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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.



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.



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.



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

Decompression Habitats Are Ascendent

Armed with reliable rebreathers, expedition-grade scooters, electric heating, helium mixes, high-powered dive computers, and those all-important P-valves, today’s cave explorers are giving our collective underwater envelope a hard shove (deeper and longer), all the while enduring increasing hours of long, cold, boring decompression. That’s the reason that the use of deco habitats—first pioneered by Dr. Bill Stone in the late 1980s—is on the rise. Here anesthesiologist-cum-cave explorer Andy Pitkin explains everything you need to know about modern deco habitats from their history, construction, and positioning to ensuring adequate, safe breathing gas flow.

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By Andy Pitkin

Cold. Hungry. Uncomfortable. Bored. These adjectives can aptly be applied to the vast majority of divers during the decompression portion of advanced technical dives. The commercial diving industry, less concerned about divers’ comfort and more interested in safety and efficiency, has long incorporated decompression in a dry chamber for anything other than shallow diving operations. Unfortunately, with the notable exception of Bill Stone’s 1999 Wakulla 2 project, surface decompression in a pressurized chamber has been impossible for technical divers. The next best thing is a habitat.

The ‘habitrough’ used by Sheck Exley at Cathedral Canyon during his exploration there in the 1980s. Photo by ?

Habitats are gas-filled spaces underwater that allow a diver to remain at pressure while getting part or all of their body out of the water. The name comes from experimental living quarters such as the Sealab series where divers would remain underwater for a number of days for (usually) scientific purposes. The habitats used for decompression by technical divers are much more modest, and this article will discuss the theoretical and practical considerations of decompression habitats, some of which are obvious, and some have had to be learned through real-world experience. 

Paul Deloach (left) and Sheck Exley (right) decompressing in the habitat designed by Bill Stone and funded by Rolex for the Wakulla Spring 1987 exploration project by the US Deep Caving Team. The 40m (120 ft) chain hoist allowed the divers to adjust the depth of the habitat as their decompression progressed. Photo taken by Wes Skiles and courtesy of Bill Stone/US Deep Caving Team, Inc.

Advantages Of Habitats

The benefits of a decompression habitat are so self-evident that they hardly need to be mentioned. The most obvious is warmth, because of the much lower loss of body heat in a gaseous environment compared with immersion in water. Even if only the diver’s head is out of the water there is a significant improvement in both subjective and objective thermal homeostasis. Being out of the water reduces both the risk of oxygen toxicity and the severity of the consequences of a seizure, which is likely to be fatal underwater but could probably be survived in a habitat.

Eating and drinking is much easier, and the ability to talk, listen to music, watch movies and pass the time in relative warmth and comfort makes a long decompression of many hours much easier to tolerate, as well as being considerably safer. A habitat also can be used as a makeshift on-site recompression chamber, which could at least allow a diver’s symptoms to be stabilized while arrangements are made to support the necessarily lengthy in-water decompression phase.

Andrew Pitkin illuminates the 15 m/50 ft habitat at Lineater Spring. This habitat is 700 m/2300 ft underwater from the cave entrance. The aluminum “floor” greatly assists entry and egress from the habitat. Photo by Kyle Moschell / Karst Underwater Research.

Securing the Habitat

Decompression habitats have occasionally been installed in open water; examples include Martin Robson’s exploration of the Blue Lake in the Russian Caucasus mountains in 2012 [1] and Michael Lombardi’s Ocean Space Habitat, also in 2012 [2]. The overwhelming majority have been used in cave diving, because underwater cave exploration often mandates lengthy decompression and the environment usually guarantees that decompression will occur in a specific location. The wide variety of underwater caves has resulted in many different approaches to construction, from sealing a natural airspace formed by a dome in the ceiling using a tarpaulin (“habitarp”), upturned rubbish bins (“habibin”) to large custom-designed and manufactured enclosures. A volume of gas large enough to be useful has considerable buoyancy, which must be restrained either from above by the cave ceiling or from below using the floor or wall of the cave passage. A 1000 liter (264 gallon) IBC container often used for this purpose has a buoyancy of 1000 kg (2204 lbs), and many habitats are larger. 

Unless it is constrained by the cave ceiling, the anchoring system must be very strong and reliable. Natural anchors such as rock projections and large boulders are better for conservation, but they may not be available in the required location, necessitating placement of artificial anchors in the cave wall or floor. These are very similar to anchors used in vertical dry caving, and can be screw anchors, expansion bolts or even glue-in types, typically made of stainless steel (or titanium, if money is no object!).

Air-powered drills are much less expensive than battery-powered underwater drills but can use a large amount of compressed air. When our group (Karst Underwater Research or KUR) placed a habitat 2 km/6562 ft from the entrance of a cave in 2013 (when no suitable battery-powered underwater drill was available), the large volume of bubbles released from the air drill we used to make the holes for the anchors percolated so much silt from the walls and ceiling that the water visibility was reduced to almost zero for about a third of the exit distance. Whatever method of fixing the habitat is chosen, it needs to be very secure, as the consequences of an anchor coming loose could be extremely severe.

The 16 m/52 ft habitat in the Pearse Resurgence. Photo by Simon Mitchell.

The depth of the habitat may be a compromise between what is ideal for decompression and what is dictated by the location. Since the final decompression stop is the longest, the habitat is often targeted as close to 6 m/20 ft deep as possible. Some advanced projects, most notably the Wet Mules’ exploration of the Pearse Resurgence in New Zealand, have used multiple habitats at various depths because of the extreme maximum depth of more than 240 m/787 ft and cold water 6°C/43°F. 

To maximize the air space, the habitat container needs to be as level as possible. In other words, the water level can be no lower than the highest point of any of the sides where gas can escape, and this consideration may be more important than installing it at the ideal depth. When a habitat is anchored from below, it is usually easiest to start a little deeper than the intended depth and then adjust to the correct depth before the container is completely filled with gas.

Our group typically uses polyester static caving rope (nylon lengthens about 5-10% on getting wet) with equalized double anchors at the bottom (double figure 8 or bowline on a bight) and ‘super Münter’ adjustable hitches at the top for easy adjustment of length. When the anchor points have been close to the bottom of the habitat, we have had a lot of success with appropriately-rated webbing ratchet straps.

The 30 m/100 ft and 40 m/130 ft habitats in the main shaft of the Pearse Resurgence during the Wet Mules’ exploration in 2021. The cable carried power for drysuit heating and a simple buzzer communication system. Photo by Simon Mitchell.

The Use of Containers

Many factors will influence the choice of a container for a habitat, but they can be reduced to two primary ones: location and cost. Inflatable habitats—for example modified commercial lift bags—have the advantage that they can be rolled or folded up to fit through narrow parts of the cave. We have found that a large golf club case works as a streamlined container for an inflatable habitat that can be swum or towed by a DPV.

A rigid habitat, typically an industrial or occasionally purpose-built container, is much more cumbersome to move into a cave, and these are typically installed close to the cave entrance, which obviously has to be large enough for it to fit through. Experience has shown that any modifications to the container (e.g. rings or hooks for hanging equipment) are vastly easier to perform out of the water before the habitat is installed, especially if any kind of adhesive is required. A reliable valve near or at the highest point in the habitat is very helpful for removing gas when the habitat needs to be adjusted or removed but, with a little practice, gas can be siphoned out by two divers and a short length of garden hose. 

Unless the cave floor is close to the bottom of the habitat, the occupants will need either a floor or seats to keep them out of the water. The size and positioning of seats is a compromise between comfort and ease of entry into the habitat. 

Breathing Gas

The easiest and most inefficient option is for divers to use a conventional open-circuit, second stage regulator, with the cylinder being hung in the water below the habitat. Using a conventional diving rebreather may be difficult because of space limitations, prompting some home-made designs which are usually of the chest-mounted (or ‘laptop’) configuration. They can also be suspended at any convenient place in the airspace, because there is no hydrostatic counterlung loading.

The most efficient and comfortable option is for divers to breathe the habitat atmosphere itself, which immediately presents three new considerations: oxygen addition, carbon dioxide (CO2) removal, and gas monitoring. Let us look at each of these in turn.

Adding Oxygen

The above-mentioned 1000 liter IBC container, large enough for two divers, positioned at 6 msw/20 fsw, and filled with the surface equivalent of 1280 liters of oxygen and 320 liters of nitrogen, would entail an oxygen fraction of 0.8 and a partial pressure of oxygen (PO2) of 1.28 ata. We can conservatively assume that a decompressing diver will have an average oxygen consumption of about 1 liter/minute, and therefore two decompressing divers would consume about 120 liters of oxygen per hour. After one hour, the oxygen fraction within the habitat would have dropped to 0.78 and the PO2 to 1.25 ata, assuming the resulting CO2 does not remain in the airspace. This simple calculation, which is supported by practical experience, shows that elaborate arrangements for maintaining habitat PO2 are unnecessary and can be accomplished by simply purging an oxygen second stage intermittently within the habitat (e.g. every 30 minutes or more).

Removing CO2

There are only two ways of removing CO2 from an enclosed airspace: replacement by adding gas free of CO2, and chemically removing the CO2 from the atmosphere using a CO2 absorbent (scrubber). The first method is often used in hyperbaric chambers—which share many of the practical problems of underwater habitats—because it is safe and simple. Unfortunately for technical divers, it is too inefficient to be practical in most circumstances. Going back to our example above, our two divers will have exhaled about 96 liters of carbon dioxide in the first hour, assuming a typical respiratory quotient of 0.8, resulting in an ambient CO2 concentration of 6.5% (surface equivalent by volume). By this point, both divers would  likely be feeling significant adverse effects. 

If we assume that the CO2 in the habitat atmosphere should be maintained below the 0.5% surface equivalent value commonly used for rebreather scrubber testing, flushing of the habitat would have to be started after less than 5 minutes. The rate of continuous flushing to keep the CO2 in an enclosed pressurized airspace at a constant level is given by the following equation [3,4]:

where Qgas is the rate of gas ventilation, Pamb is the ambient pressure, VO2 is the total oxygen consumption of the divers, R is the respiratory quotient, F is a mixing factor (1 = ideal mixing) and PCO2 is the desired ambient partial pressure of carbon dioxide. 

For our two divers, the habitat would have to be flushed at a rate of 512 liters per minute or 19.5 cu ft per minute (surface equivalent) to maintain the CO2 at a surface equivalent of 0.5%. Note that the amount of gas required is independent of the volume of the habitat. This is logistically unsustainable in most situations: a typical 80 cu ft (11 liter) aluminum cylinder would last less than 3 minutes. This shows how difficult it is to maintain low CO2 levels with flushing of the gas space. Even if the CO2 is allowed to rise to a surface equivalent of 2%, which would cause some breathlessness but might be tolerable, the same cylinder would still only last about 16 minutes.

For the Wakulla project in 1987, Bill Stone calculated a 32 cu.ft./min (906 liters/min) gas flow requirement for an exploration team in that habitat positioned at 60 ft/20 m depth [5]. Two industrial Ingersoll-Rand surface compressors were easily able to meet this demand via a 400 foot long, ¾ inch internal diameter hose with manual shutoff valves and check valves fitted at both ends to prevent inadvertent venting of the habitat atmosphere when the compressors were not running. No direct measurement of habitat CO2 levels were made; the divers were able to purge the gas in the habitat whenever it seemed excessively ‘stuffy’.

The only other way to reduce the CO2 in the atmosphere is to remove it chemically, turning the habitat into a giant shared rebreather. This is relatively a simple engineering task, using a sealed 12V motorcycle radiator fan to blow habitat gas through a scrubber bed, ideally with some form of speed control to allow the flow rate through the absorbent to be controlled by the diver(s). It can be powered from portable battery packs (such as those used for dive lights or undersuit heating) or a cable from the surface. Such a device needs to be transported to the habitat inside an appropriate container or designed into a pressure-proof housing (see picture).

A habitat scrubber built into a pressure-proof housing, with a venting valve on the left end cap. The white cylinder in the center is the axial scrubber basket, the space on the right is occupied by the fan, and the space to the left contains oxygen sensors and monitoring. The whole unit is neutrally-buoyant and measures 14 in/36 cm long with a diameter of 6 in/15 cm. Photo: Andrew Pitkin

Monitoring Your Gas

When KUR started building habitat scrubbers about 10 years ago, we used a prototype CO2 monitor for a rebreather to help decide how fast to run the scrubber motor. The monitor, which used infrared absorption spectrometry to measure CO2, was power-hungry and would exhaust all of its battery capacity in a few hours if left on continuously, so we would only switch it on intermittently. To pass the time while it was warming up, we would attempt to guess what the reading would be, and after a few iterations we became surprisingly good at estimating the CO2 level subjectively by how ‘stuffy’ the habitat atmosphere felt. Switching on a habitat scrubber fan feels pleasantly like someone opening a window, but the insidious accumulation of CO2 when the scrubber is off is much harder to notice. As an aside, I believe there is some potential for research into whether divers can be trained to recognize increasing levels of inhaled carbon dioxide from scrubber breakthrough. Handheld CO2 meters are available, and we are currently evaluating some of these for use in our habitats. Many are not suitable for the environment or will not give accurate readings in the presence of 100% humidity.

Oxygen measurement is simple, as in any rebreather, and can easily be combined with the scrubber assembly so that the sensors sample the gas being circulated by the fan. 

Ensuring Diver Safety

The limited space in most habitats often precludes the use of the divers’ main scuba system, in which case this must be removed when entering the habitat. When leaving for the surface (or a shallower habitat) this must either be redonned or a separate (often simple open-circuit) scuba used. These transitions present some hazards, especially if there is no solid floor beneath the air space with the potential for critical items to be dropped out of reach. A support diver is very valuable to assist a mission diver with entering and exiting the habitat and retrieving any items that are inadvertently released.

Explorer Matt Vinzant decompresses inside the 15m/50 ft habitat at Lineater Spring wearing his dual rebreather rig. This habitat is custom-built to allow two divers with redundant rebreathers to sit reasonably comfortably inside. Photo: Andrew Pitkin

As mentioned above, the positive buoyancy of a habitat can easily exceed 1000 kg (10kN) so all anchors, ropes, and connectors such as carabiners and maillon rapides should be appropriately rated for the application. The consequences of a habitat breaking loose in an uncontrolled ascent could be very severe and even fatal.

One concern, especially if the habitat atmosphere has a significantly elevated PO2, is fire safety. With the bottom of the container open to the water, its atmosphere necessarily has 100% humidity, which has been shown experimentally to dramatically inhibit flame spread due to the latent heat of evaporation of water. While practical experience has been reassuring so far, the relative balance of fire-promoting conditions and humidity within a habitat has not yet been scientifically studied, so I would advise great caution with any potential ignition source, especially electrical switches, brushed motors (potential arcing), and dive lights (overheating).

Communications

The Wakulla 1987 project, pioneering in so many ways, introduced the use of habitat to surface communications with two phone lines, one of which was able to be used for long-distance calls, although the pushbutton phone used for the latter became unreliable after a time because of moisture ingress affecting the pushbuttons. Our group, like some others, has adopted single-wire earth-return telephones (also known as Michiephones) for communication with the surface. These are simple, robust, and require only a single wire to be installed to the habitat, although we sometimes use two-conductor military field phone wire with the conductors paralleled for redundancy. You can see them being used in this Alachua “habichat” video.

We have also used the combination of an LTE modem, power-over-ethernet switch, rugged ethernet cable, and a wi-fi access point in a pressure-proof housing to provide internet access within a habitat close to the entrance. While attractive, this option is not suitable for long-term installation and the effort of setting it up for each dive makes our dive teams generally prefer the single wire phone option. Other systems, such as two-wire intercoms for offices or door entry have also been used successfully. All these devices need to be able to function at elevated atmospheric pressure with 100% humidity.

Neville Michie (pronounced ‘Mickey’), an Australian caver, designed a simple and reliable earth-return telephone in the 1970s, although the concept is much older, dating back to the 19th century. Many similar units have been constructed for use around the world by cave rescue teams. Picture: Andrew Pitkin

Is There A Deco Habitat in Your Future?

We have already seen one version of the future: the Wakulla 2 project’s surface decompression chamber system with a transfer capsule (“bell”) to transport the divers under pressure from the water into a dry decompression chamber on the surface. Unfortunately very few sites have the geography, and even fewer divers the financial means, to support it. 

The ultimate habitat? An exploration diver entering the personnel transfer capsule (PTC) during the Wakulla 2 project in 1999. Because of the topography of the cavern, an angled Tyrolean of ¾” (19mm) rope was rigged along its roof to allow the PTC to reach the divers at 30 m/100 ft depth and get them out of the water several hours earlier than if the PTC was lowered vertically. Photo taken by Wes Skiles and courtesy of Bill Stone/US Deep Caving Team, Inc.

Some explorers have started experimenting with small one-person collapsible habitats which with advances in materials technology can be made more compact and lighter. I foresee more use of purpose-designed enclosures, especially collapsible ones that can be deployed in multiple locations. Underwater rotary hammers are now available which, although expensive, allow rapid placement of anchors in hard limestone. I also anticipate more habitats deployed in open water, like Michael Lombardi’s system-see below.

For deep cave exploration, habitats offer safety, some very welcome mouthpiece-free time, a chance to eat and drink, and even entertainment. More importantly, they allow the diver to warm up and stay warm at a critical phase of the dive, promoting (presumably) better perfusion and faster off-gassing. For these extreme dives, habitats truly change the game.

See Companion article: Portable Habitats—New Technical Diving Capabilities are Well Within Reach  by Michael Lombardi

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References

[1] Blue Lake: the habitat. 

[2] Lombardi M. Portable Habitats: New Technical Diving Capabilities are Well Within Reach. InDEPTH V 4.11

[3] Nuckols ML, Tucher WC, Sarich AJ. Life Support Systems Design: Diving and Hyperbaric Applications. Pearson Custom Publishing, Boston, USA, 1996.

[4] Gerth WA. Chamber Carbon Dioxide and Ventilation. NEDU TR 04-46. Navy Experimental Diving Unit, Panama City, FL, USA, 2004.

[5] Stone WC. The Wakulla Springs Project. U.S. Deep Caving Team. January 1st, 1989. ISBN-10: 0962178500. ISBN-13: 978-0962178504.


Andrew Pitkin learned to dive in 1992 in the cold murky waters of the United Kingdom and started cave and technical diving in 1994. His first exposure to exploration was in 1995 when he was one of a team of divers who were the first to reach the bottom of the Great Blue Hole of Belize at 408 fsw (123 msw). Subsequently he has been involved in numerous cave exploration projects in Belize, Mexico and Florida.

From 1996-2000 he was employed at the Royal Navy’s Institute of Naval Medicine, running a hyperbaric facility, treating decompression illness, participating in research into outcome after decompression illness, submarine escape and testing of new military underwater breathing systems. He is one of a handful of civilians to be trained by the Royal Navy as a diving medical officer.  

He moved to Florida in 2007 and is currently on the faculty of the College of Medicine at the University of Florida in Gainesville. With Karst Underwater Research he has participated in numerous underwater cave exploration and filming projects. Like many explorers, he spends much of his spare time developing and building innovative equipment for exploration purposes.

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