in small quantities in various lengths. Scuba hoses tended to start to leak after a year or two. The others are still usable after a quarter century in tropical conditions.

Hose clamps may seem mundane but are worth considering. A hose coming off with scuba is an inconvenience. With an RB, it is a life-threatening disaster. Initially, we used the spring steel ratchet-type clamps used on scuba regulators, but I didn’t fully trust them, and they rusted. We tried chrome plating them and found they would then break unexpectedly due to hydrogen embrittlement. In the end, we went to good quality heavy duty SS worm screw type clamps. Some SS clamps have ordinary mild steel worm screws which rust, electrolyze, and break after a while in marine use. Lightweight SS clamps can also break for no apparent reason due to stress fatigue. SS is prone to this. SS fitting used in sailboat rigging are notorious for letting go at the most inopportune times.

Mouthpiece parts. Body is teflon coated cast aluminum, other parts PVC.   Threaded parts are sealed with non-acidic silicone sealant.  Inlet check valve is in the PVC part at left.  Outlet check valve is inside the body just above the hose connection at middle right.  Close-off valve is at top.

A lot of people seem to now use the nylon ratchet type ties for clamping hoses, but I wouldn’t trust these either for critical applications. Nylon is subject to cold flow under stress, and after a while they become looser. Using them as a backup next to a good SS clamp however, might not be a bad idea.

The counterlung was a bag made of two pieces of plastic material electronically welded together over a 1/4″ surface around the entire perimeter about 1/2″ in from the edge which was then sewn together and wrapped in a heavy edging tape. Originally, I used clear vinyl for the material. This worked well enough, but later I found a lighter, more flexible, translucent fiber reinforced plastic material which was better.

Being clear or translucent offers two advantages. It lets light in and so reduces the growth of microorganisms. It also makes it easy to see if any water has accumulated and get rid of it before it gets to a level where it might be drawn into the absorbent canister. Places which make awnings, boat covers, etc. or those who make plastic zipper bags, folders, and the like can easily and inexpensively do this kind of work. All you need is a paper pattern of what you want. One-offs for prototyping are not expensive and with a $100 or so for a die for the welding, they can pop out small quantity runs dirt cheap. Usually they also have samples and catologues of all sorts of material to choose from. Teflon coated nylon is now available and might be very good for this application. I mention all this because some homebuilders may be interested.


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The counterlung had a single hose attached about 1/3 of the way up from the bottom on the front (away from the body) side. A drain plug was at the bottom near the lower left corner. At Beckman we added an overpressure relief valve near the top. The fittings all used a flange and threaded collar-type attachment similar to a kitchen sink drain. The flange incorporated a groove and o-ring in its face which ensured a firm grip and seal with the counterlung material. The fittings were machined from PVC, except for the small drain fitting, which was SS or chromed brass with a 1/4″ plug on a short lanyard. It was basically the same as a control gland used in underwater camera housings.

The overpressure relief valve released at somewhere around 0.75 psi. Its practical use was only the prevention of possible counterlung rupture if gas was accidentally valved in with the mouthpiece shut-off valve closed. It was introduced at the suggestion of experienced OC divers who, not being used to getting rid of excess gas via the nose during ascent, tried to exhale against a full counterlung and couldn’t.

Counterlung.  Hose connection is near center, drain at lower right.   Grommets for attachment at corners.  Material is a fiber reinforced translucent plastic.  Probably vinyl.

The counterlung was attached by grommets at each corner which mated with twist studs mounted on the shoulder straps at the top and on short adjustable straps paralleling the backpack waist strap on each side at the bottom. The counterlung volume we used was about 4 L.

There has been some discussion on the list recently re: the relative merits of chest mounted (resistance on exhale) vs. back mounted (resistance on inhale) counterlungs. In an earlier post, which has been quoted in the recent discussion, I opted for chest mounting as preferable because the mechanics of breathing musculature is such that the power available for exhalation is greater than that for inhalation. The counter argument is that resistance on exhale reduces the volume of exhaled gas leading to CO2 retention.

First, we need to keep in mind all this is somewhat hypothetical, and in the real world both configurations have been used successfully. With sustained high level exertion where any advantage might be important (and in the continuing absence of any proper comparative testing) I would opt for the chest mount for two reasons: It’s less tiring to put a bit of extra effort into exhaling than it is into inhaling, and the bottom line at the extreme for ventilation lies with how much gas you can move in and out in a given time. Given equal resistance in either direction, the more powerful exhalation cycle will move the greater amount of gas. In the end, over a few breaths, expiration and inspiration must be equal. Restriction of either sets the limit, so if there has to be a restriction I would rather it be on the side which can best handle it.

We sold a number of Electrolungs to commando-type users. One of their prime concerns was breathing resistance in sustained hard swimming. After trying it, all gave it their thumbs up in this respect. In out-of-water chamber tests at 1000 fsw/306 msw pressure, breathing resistance during exercise was encountered. This was in the breathing circuit itself and could of course be relieved somewhat by bigger hoses, larger absorbent bed cross section, etc., but as the market for that capability was effectively nil, it was never pursued.

Counterlung details.  Bar on inner collar of hose fitting is to prevent  possible stoppage of gas flow by the opposite side of the counterlung covering the opening while gas still remained in the upper part of the bag.  Note the welded seal near the edge of the bag.  All hose fittings on the Electrolung had a groove like this one.  The groove is beneath where the hose clamp goes, and hose material is squeezed into the groove by the clamp making for a much more secure attachment than a smooth surface.

One memorable experience with the commando types took place in the Bahamas. A British Royal Marine Commando attached to the Canadian Navy flew down to join me on my vessel and try out the Electrolung. He was a big, bullet-headed guy, built like a fridge with a head. After a couple days’ instruction diving, he wanted to do a long, hard swim with it, and as there was no one else to do it with him, I ended up going along. He went for a couple of miles virtually, flat out. Luckily the water was crystal clear, so I managed to at least keep him in sight. When we came to the surface, the bastard wasn’t even winded. He was satisfied it could do the job and just wanted a smoke. It was flat calm, and the water was only about 30 feet deep, so I had a skiff following us with his smokes and we could ride back.

Although I still don’t know for absolute certainty whether chest or back mount is optimal, I do know that chest mount is good enough. What I really do like about it is that it is easy to see if there is any water in it and easy to pull the plug, squeeze the bag, and expel it.

We did have a couple of experienced open circuit (OC) divers, new to the Electrolung, let water leak in around their mouths until it was gurgling away with each breath. They continued until they had largely flooded the absorbent canister and eventually got a mouthful of absorbent cocktail. They were quite irate about all this and swore it was the fault of the Electrolung. This kind of thing is a recurrent problem with rebreathers (RBs). Experienced OC divers have habits which don’t go with RBs. They also tend to think of themselves as expert divers rather than as novice RB users. As a result, they often don’t really listen and don’t take advice well, and they tend to blame the device if anything is not right, rather than realizing that they have to learn to use it right.

The actual breathing circuit for the Electrolung was: Exhale directly to counterlung via bottom mouthpiece hose. Inhale draws gas from counterlung back out the same hose into the left mouthpiece hose then to the bottom of the canister via a central tube inside it. At the bottom, the gas emerges into a plenum which distributes it over the inlet surface of the absorbent column. After passing through the absorbent, it emerges at the top into the chamber where the sensors and solenoid are located. From here, it continues via the right mouthpiece hose into the mouthpiece itself.

The Absorbent Canister/Electronics Housing

In the Electrolung the CO2 absorbent and electronics were housed in a transparent acrylic (Plexiglas) cylinder which, together with the gas bottles, was worn as a backpack. This cylinder was 24″ long x 4″ I.D. with 1/4″ thick walls. It was divided into two pieces. An 18″ section contained the absorbent bed with a 4″ space at the top which accommodated the sensors and O2 solenoid. Above this, a further 6″ section separated by a bulkhead contained the electronics. A small knob on the electronics section operated a double throw, center off switch controlling power to the electronics via either set of batteries. This was reachable behind the head if development of your biceps didn’t prevent it. Actually, geeks do make the best rebreather divers anyway, but out of respect for the temporary cease fire, I will refrain from further comment along that line.

Backpack viewed from behind.  Inert gas cylinder on left, O2 on right, absorbent canister between.  Electronics section at top.  Breathing hoses attach on the opposite side just below the electronics bulkhead.
Bottom of absorbent canister.  Note spring loaded bulkheads and screen to retain absorbent, central gas inlet tube from above and tie rod which threads into top of electronics section holding everything together.

The ends were sealed by 1/2″ thick PVC O-ring sealed plug-type closures. A thick PVC double plug-type bulkhead joined and partitioned off the electronics section from the absorbent section. The entire assembly was held rigidly together by a central 1/4 SS tie rod running from top to bottom. It had a large external knob at the bottom for tightening and loosening which was affected by screwing into a metal socket on the top closure. O-ring seals were used to seal the tie rod penetration of the bottom closure, the metal tie rod socket on the top closure and where the tie rod passed through the electronics bulkhead. This last was only to prevent capillary action from possibly drawing any water from the absorbent section into the electronics section. The electronics section atmosphere was vented to the absorbent section through a small canister of silica gel via a separate bulkhead penetration and a small standpipe as mentioned earlier in the description of the electronics.

Top of absorbent canister looking into space where sensors and solenoid fit when assembled.  Inhaled gas comes in on right, down the central tube to the bottom, then back through the absorbent, past the sensors, and out to the mouthpiece via the opening on the left.  Note top absorbent bulkhead in background and threaded top end of central tie rod in foreground. The shoulder in main cylinder is because acrylic tubing varies somewhat in size and we machined it out to a standard I.D. so the electronics bulkheads and end closures could be made to a standard size.

The O-ring seals for the end closures and the join at the electronics bulkhead were all radial type seals which automatically affect a proper seal when they are plugged into the cylinder. Sealing is effectively independent of how tightly or loosely things are clamped together. With this type of seal and operating at ambient pressure, the possibility of leakage around a seal is vanishingly small.

Front side of Electrolung backpack showing shoulder and waist strap arrangement.  Adjustable tabs carrying the twist studs for attaching the counterlung at the shoulders and waist provided for its positioning adjustment.

A brief aside for homebuilders: Although O-rings are marvellously effective seals and are universally used in all types of underwater equipment, it is remarkable how often manufacturers use them improperly. O-ring suppliers have various free pamphlets and data sheets on proper application of O-rings which includes data on the correct shape and tolerances for the grooves which accommodate them. It is well worthwhile to avail yourself of this information.

Common errors in O-ring usage often seen in marine equipment are: Grooves too deep, resulting in inadequate sealing pressure. Too shallow, resulting in too much compression of the seal, leading eventually to fine, radial cracking of the O-ring itself and consequent leaking. Too narrow, which interferes with proper compression and sealing at low compression and distortion toward a square cross section under full compression, this leading again to radial cracking of the seal. Finally, and most ignorant of all, is the use of rounded U-shaped grooves which defeats the whole principle and advantages of the circular cross section and turns it effectively into a flat gasket.

The absorbent canister portion of the cylinder was a 12″ section toward the bottom defined by two 1/4″ thick acrylic or PVC internal bulkheads perforated with an array of holes. Plastic screen was used to keep the absorbent from falling through the holes. The top bulkhead was fixed in place. The bottom one was free to move but held in place against the absorbent column by a large spring. This served to keep the absorbent compacted without channelling despite any minor settling of the granules after filling the canister.

We used Baralyme as an absorbent. This is a National Cylinder Gas trade name for Barium Hydroxide. It was widely used in hospitals and came in hermetically sealed one quart cartons of the type used for milk. The Electrolung canister held two cartons which would be sufficient for six hours of moderate activity. We changed them after four hours. Baralyme came with a color indicator—pink when fresh, blue when expired. It is less caustic than soda lime and worked well for us.

The author in the Electrolung showing the counterlung

As described earlier, gas was drawn from the counterlung to the space at the bottom of the canister down a central 1″ I.D. tube leading from the inlet hose attachment down to the bottom end of the canister section. From there it passed back up through the absorbent into the sensor/solenoid chamber and on via the inhale hose to the mouthpiece.

The gas supply cylinders were mounted on either side of the absorbent canister/electronics cylinder using spacer blocks conforming to the curvature of the respective cylinders. The three cylinders were secured rigidly in place by two large SS hose clamps. One was adequate for the purpose, so there was backup in the event of one breaking. The spacer blocks also served as attachment points for the harness, which was a U.S. Divers, one of the types widely used before B.C.’s took over this function. There was a wide vinyl strap for each shoulder plus a waist strap. The overall configuration of the Electrolung backpack was similar in many respects to that of the small triple tank OC rigs favored by the French at that time. It rode well on the back and was quite comfortable. All up, the weight of the Electrolung was about 30 pounds/14 kg.

Some people expressed concern about the use of acrylic, fearing the possibility of breakage. This is one of those things which is more apparent than real. In this case, it is protected on one face by the wearer’s body, on either side by steel gas cylinders, and at top and bottom by thick PVC ends. The only real exposure to any possible impact was the curved surface of a 4 1/2′ O.D. ¼”- thick cylinder, which would be extremely hard to break. We did look at using polycarbonate (Lexan) which is literally bullet proof but found it crazed and crumbled into small pieces when exposed to hydroxides. 

It would of course be easy to make the whole thing out of PVC, but I feel the advantage in being able to see condensation, water, and the condition of your absorbent more than outweighs the non-problem of smashing heavily into things while going backwards. Beckman offered a fiberglass fairing for those who might be concerned with this, and of course some then bought it because they liked the way it looked and others did so because they felt that if it was offered they probably should get the complete setup.

Note the absence of open circuit bailout. Limited open circuit bailout was available by manual O2 and inert gas control valves combined with use of a 50% Helium 50% air mix [trimix 10.5/50] for inert gas. This provided manual control of a breathable inert mix permitting ascent to a depth of 50 ft/15m above which manual control of oxygen could be used.—W.S.

A final aside for homebuilders: The Electrolung was really a homebuilt which became a commercial product. It was built entirely with a drill press, lathe, and jigsaw, plus a bench grinder for shaping and sharpening lathe tools as the only power tools. For anyone attempting to build any kind of underwater equipment, a metal lathe is really a must. You can easily make all kinds of cylindrical housings, O-ring sealed fittings, ports and closures, and any kind of threaded fitting you might need with one. 

A small lathe with a five or six inch swing over the bed will enable you to make housings and ports up to 10-12″ in diameter. Good quality, Chinese-made lathes suitable for this kind of work are now available for about US$1500. For another few hundred dollars, you can also get a milling attachment as well, which is a useful addition. Teaching yourself how to use it is not hard. Good textbooks which cover this kind of machine tool work are readily available and easy to follow.

Here is the Electrolung patent. It contains much more detail including various drawings.


  • Halcyon Sidemount
  • Fourth Element
  • DIVE RITE

Reflections and Speculations

Although development of the Electrolung was interesting, even exciting, in itself it was just an interesting incident in a bigger, far more interesting, and significant picture. Like most historical events, I suppose, what was happening to the participants at the time didn’t appear to be so remarkable as it later does with the broader perspective of hindsight. The larger perspective on what is taking place right now tends to be somewhat obscured by the ordinary events of living. Except for rare instances, whatever we are doing, however interesting and exciting it may be, tends to still feel like life, not like history in the making.

In retrospect however, I have come to realize that from the mid 1950s through the mid 70s, something really remarkable was taking place in diving. During that period, diving grew from the obsession of a small group of generally impecunious young people mostly in Florida, California, France and Italy to a global industry catering to well-to-do hobbyists. Remote tropical islands all over the world began to sprout airports and dive operations and diving became strongly oriented to travel to exotic locations. Though all this was in itself remarkable, something truly unique was at the real heart of what was happening.

For the first time in history, humans could freely enter, explore and personally experience the oldest, richest, most beautiful and exotic communities in nature, tropical coral reefs. Coral reefs are truly remarkable places. Nowhere else can one experience such an abundance and diversity of life. Nowhere else is it so colorful, exotic and so easily experienced at close range.

Diver with Electrolung.  Note pink Baralyme absorbent.  The acrylic bubble was being used for a shark cage.

Diving on a reef is like a trip in a time machine to a time before humans existed and nature ruled in primeval pristine abundance. Fossils of many reef creatures from as much as 60 million years ago are essentially the same as those on reefs now. In fact, some Pacific reefs have existed as reefs for that period of time.

For a biologist, being among the first to dive on reefs was a most extraordinary experience. In a way, it was a bit like landing on another planet. On nearly every dive, we were going where no human had ever been before. The discovery of phenomena of life, as well as strange and beautiful creatures whose existence we never even suspected was an everyday occurrence. At the time this kind of experience was so commonplace, tropic seas so vast and remote, and so few people were doing it, that it began to seem as if this was just the way things were, and this kind of experience would continue on indefinitely.

Already, however, this era has become history. Although there are vast amounts still to be discovered about the details and inner workings of reefs, still undiscovered species are getting harder and harder to find and remote locations are becoming less and less remote. The experience of being among the first to explore the richest realm of nature has come and gone, not to be repeated.

On reefs, one niche still remains. Actually it is a really big one. The zone below 200 ft/61 m, down to the deepest limits of what you might call part of the reef community at about 600 ft/184m, is still largely unexplored. Although it is not as rich in life as the shallower regions, it is still incredibly rich in life and is an area about which we know very little.

As far as I am aware (in 1998), the only person on the planet regularly exploring this zone is icythologist Richard Pyle. What he is doing is really exceptional, and he is doing it essentially on his own. While discussion on the Rebreather List is largely restricted to the technology and physiology as an end in itself, what Rich is doing goes well beyond this. As well as making more deep free dives than anyone ever has before, he is coming back with knowledge and specimens from every dive. What he is doing is a permanent contribution to knowledge which will stand long after any of today’s diving records are broken and forgotten. I have never met him personally, and am commenting only out of recognition of something exceptional.

Over the past 25 or 30 years advances in diving technology have been almost entirely small and incremental. The only real exception I can think of is the development of dive computers. It appears we are up against the realities of human physiology. With every increase in depth and bottom time, the cost, complexity, effort, and risk increases exponentially while the return of useful achievement remains more or less linearly related to bottom time.

The future, it seems, lies in other directions—especially robotics. Here the advances have been impressive, and future development promises to become even more so. Already we are at a point where more and more functions which previously required a diver can be effectively carried out by ROV’s. It is not hard to foresee that in a few years most of what we do at great effort and risk by diving can and will be done by nerds at consoles. In fact, right now in the Gulf of Mexico, Shell and BP are drilling in 5000-6000 fsw/1532-1839m and all work at the wellhead is done by ROV’S.

If you find this kind of scenario uncomfortable, don’t let it worry you. Long term prediction, no matter how well reasoned and seemingly inescapable, has a way of almost always being wrong. So much so that I have often wondered if beneath the facade of Newtonian certainty of our universe, somewhere in the iffy probabilistic realm of quantum mechanics, there is not a relationship which dictates that the very act of prediction sets in motion forces which generate a different outcome. So, if you don’t agree with my prediction, the good news is that I may well have avoided it by predicting it.

Fortunately, the real outcome is usually even more interesting than any of the predictions.

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How  did Starck and his colleagues manage their Electrolung decompression? Stay tuned for Starck’s follow-up story: Confronting the Unknowns of Decompressing on The First Electronic Rebreather in a coming issue of InDepth.

Dive Deeper:

Alert Diver (2017): Oxygen Sensing in Rebreather Diving by Michael Menduno

Popular Mechanics (July 1965): Half A Mile Down With Scuba by C.P. Gilmore

Reprinted with permission from the Historical Diving Society USA, The Journal of Diving History Volume 23 #85 4Q 2015.

SDM’s Publisher/editor Paul Tzimoulis’s account of diving to 300 ft/92m on the Electrolung (1970)
A review of the developments before the Electrolung including the work of Alan Krasberg

To become a member and access our treasure trove of dive history, visit us at: www.HDS.org. We are also on Facebook: Historical Diving Society USA

Download aquaCORPS Journal #7 C2 (DEC93): aquaCORPS’ rebreather issue See “Electrolung,” by Walter Stark p. 6 & 8. Proudly sponsored by Divesoft (Original magazine with two Divesoft inserts).


Walter Starck is one of the pioneers in the scientific investigation of coral reefs. He grew up in the Florida Keys and received a PhD in marine science from the University of Miami in 1964. Since 1978, his home has been in north Queensland, Australia. Throughout his career in marine biology, participating in expeditions around the world,  Dr. Starck has been extensively involved with development of the technology required to facilitate his activities. In several instances patented inventions and commercial products have resulted. In addition to the optical dome port and the Electrolung other noteworthy achievements in this area have been: The Bang stick, a hermetically sealed underwater firearm for hunting and defense. Underwater housings for numerous cameras and instruments. Underwater lighting systems. A multipurpose commercial waterproof electrical connector.  Design of the unique research vessel El Torito, a 9 meter high-speed diving launch, a 24 passenger eco-tourism vessel, and the Oceanic 8000 Longboat.  The longboat was a long narrow high efficiency powerboat inspired by the efficiency of the log canoes of the Solomon Islands.  He has also built and flown an amphibious aircraft of advanced canard wing design using high technology composite materials. Recently (Aug 2017) he was senior author on an extensive update on the Alligator Reef study that brought the total species list for that locality up to 618 species.

Dr. Starck has authored over 100 articles and books, which include numerous technical and peer reviewed scientific studies as well as many articles in leading popular publications. His photography has been widely published in conjunction with his writing, and he has produced nearly 20 films and videos. Throughout his extensive career, he has managed to inspire not only admiration, but also the ire of some detractors who have taken umbrage at his efforts to inject what he believes to be “a rational perspective on human ecology into the eco-mania that has become epidemic in our struggling Western economies.” His criticisms of the “poor science and blatantly false claims widely used to support various environmental agendas” have earned him some criticism.

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Equipment

Configure Me This: The Annotated Sidemounter

We make a deep dive into the world of sidemount diving, and examine seven leading sidemount systems with guest editors Steve Davis and Stratis Kas.

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This feature was created by Steve Davis, Stratis Kas and Michael Menduno. Introduction by Michael Menduno. Special thanks to Michael Thomas and brand representatives who helped us. Cover collage by SJ Alice Bennett with photos by Jason Brown of Bardo and Stratis Kas. Images by Stratis Kas unless noted.

🎶 Pre-dive Clicklist: Dave Brubeck – Take Five 🎶

At last, I am pleased to present InDEPTH’s long awaited sidemount edition, which has been in the works for over a year. The purpose of the issue is to celebrate the art and practice, as well as the equipment and culture of sidemount diving. Call it, “The Joys of Sidemount.” But whether you already follow the Bogarthian way, or simply regard it all as “widemount,” I promise you there is something for you in this issue.

The seed of an idea for a sidemount issue grew out of two things: First was the popularity and interest of our NOV 2021 feature that celebrated our innate gear headedness: “Annotated Tekkie,” which examined the question of what and how much kit is required to safely explore our underwater world. Diving is a technology-dependent activity to be sure.

Second, was, that we’ve been seeing tremendous growth and interest in sidemount diving we’ve been seeing. Like technical diving itself, what started in the cave community with small groups of experienced divers experimenting with DIY sidemount configs some three plus decades ago, has now blossomed into a mainstream, commercial tech diving activity in both cave and open water. Sidemount diving has now even spilled over into recreational diving—the configurational equivalent of recreational nitrox in the era of tech mixed gas diving.

One of the important inflection points was bringing sidemount diving out of the caves where the configuration was born, into open water. Sidemount pioneer Jeff Loflin was one of the individuals instrumental in bringing sidemount to recreational divers and the open water tech community. He explained it this way to me in our interview, “We were taking sidemount from the dark, and bringing it into the light.”

“We were taking sidemount from the dark, and bringing it into the light.”—Jeff Loflin

Interestingly, over the last decade, sidemount rebreathers, used as both a primary or a bailout, or both, have also gained significant traction driven by innovative vendors like KISS/XDEEP, SF2, Divesoft, and others. In fact, seven vendors are currently producing sidemount rebreathers. Note that we have also seen an increase in the number of chest-mounted units that can be integrated with sidemount systems. However, we decided to focus on open circuit sidemount for the purpose of this issue, andl address sidemount rebreathers at a later date.

Today, virtually all of the tech diving agencies offer open sidemount courses, with the possible exception of NAUITEC. For example, these range from “Intro to Tech” level sidemount courses, at RAID, which are aimed at getting divers into sidemount at the beginning of their tech journey. At the other end of the spectrum, Global Underwater Explorers (GUE) only offers a cave sidemount course to those who have completed GUE’s advanced cave training (Cave 2) and have at least 50 post-class cave dives.

Similarly, the majority of the major recreational agencies offer a recreational sidemount class for open water divers, including CMAS, NAUI, PADI, RAID, SDI, and SSI. Collectively, these agencies are issuing thousands of certifications a year.

So how big is sidemount diving? Good question, right?!?

How big is tech diving?!? We don’t really know. As an industry we suffer from a severe case of data insufficiency syndrome (DIS). [We can combat this—see our survey below!] However, in speaking with insiders involved in the sidemount business, some rough comparisons emerged.

Sidemount diving is likely bigger than rebreather diving, which according to estimates presented at Rebreather Forum 4 in April, likely represents about 15-20,000 divers globally. This seems like a plausible estimate given the significant cost difference, familiarity with the equipment, and perhaps greater accessibility to training and equipment with open circuit sidemount compared to rebreathers.

Sidemount rebreathers have gained significant traction. Here is Divesoft’s Joseph Bosquez diving a Liberty Sidemount in Ginnie Springs, Florida. Photo by Marissa Eckert.

Another comparison: sidemount diving is likely bigger than cave diving! “Sidemount is not just used for cave diving anymore, but I would say that at least 80% of cave divers have at least tried, or have sidemount in their tool kit. Fifty percent or more dive it exclusively,” Dive Rite general manager Jared Hires explained. Add in tech sidemount diving in open water and wrecks, and the growing number recreational sidemount divers, and we are likely to arrive at what sidemount instructor Steve Davis of Sidemount Pros estimates as “Cave divers plus some.” 

On the recreational side, a PADI exec told InDEPTH that PADI’s Sidemount Standard Specialty is now as popular as its Dry Suit Specialty, which is PADI’s third most popular Specialty Diver course, behind Enriched Air (Nitrox) Diver and Deep Diver, and ahead of Peak Performance Buoyancy. No drills on the knees, puhleez! I kid our friends at PADI.

Despite its growth and relative new-found prominence, sidemount still remains a kind of a best kept secret—the elephant in the restriction?—by which I mean many tekkies (including myself until recently) lack awareness and working knowledge of sidemount diving and may not have even tried it.

Not surprising divers being what they are, even among sidemount instructors and users, there’s a range of opinions on its use and application. On one hand, some view sidemount as a specific tool to be used exclusively for diminutive karst cave, on the other, many consider sidemount more of a platform choice. In fact, Davis, who you’ll hear more about in a minute, offered this question and assertion in our interview, Speaking Sidemount, and then went on and made his case:

 “Is backmount the best tool for cave diving? No, it’s not. Sidemount is far and away the best tool for cave diving!”—Steve Davis

Certainly, many including GUE, would challenge this assertion. But on the other hand, who died and left legacy twinsets in charge? Jacques Cousteau? Is backmount intrinsically safer? Does it offer better performance? If so, please share the data with everyone. There is room for respectful debate. Of course, every platform has its strengths and weaknesses, and environmental conditions, and individual physical and preference differences play an important role in choosing the appropriate platform. However, because of the lack of awareness, knowledge, and direct experience in the overall diving community, the limitations of sidemount get exaggerated, and its application, perhaps, under appreciated.

“There’s absolutely nothing I can’t do in sidemount that you can do in doubles, and there are a shit ton of things that I can do in sidemount, that you can’t do in doubles.”—Edd Sorenson

As instructor trainer and cave rescuer Edd Sorenson explains in his Who’s Who interview, “People used to tell me all the time all the things I couldn’t do in sidemount, and still do. ‘You can’t dive off a boat, you can’t do this, you can’t do that, you can’t scooter, you can’t double stage, you can’t quad.’ I would tell everybody the same thing. There’s absolutely nothing I can’t do in sidemount that you can do in doubles, and there are a shit ton of things that I can do in sidemount, that you can’t do in doubles.” Note in his interview Sorenson recounts taking GUE founder and president Jarrod Jablonski for his first sidemount dive. A trust me dive with Edd? Trust me, you don’t want to miss it.

Not surprising, it seems that many, or most of the dedicated sidemount divers we spoke to for this issue (See: The Who’s Who of Sidemount Diving) offered some version of the following, and or, would likely agree with this meme:

Once You Go Side, You’ll Always Dive Wide.

Ha! I kid our backmount sisters and brothers. None the less, it’s clear that sidemount is here to stay!

Now I’m not a sidemount diver, yet (it’s on my list), and only one member of InDEPTH’s team dives sidemount, in caves. So, to help us with the issue, we enlisted the help of two sidemount experts to serve as guest editors. We reached out to New Zealand-based sidemount instructor, ambassador, and host of the “Speaking Sidemount” podcast Steve Davis, principal of Sidemount Pros. We also enlisted the help of Greek photographer/filmmaker, author, long time InDEPTH contributor, and cave and sidemount instructor, the inimitable Mr. Stratis Kas. Both of them helped with the selection people and sidemount systems we planned to highlight, and authored content for the issue.

Fourth Element

Beginning with this lead story, “Configure Me This,” we explore seven recognized and storied sidemount systems, all of them a little different. Who knew there were that many? In contrast, it’s fair to say, backmount configs have generally become fairly standardized in the global tech community. Please note, that the majority of models are shown wearing their personalized branded gear not necessarily stock products.

Next, we offer the perspectives and stories of 34 leaders in the field, in a piece titled, “The Who’s Who of Sidemount Diving.” We also dive into the philosophy, culture, and practice of sidemount diving in “Speaking Sidemount” and “The What, Which and Why of Sidemount Diving’‘ with our guest editors.

In addition, we offer A Brief History Sidemount Diving with Lamar Hires, Bill Renaker and Patrick Widmann, and specifically the evolution of cold water vs warm water sidemount titled, “The Evolution of Sidemount System Design: Two Distinct Paths Shaped by Florida and Mexico,” by sidemount instructor and author Andy Davis. Finally, the issue wouldn’t be complete with a bit of DIY sidemount heresy from long time scuba engineer and troublemaker, Dave Mclean, call it Sidemount Heresy! Trust me, you’ll be better for the exposure!

Here then is InDEPTH’s celebration of sidemount diving and culture. We want to thank our forward thinking sponsors: DAN EuropeDive Rite, DivesoftFourth ElementHalcyon, Shearwater and XDEEP for making this issue possible. We also want to thank Nicole Alarid, Orie Braun, Jared Hires, Nick Hollis, Michael Thomas, and Patrick Widmann, who helped us sort out sidemount configurations and Elena Vivaldo for researching recreational sidemount. And of course, we want to thank our models: Robert Thomas (CDG), Ricardo Castillo (Dive Rite), Emöke Wagner (Halcyon), Melodie Trevino (Hollis), Marcelin Nebenhaus (Razor), Mélissa Bezaz (Toddy) and Tamara Adame (XDEEP). Looking good divers! We do plan to create a free downloadable poster from this issue. Watch this space!

Please note, we realize this issue primarily represents the views of advocates who are making a case for sidemount. We haven’t focused on stories of divers who have had bad experiences with sidemount or spent much ink delving into its downsides, to the extent that these exist. Consequently, I invite readers who feel so moved to submit their views and or experience, call it, “The Trouble With Sidemount ” or “Never Do This is Sidemount..” . I’m joking about the titles but serious otherwise. We will happily run thoughtful stories. Let’s get a conversation going.

Finally, we would like to acknowledge that as human diving journalists and artists on deadline, we have likely made errors and omissions, and/or failed to identify important items that our geeky readers will no doubt discover. Our apologies in advance. If you do find any errors, omissions, or needed tweaks, please let us know, and we will endeavor make corrections. Thank you.—Michael Menduno/M2

Please take a minute and complete our new: Sidemount Diving Survey. We will report the results in a coming issue.

Use the following navigation links to dive into your favorite configurations 

Cave Diving Group

Dive Rite Nomad Ray

Halcyon ZERO GRAVITY™ Sidemount System

Hollis Katana 2

Razor

Toddy Style

XDEEP Stealth 2.0 Tec

  • Fourth Element
  • DIVE RITE
  • Halcyon Sidemount

Cave Diving Group (CDG) Sidemount System 

The CDG sidemount system has evolved many times since the 1960s but it’s still a lightweight system for diving sump’s generally found after significant dry caving. The important thing to remember is that it is not just one system. Each diver will have a slightly differently made harness but basically doing the same thing. 

Photos courtesy of the CDG.
Weight System
  • Traditionally the weights go around the waist on a CDG harness, a more modern version of the CDG harness such as the RMTD harness (in the photo) weights can be fitted down the spine. 
Harness
  • The harness itself is a simple design with minimal adjustment but designed to walk or cave while wearing wetsuits and small cylinders. It can also be used with larger cylinders. Steel cylinders are the norm for uk cave diving. The harness has fixed D rings and no sliding D rings for use with Aluminium cylinders. 
  • Some CDG harnesses are adapted to use with vertical access equipment (SRT) so the caver/diver can descend the vertical dry cave then use the same harness to dive. 
BCD/Wing
  • The CDG harness has no wing, because generally buoyancy is not required when using wetsuits and small cylinders in sumps. It was common for divers in British Sumps when using drysuits to use the suit for buoyancy and use no wing. In more recent years a small wrap around wing such as the X deep classic or home made equivalent can be easily added if the diver is using bigger cylinders or carrying equipment through the sump to further dry cave exploration and requires buoyancy.
Commentary

The CDG harness is designed to take weight loading on the divers’ shoulders and hips. Karabiners are generally used for attaching cylinders to the harness. A bolt snap is not a weight loading piece of equipment and should not be used to lower equipment down and then up vertical sections of dry cave, karabiners also work when covered in mud or grit! It’s not a harness that will get cylinders in that perfect internet trim but it is a harness designed to explore flooded cave sections found a long way underground. All modern harnesses took the sidemount system from the original Cave Diving Group harness design. 

Dive Rite Nomad Ray

The Nomad Ray is the latest sidemount system offering from Dive Rite. It includes a number of notable upgrades to the Nomad range and was designed to be easily adjustable, suitable for a range of body types and diving applications, including the use of both steel and aluminum cylinders.

Weight System
  • The Nomad Ray has three padded, internal pockets each of which can hold up to 5 lbs/2.3 kg of either hard or soft weight.
  • The weight pockets are located on the back of the wing and accessed via a zipper, making for easy weight adjustments while fitted.
Photos courtesy of Dive Rite.
Harness
  • The harness consists of independent shoulder, waist/hip, and crotch straps each of which are adjustable to fit the system to a variety of diver body types.
  • The harness incorporates custom hardware including two fixed angled D-rings on the shoulders, two fixed angled D-rings on the waist/hip strap plus, two sliding waist D-rings, two chest bungee-retainer slides, two rear drop D-rings for primary tank attachment, and one “dog-bone” crotch strap double D-ring for accessories, plus a DPV D-ring.
  • The harness is in Y configuration and attaches to the BCD/wing via woven D-rings at the top and through woven loops and screwed connection at the bottom.
  • For the fashion conscious, the wing comes in six colors.
BCD/Wing
  • The BCD wing is shaped to provide lift at the lower lumbar region. It comprises a Denier outer shell with an inner bladder and has 42 lbs/19 kg of lift.
  • The upper backside panel is abrasion-resistant for restrictions.
  • It incorporates a standard inflator that can be routed from the left (standard) or right side.
  • The BCD includes two pull dumps which are also OPVs. One is located at the inside top center of the wing with a left shoulder pull dump activated via a sheathed cord. The second is located at the bottom left of the wing. The locations of the dumps may require an alteration of diver position to dump all of the gas from the wing e.g. roll to the right to dump from the left side lower dump. Note this would also align with dumping gas from a drysuit. 
  • “Belly-band” bungees locate and wrap the outer wing extremities to the diver’s body. The bungee includes a bungee hook attachment and two wing grommets for easy and quick adjustments by simply moving an overhand knot on the bungee.
Commentary

The Nomad Ray is a significant upgrade for Dive Rite and evolving to meet the needs of modern sidemount divers. In particular, the easy-access weight system, shoulder dump, easy-to-adjust “belly-bungee,” plus the method of attaching the harness and the independent crotch strap to the wing, are notable improvements.

Halcyon ZERO GRAVITY™ Sidemount System

Halcyon’s Zero Gravity sidemount system was a collaborative creation with our Mexico distributor, Zero Gravity. The Zero Gravity system puts the buoyancy over the hips and along the sides of the rib cage where buoyancy is desired. Extremely streamlined, the air chamber positioning gives it a flat profile on the back of the diver. The Zero Gravity is suited for double aluminum 80’s and a wetsuit or small steel cylinders used with dry suits and heavier undergarments. The Zero Gravity features an adjustable harness, and an optional weight pouch that will hold up to 15 lbs./6.8 kg, and optional inflator placements on the right or left side.

Weight System
  • The added weight attachment is designed and built to lay on the diver’s back in line with the spine. Like our traditional weight options, this allows the diver to keep perfect trim while adding weight. With the three-pocket design, you can add up to 5 lbs/2.3 kg in each pocket as needed and disperse them at different placements for that balanced distribution. The velcro additions allow for secure storage and keep even soft lead pouches from moving around while diving. 
Top left and right photo courtesy of Halcyon Dive Systmes.
Harness
  • A harness system with multiple point adjustment is styled after the standard backplate harness, with the lower portion of the strap going behind the diver, allowing for greater freedom of movement and comfort, especially for the women in the diving. This harness system is easily changeable when needed over time, allowing the diver to modify the colors of the webbing if desired for personal customization.
  • The newly redesigned door handles provide divers with several options when choosing the right cylinders and placement. With the added grommets, the diver can adjust the door handles into three different locations on the back, as is, higher, or in towards the diver’s back. This placement is essential when setting up the cylinders. The added feature of using curved door handles over traditional square door handles is that when the cylinders are clipped to the system, the bolt snaps do not snag or get hung up in a corner. This keeps the cylinders and the bolt snaps in a free space for easy reaching, clipping, and removing as needed throughout the dive. 
  • The adjustable bungees make it easy to use with different regulator configurations, whether the regulator is pointed up or down. The waist strap D-rings on the right and left hip allow for AL80’s to be clipped forward as they become buoyant. We achieve this with a non-fixed D-ring by adding an O-ring to the design. This allows the D-ring to be in an upright position for easy finding and clipping while at the same time allowing the D-ring to move freely if in a tight passage or corridor without becoming a snag point. 
  • The uniquely positioned cylinder bungee runs across the back of the system and diver giving you more stretch when wrapping your valves while keeping your cylinders high, tight, and secure. This placement of the bungee allows the diver to adjust the bungee length very easily, whether in training or for the more experienced sidemount divers. This bungee is very easy to change out by the user if they prefer a lighter or heavier bungee without compromising the design of the system.
BCD/Wing
  • The streamlined design with a unique U-shaped bladder provides an almost free and clean back that reduces your profile and drag. The U-shaped bladder allows stable, uniform lifting along each side of the diver where they need it and across the rear of the diver so they have adequate lifting potential without sacrificing profile and trim. This keeps the profile of the diver very low enabling them to access smaller passages without rubbing the bladder. Top to bottom while trim position does not change even as the bladder is fully inflated. This was a key component in the design for low-profile passages and doorways.
Commentary

“As an avid sidemount diver and explorer in North Florida, I have used a variety of side mount systems on the market. It wasn’t until I dived the Zero Gravity system, that I was able to pass through several low passage tunnels without feeling the top of the system scraping along the way. The low profile of the system, even when fully inflated, allowed me as the diver, to keep a low profile in the cave system. This truly became a game changer when it came to exploring new tunnels.” Orie Braun, Halcyon Sales Manager

Hollis Katana 2

The Katana 2 is the latest sidemount offering from Hollis and brings them into line with the other top contenders in the sidemount space.

Designed with support from cave explorer Edd Sorenson, the Katana 2 incorporates a number of innovative features including a “Quick-Fit” system for easy adjustment, the ability to configure as a H or Y style harness, out of the box support for the KISS Sidewinder, and a BCD top dump valve with shoulder pull.

Photo courtesy of Melodie Trevino.
Photo courtesy of Hollis.
Weight System
  • The Katana 2 weight system features a drop-in Velcro pouch design with 4 x 5 lbs/2.3 kg pockets along the spine. The spine positioning of the weight system can assist with trim adjustment as required.
Photos courtesy of Hollis.
Harness
  • The Katana 2 sidemount harness features an innovative “QFS” Quick Fit System which allows users to easily tailor the one-size-fits-all harness to their specific size.
  • The design also allows for either an “H” or “Y” style harness configuration. H-style is most commonly used when donning the system in a drysuit and can be less restrictive, while the Y-style is most similar to a backplate configuration and provides a snug but comfortable fit when properly sized. Both are suitable for various types of diving and user preferences.
  • The Katana 2 is the first harness with attachment loops built-in to support the popular KISS Sidewinder CCR and similar units now in the marketplace.
BCD/Wing
  • The wing lift capacity is 40 lbs/18 kg and features a tapered design to keep the diver profile as low as possible, without the “turtle shell effect” which causes drag. 
  • The top of the wing is two- dimensional, versus the bottom which has a three-dimensional design for targeted lift placement to promote better horizontal trim. D-ring attachment points for accessories are placed on both sides of the wing, which are easier to reach than the traditional crotch strap attachment. The wing is also available in either a single or dual bladder option.The Katana 2 wing has a 1000D Nylon Cordura outer shell for abrasion and puncture resistance, and a 420D nylon inner bladder. The wing includes a top dump with shoulder pull that allows venting of gas while in trim.
Commentary

“The coolest thing about the Katana 2 is the way it ships out of the box. You can customize it to your preference thanks to the innovative harness and adaptable wing design. Technical divers love to customize their gear … and they can make all of the adjustments themselves without permanent modifications on this rig.” Hollis Brand Manager, Nick Hollis

Razor

The Razor Harness embodies simplicity and elegance with just two continuous pieces of webbing and one closure point. Its design is minimalistic, yet strong, rugged, and reliable. The harness offers a comfortable and custom fit for divers of all sizes, thanks to its quick and easy setup and adjustable, standardized hardware.

Weight System
  • The Razor Harness allows precise placement of weights for optimizing trim. Additional weights can be easily added to the Razor Pocket Weight, the Waist Strap, or both if more than 13 lbs/6 kg are needed.
Harness
  • All attachment points, such as D Rings, on the Razor Harness can be swiftly and easily adjusted for personalized equipment placement. Each Shoulder Strap/Waist Strap can be adjusted at the Mini Back Plate.
  • The length of the Lumbar/Crotch Strap can be adjusted at the Delta Shoulder Plate. The height of the Waist/Hip strap can be adjusted at the Mini Back Plate. Extra attachment points can be added if necessary.
BCD/Wing
  • The BCD/wing is simple to use and attach or detach from the Razor Harness, secured by just two button head bolts. The mounting position can be easily adjusted to accommodate different-sized divers.
  • The wing is exceptionally durable, constructed with three layers: two outer layers of abrasion-resistant 1000 denier ballistic nylon with a layer of heavy gauge polyurethane in between. The wing is ultrasonically welded, and all edges are finished with edging tape. All attachment points feature reinforced grommets for added strength.
  • The primary wing provides 45 lbs/20 kg of lift and is equipped with a low-profile, heavy-duty manual dump/OPV valve from DSS, along with a standard power inflator. This allows inflation of the primary wing either using the power inflator or orally.
  • The fittings for the dump valve and corrugated hose/power inflator are interchangeable, enabling divers to use them on either the left or right side of the wing according to their preference.
  • The Redundant wing also provides 45 lbs/20 kg of lift and is fitted with a very low-profile “coin” dump/OPV valve and oral inflator.
Commentary

The Razor Harness and BCD/Wing system offers divers a sleek and minimalist design without compromising on functionality or durability. Its simple yet robust construction provides a comfortable and secure fit for divers of all sizes. The weight system allows for precise weight distribution to optimize trim, while the adjustable attachment points offer personalized equipment placement. The BCD/wing is easy to attach and remove, and its rugged three-layer construction ensures long-lasting performance. With interchangeable fittings and low-profile valves, the system offers versatility and convenience. Overall, the Razor system combines simplicity, reliability, and versatility, making it an excellent choice for divers seeking a streamlined and efficient diving experience.

Toddy Style

The Toddy Style Sidemount System offers divers a unique and innovative approach to sidemount diving. Its “sandwich” style weight system, adjustable harness, and thoughtful design elements enhance comfort, balance, and ease of use. With the ability to customize weight distribution, easily adjust harness straps, and utilize specialized clips, this system provides divers with a streamlined and efficient sidemount experience. The Toddy Style Sidemount System is a reliable choice for divers seeking enhanced maneuverability and minimal drag during their underwater explorations.

Weight System
  • The “sandwich” style system consists of a thin backplate that holds the wing together while protecting it from accidental restriction impact. A backmount-style backplate finishes the sandwich-style system. These backplates also act as a weight system. If the user desires, additional backplates can be added, fitting perfectly on top of each other and distributing the weight evenly over the diver’s back, rather than concentrating it on the spine. This allows for better balance and reduces the “rotation” effect.
  • For divers who want to travel with the system, there is a more traditional sidemount weight system. It consists of a weight holder that can be added to the backplate, again placed away from the spine, to enhance balance and minimize the “rotation” effect.
  • Finally, there is a flexible butt extension with a special mount for precise addition of weight to achieve optimal trim.
Harness
  • The system’s shoulder straps are easily adjustable using a Velcro system that secures it in place with wide elastic wraps.
  • The shoulder straps are connected by a removable bungee that keeps them in place when divers turn on their side. It also acts as a temporary clipping place for items such as pigtail marker clips, lights, etc.
BCD/Wing
  • The inflator extends from behind, near the neck area, resembling the position in a backmount system. There is no valve, but instead, a direct, extremely durable, and resistant connector. This eliminates a fragile point present in all other systems.
  • The wing’s bladder is easily accessible and can be changed even in the field.
Regulator accessories
  • The system also utilizes custom-made clips for the regulator’s second stages. These clips allow divers to secure the regulators to their chest D-rings, resulting in a more streamlined profile and easy access in case of an emergency.
Commentary

The Toddy Style Sidemount System stands out as a top choice among cold water backmount divers looking to transition smoothly to sidemount diving. It offers a convenient alternative because of its compatibility with both existing regulators and hose lengths, as well as not requiring dedicated cylinders. The presence of a backplate, reminiscent of traditional backmount setups, provides a sense of familiarity and ease of adaptation. Whether for cold water diving or any other diving environment, the Toddy Style Sidemount System offers divers a reliable and comfortable sidemount configuration to enhance their underwater experiences.

xDEEP STEALTH 2.0 Tec

The STEALTH 2.0 TEC was designed for deep decompression diving and extended cave penetrations. It provides 42 kbs/19 kg of lift to support multiple cylinders required for advanced diving. It effectively manages gas movement and position to ensure stability, balance, and trim at any inflation level.

Photos courtesy of Mekan Photography.
Weight System
  • The Xdeep Stealth 2.0 features a customizable central weight system located on the spine. It can be adjusted to accommodate the maximum weight preferred by the diver.
  • Additionally, there are droppable weight pockets available in different sizes: S-size (2 x 4.4 lbs/2 kg), M-size (2 x 6.6lbs/3 kg), and L-size (2 x 13.2 lbs/6 kg).
  • For fine-tuning trim purposes, there are also trim pockets in M-size (2 x 4.4 lbs/2 kg) or L-size (2 x 6.6lbs/3 kg) that can be placed anywhere on the harness.
Harness
  • The harness is one universal size with a wide adjustment range. It consists of independent shoulder, waist/hip, and crotch straps, each made of different thicknesses and fully adjustable with tri-gliders to fit various diver body types.
  • The harness utilizes custom hardware, including two fixed large D-rings on the shoulders, two fixed D-rings on the waist/hip strap, and two rubber sliding waist D-rings (metal version available as an upgrade).
  • It also has two rear drop square attachments for primary tank attachment, a crotch strap D-ring for accessories below the dump valve, and a DPV D-ring.
  • The Y-configured harness attaches to the BCD/wing at the top.
BCD/Wing
  • The BCD wing is designed to provide lift at the lower lumbar region. It is constructed with a Cordura 1100 dTEX outer shell fabric and a Nylon 440 dTEX inner bladder with a 0.2 mm TPU coating. It offers 42 lbs/19 kg of lift.
  • The upper backside panel is abrasion-resistant for durability.
  • The BCD includes an inflator available in three sizes, with a standard length of 16″/41 cm (14″/36 cm and 19″/48 cm available upon request).The inflator can be routed from the left (standard) or right side.
  • One central low pull dump also functions as the system’s OPV.
  • Bungees keep the wing attached to the waist harness, ensuring a snug fit around the diver’s body.
  • The wing’s color combinations can be customized on Xdeep’s website: https://tuneup.xdeep.eu
Commentary

The XDEEP Stealth 2.0 is a well-designed and versatile diving system that caters to the needs of deep decompression diving and extended cave penetrations. Its robust weight system, adjustable harness, and efficient gas management make it a reliable choice for advanced divers. The BCD/wing combination offers ample lift and durability, while the customizable options allow divers to personalize their gear. Note that the modular system can also be configured from recreational sidemount. Overall, the XDEEP Stealth 2.0 combines functionality, comfort, and performance, making it a strong contender for serious diving adventurers.

Please Take a Minute And Complete our New: Sidemount Diving Survey. We will report the results in a coming issue.

Steve Davis  is the producer and host of the acclaimed podcast, “Speaking Sidemount,” author of the books, “The Canterbury Wreck – A Diver’s Guide” and the eBook, “Sidemount Fundamentals.” He is a specialist sidemount diver/instructor, dives exclusively in sidemount, and is the principal instructor and founder of Sidemount Pros. Steve travels the world diving sidemount in caves, wrecks, and open water. Through Speaking Sidemount Steve’s mission is to share his passion for sidemount diving and provide a medium for the world’s top sidemount divers, instructors, and explorers to share their experiences and thoughts on sidemount diving.

Stratis Kas, a Greek-Italian professional diving instructor, photographer, film director, and author, has spent over a decade as an esteemed Advanced Cave instructor, leading expeditions to extreme locations worldwide. His impressive diving achievements have solidified his expertise in the field. In 2020, Kas published the influential book “Close Calls,” followed by his highly acclaimed second book, “CAVE DIVING: Everything You Always Wanted to Know,” released in 2023. Accessible on stratiskas.com, this comprehensive guide has become a go-to resource for cave diving enthusiasts. Kas’s directorial ventures include the documentary “Amphitrite” (2017), shortlisted for the “Short to the Point” Film Festival, and “Infinite Liquid” (2019), which explores Greece’s uncharted cave diving destinations and was selected for presentation at Tekdive USA. Kas’s expertise has led to invitations as a speaker at prestigious conferences, including Eurotek UK, Tekdive Europe and USA, Tec Expo, and Euditek.  For more information about his work and publications, visit stratiskas.com.

Michael Menduno/M2 is InDepth’s editor-in-chief and an award-winning journalist and technologist who has written about diving and diving technology for more than 30 years. He coined the term “technical diving.” His magazine “aquaCORPS: The Journal for Technical Diving” (1990-1996) helped usher tech diving into mainstream sports diving, and he produced the first tek.Conferences and Rebreather Forums 1.0 & 2.0. In addition to InDepth, Menduno serves as an editor/reporter for DAN Europe’s Alert Diver magazine, a contributing editor for X-Ray mag, and writes for DeeperBlue.com. He is on the board of the Historical Diving Society (USA), and a member of the Rebreather Training Council. Menduno is the organizer of Rebreather Forum 4.

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