Connect with us

Equipment

The RB80 Semi-closed Rebreather: A Successful Exploration Tool

What rebreather has arguably logged the most exploration kilometers since its market introduction in 1998—an estimated 160 km plus (100 miles plus for you Imperialists)—and continues to rack up the klicks? It’s Halcyon’s RB80 passive-addition, semi-closed rebreather. Here WKPP greybeard and RB80 instructor trainer David Rhea reports on the RB80’s history, design & workings, training, and he offers the lowdown on its new sidemount progeny, the RBK. Looking for an electronics-free, sidemount bailout rebreather? Halcyon may just have your number.

Published

on

By David Rhea

Header image by David Rhea

Full Disclosure: Halcyon Dive Systems is a sponsor of InDepth.

In the early 1990s, the cave exploration conducted by the Woodville Karst Plains Project (WKPP) in the Woodville Karst Plains of Florida, especially Wakulla Springs, was becoming quite complicated. With dives averaging depths of 89 m/290 ft, with penetration being measured in miles, and decompression taking hours, it was becoming obvious that rebreathers would be necessary to move forward. In 1996/97, the WKPP began using a semi-closed circuit rebreather known as a Passive Variable Ratio-biased Addition Semiclosed Rebreather (PVR-BASR), nicknamed “The Fridge,” to extend their exploration and decompression obligations. This piece of equipment was a very large, bulky, and complex unit, and while it was uncomfortable above and below the water, the PVR managed to do what was intended and allowed for further exploration.

George Irvine and Anthony Rue with the Halcyon “Fridge” rebreather prepping for a WKPP dive. Photo from the GUE archives.

In 1996, a team of European explorers called the European Karst Plains Project (EKPP), who utilized the “Doing It Right” (DIR) techniques and philosophy of the WKPP, began using a semi-closed rebreather called the RB-2000. The unit was developed by the EKPP founder and director Dr. Reinhard Buchaly, who was inspired by the great success of French cave explorer Olivier Isler had at Doux de Coly and other cave systems using a custom made, triple redundant, semi-closed rebreather, the RI 2000 designed with the help of French engineer Alain Ronjat.

The RB-2000 unit was much smaller than the PVR-BASR, and utilized a very clever, intuitive, and reliable design. This design complemented the DIR philosophy used by both teams, and would become the choice for both groups moving forward.  By 1999, WKPP explorer Jarrod Jablonski and Robert Carmichael, the owners of Halcyon Dive Systems, worked out a deal with Buchaly to have Halcyon manufacture, sell, and service an American version known as the RB80.  

Designing a Semi-closed Rebreather

Being a cylindrical design 185 mm/7.28 in. in diameter, and 660 mm/25.98 in. tall —virtually identical to the size of an aluminum 80 cylinder—helped the RB80 get its name. The RB80 was designed to fit between the cylinders of a double tank configuration utilizing a specially designed frame, manifold, and switch block system. The economy of parts allows for maximum efficiency with only about 130 parts total. 

Images (L2R): The RB80 ready to rock, (R) Blake Wilson headed for the spring. Bottom: The RB80 dissembled. Note the countering bellows in foreground center right. Photos by David Rhea.

This design complemented the DIR philosophy of maintaining all of the diver’s back gas for emergencies, utilizing stage tanks for exploration and decompression. The gas switches utilized the same procedures taught by Global Underwater Explorers (GUE) but instead of swapping regulators from the mouth, the diver plugs a special QC6 swagelok fitted hose from the stage bottle into a switch block that feeds the gas into the rebreather. 

The stage bottle regulator is a standard open circuit (OC) configuration with the addition of one extra hose with a QC6 connector. The switchback also allows for a hose from the back gas to be plugged into the block in case of a stage failure or other emergency. 

Figure 1. Schematic of the RB80.
1. Dive/surface loop with non-return valves
2. Exhalation hose
3. Counterlung fore-chamber
4. Non-return valve to discharge bellows
5. Discharge bellows
6. Overpressure valve
7. Main counterlung bellows
8. Addition valve
9. Scrubber (axial flow)
10. Inhalation hose
11. Breathing gas storage cylinder
12. Cylinder valve
13. Regulator first stage
14. Submersible pressure gauge
15. Bailout demand valve

The vertical design of the RB80 has a very clever water removal tube that runs directly through the center of the scrubber bed and vents water along with a small volume of discharge gas. The unit is a passive addition semi-closed design, with no depth compensation and is tied to the diver’s respiratory rate. Roughly 1/10 of the respired volume of breathing gas is discharged into the water with each breathing cycle via the inner bellows and a familiar over pressure relief valve (OPV), commonly used on most dry suits. See Figure 2 and 3 below.

The RB80 utilizes a dual bellows counterlung system (versus traditional counter lungs), which reduces the loop volume each breathing cycle. When the loop volume is sufficiently reduced, it triggers the injectors made of components of an open circuit regulator that function quite similarly. Once the injectors fire, the loop volume is replenished with fresh gas. The unit has dual injectors for redundancy, which can be isolated at the switchblock if necessary.

The scrubber bed lies above the bellows in this vertical design and is manually filled by the diver before each dive. The scrubber is a 3.2 kg/7.05 lb design and will last approximately ten hours, based on more than twenty years of operational experience. Note that semi-closed rebreathers generally get longer duration on a scrubber given that a percentage of the breathed gas is expelled and replaced with fresh gas. 

The mouthpiece design incorporates a bail out valve (BOV) allowing the diver to switch from the rebreather to OC at the turn of a lever conveniently located in the center of the mouthpiece block. A hose routed from the right post regulator of the back gas is always live and gives gas immediately once the lever is turned. 

Assessing the Work of Breathing

New rebreather divers often state they feel the RB80 has a lot of breathing resistance. This is generally due to the fact that they are accustomed to modern OC second stages which deliver almost effortless on-demand breathing. Typically, modern second stages use VIVA (Ventura-Initiated Vacuum Assist) technology. This technology, along with the geometry of the second stage and the fact that the second stage is balanced, make for incredible light cracking effort. Upon inhalation, the initial cracking effort lowers the gas volume in the case, which pulls down the flexible diaphragm, activating the lever that opens the valve and allowing gas to flow to the diver. The VIVA then keeps the gas flowing at the same rate without the need for the diver to continue to draw on the regulator.  

By comparison with the RB80, the diver is simply pulling the gas through a one-way check valve, drawing the available gas in through the right inhalation hose, beyond the valve, into the mouthpiece block, and into the divers mouth. This entire system of gas flow does not have any boost effect like its OC sister and therefore feels as if you are working hard when, in fact, it is quite effortless. As a “virtually” closed loop, one simply draws the available gas through the inhalation hose into the diver’s mouth, and then exhales out a one-way valve at the mouthpiece, back through the left exhaust hose into the breather where the gas is scrubbed of CO2, water is removed, and the gas is replenished. 



Similar to OC, the rebreather does vary in breathing performance based on the diver’s position. If you have ever stood on your head diving OC, you feel a change in performance, as the second stage is much deeper than the lungs. The RB80 historically being a back mounted rebreather keeps the unit at a slightly shallower depth than the diver’s lungs, making inhalation slightly harder and exhalation slightly easier.  This difference is virtually indistinguishable; however, extreme head down or head up positions can seriously affect rebreather breathing efforts.  When worn in a side mount or stage position, the unit is in equal position with the lungs, making for very easy breathing. Fortunately, when a  diver is in near perfect trim, the RB80 performs best, as this is the ideal position for ease of breathing. 

Extending a Diver’s Breathing Gas

The RB80 is a serious gas extension device, providing 8-10 times the gas mileage of OC. By rebreathing one’s gas and only losing 1/10 of each breathing cycle into the environment, the RB80 can take a single aluminum 80 cf/11 ltr stage bottle and turn it into roughly 640 cf/18m3, or the equivalent of eight AL 80s. 

In cave exploration, we always start a project by setting up the cave with “safeties.” These are caches consisting of two bottles each equipped with a stage regulator complete with an OC second stage as well as a QC6 equipped drive hose to plug into the RB80, and a submersible pressure gauge (SPG). These bottles are placed roughly every 3,000-5,000 ft/900-1,500 m in the cave, and will remain there throughout the exploration. The safeties are checked by support divers prior to every push to ensure function and adequate gas volume. The bottles are properly filled and marked with the proper Maximum Operating Depth (MOD) gas for the dive, and they are labeled “SAFETY.” 

With rebreather diving, it is paramount that adequate bailout gas be available in case of a single point failure on the rebreather. Rebreathers, while quite robust, have many single failure points, i.e., the breathing hoses, one way valves, OPV, the bellows in the case of the RB80, and even the diver’s mouthpiece. As mentioned, the injectors have redundancy and can be isolated in case of issues, and a spare mouthpiece is always carried by the diver in case of a serious tear or damage. 

Any other single point failure could render the rebreather inoperable, forcing the diver to return and complete all decompression on OC, demanding eight times the amount of gas that had been used at this point in the dive. So, in addition to 100% of the back gas being maintained for bailout, cave exploration demands the discipline of staging the cave with safety bottles, safety scooters, as well caches of decompression gas, and proper support personnel.

RB80 vs. an Electronic-controlled Closed Circuit Rebreather (eCCR)

A variety of eCCRs are available by manufacturers. These units are extremely efficient, as no gas is lost from the breathing loop. The eCCR can be 25-50 times more efficient than OC. However, in addition to the single point failures listed above, which are common on all types of RBs, the eCCR, has additional concerns that prevent it from being a consideration for many cave exploration groups like the WKPP and GUE-affiliated El Centro Investigador del Sistema Acuífero de Quintana Roo (CINDAQ) foundation, which hosts the  Mexico Cave Exploration Project (MCEP) in the Yucatan. 

Most eCCRs have three oxygen sensor cells that must be meticulously maintained and work together with a solenoid and an electronic controller, using a concept called voting logic. Together with an oxygen bottle and a diluent bottle, the eCCR mixes the diver’s gas during the dive within a (PO2) set point range that is predetermined by the divers. By having three oxygen cells, the controller will side with the two that have the most similar reading if one were to start to read differently from the other two. 

Lauren Fanning and Blake Wilson diving their RB80s at Emerald Sink, FL. Photo by Kirill Egorov.

Unfortunately, voting logic is inferior to the gold standard—triple redundancy: main unit, back up, back up for the back up—and has been known to be incorrect i.e., in the case of a double cell failure. Discipline, and pre- and post-dive maintenance, are the key to maintaining good sensor reliability.

When diving an eCCR, it is necessary for the diver to constantly monitor the gas mix in their loop in order to ensure that they safely avoid hypoxia or hyperoxia. For an easy-to-see reminder that the unit is working within the safe limits set by the diver, most eCCRs rely on a heads-up display (HUD)— generally mounted to the inhalation hose—that shows a small series of lights indicating green for good, yellow for caution, and red for danger, in case the PO2 in the breathing loop is getting out of range. This is of course driven by a controller that gives real time PO2 that can be viewed on the diver’s handset. Most eCCRs provide at least one handset as well as the HUD to ensure proper redundancy.  

One of the reasons many cave exploration groups like the WKPP strictly use the RB80 is its simplistic mechanical, reliable design. With the RB80, the gas is premixed into the stage bottles, and the back gas is always mixed for the MOD of the max depth expected to be reached during the dive. With the RB80, there is no gas mixing during the dive; the gas is plugged into the switch block similar to doing an open circuit gas switch. The gas is filled, properly analyzed, and the content label is attached to the neck of the bottle prior to leaving the dive center. 

The bottles all have properly placed MOD stickers on two sides of the bottle for easy identification by both the diver and his team mates, plus a MOD sticker placed on the bottom of the cylinder that can be identified by teammates when being viewed from behind. In the water, the proper stage bottle is selected for the MOD, and the gas is safely plugged in at the proper switch depth, but only after the bottle has been properly identified, verified by the buddy, and the drive hose confirmed with the bottle that has been chosen, similar to open circuit gas switches. 

WKPP explorer David Doolette measuring a mastodon bone in Wakulla Springs B Tunnel. Dr. John Rose in background. Photo by David Rhea

The most dangerous thing about the RB80 (and semi-closed units in general), is the oxygen drop, especially in shallow water [See the Loop Gas calculations section of the RB80 page in Wikipedia]. Due to the fact that oxygen is being consumed during respiration, and gas is discharged from the inner bellows with each exhalation, the oxygen drops slightly with each breathing cycle until fresh gas is replenished from the injectors, typically every two to four breaths. For this reason, one must be cautious when using the RB80 at shallow depths (when the ambient pressure is low) or when using mixes with a lower oxygen fraction as a travel gas. 

The drop in oxygen levels also means there is a slight increase in inert gas that remains in the loop and that needs to be taken into consideration for decompression. Both of these nuances of the RB80 are easy to calculate and adjust for prior to the dive. 

During RB80 training, both the oxygen drop and the increase in inert gas load are addressed and easily able to be factored in. The theory is discussed in an RB80 class, and software is available to easily do quick calculations.  All of this can then be programmed into GUE’s Buhlmann-based desktop decompression program, DecoPlanner, for proper dive planning.  Like most rebreathers, the RB80 has additional complexities requiring proper pre-dive assembly, testing, maintenance, and post-dive discipline. 

Training on the RB80

The WKPP was established in 1995, and from the beginning, adopted a standardized approach to gear configuration and procedures. Initially, this approach was called “Hogarthian,” after early WKPP pioneer Bill Hogarth Main. Later, project director George Irvine added to the standardization and coined the phrase “Doing It Right,” or DIR, to represent this standardized approach.  In 1998, Jarrod Jablonski founded GUE, which offered exploration-based training utilizing WKPP’s standardized approach and gear configuration. Once Halcyon started building the RB80, GUE began offering formal training. Currently they are the only training agency to do so. 

From the beginning, GUE’s RB80 training has been exploration-based, with a heavy emphasis on failure-based training i.e. dealing with equipment failures as a team, similar to other GUE courses. Exploration-level cave diving has complex exposures that require divers to return from deep inside the cave, and then make a vertical ascent to return to the surface. With the addition of the RB80, divers are able to extend their penetrations exponentially, adding as much as 12-14 hours of decompression on some dives alone. Conventional rebreather training does not properly prepare someone for these types of exposures. 

RB80 Divers on the wreck of the freighter Judge Hart in Rossport, Canada. Photo by David Rhea

GUE divers have historically been required to take Fundamentals, Tech 1, and Tech 2 with a minimum of 25 dives at each level between classes prior to beginning their RB80 training. This is in addition to the Cave 1 and 2 level training and experience required to begin cave exploration.  The investment of time, energy, and resources necessary to become a GUE/WKPP exploration cave diver makes for a very serious explorer who has the skills and experience necessary to conduct dives with this level of exposure. The failure-based training also builds the diver’s confidence, repetitive learning, and instincts necessary to safely explore. 

One of the many reasons for the long term success of the RB80 has been this extremely regimented training by GUE’s four active RB80 instructors. In addition to the most intense and demanding rebreather training available, GUE RB80 students must purchase the unit prior to taking the training. This alone narrows the attendance to only the most serious explorer, as no rental option is considered. 

Until fairly recently, GUE divers were the only ones using the RB80. Even then, only those willing to take the robust training who had an exploration mindset learned to dive the unit. Currently there are 150-200 GUE divers certified to dive the RB80. The discipline and attitude of these explorers has ensured that the RB80 has been responsible for more kilometers/miles of cave exploration than any other rebreather in the world. I estimate that more than 161.6 km/100 mi of cave passage has been explored using the RB80. 

The discipline and attitude of these explorers has ensured that the RB80 has been responsible for more kilometers/miles of cave exploration than any other rebreather in the world. I estimate that more than 161.6 km/100 mi of cave passage has been explored using the RB80.

A Specialized Exploration Tool

For 30 years the WKPP has been mapping the underwater labyrinth of the Woodville Karst Plain, having mapped over 56,609 m/185,000 ft of cave passage with more than 35,189 m/115,000 ft below 58 m/190 ft. The RB80 has been one of the most vital keys to this success, including the world record dives in Wakulla and the following traverse. It is the only rebreather used for exploration on Woodville Karst Plain projects. Presently, virtually all exploration being conducted by the WKPP below 61 m/200 ft is exclusively done on the RB80. 

WKPP dives in Florida on the RB80. Photo by David Rhea.

Over the years, and especially during the Wakulla exploration heydays, one of the growing concerns was running out of scrubber material during the dive. On the biggest dives, the entire RB80 double tank configuration would be swapped at the deep portions of the decompression for a fresh ‘breather with smaller double five-liter bottles and fresh scrubber material. Note: an advantage of the RB80 over an eCCR is that the valves can be closed and the unit reliably stored underwater like a stage bottle for bailout.  It can then be quickly turned on and dived.

In 2008, CINDAQ’s MCEP project also adopted the RB80 and has done countless hours of exploration in the caves of the Yucatán. Between January 2018 and December 2020, for example, MCEP exploration divers mapped in excess of 180,000 m/594,000 ft of new cave passage in Ox Bel Ha alone using RB80 technology. 

As CINDAQ board member and co-owner of Zero Gravity Dive Center, Christophe Le Maillot, explained, “It is such a sturdy and intuitive unit. In all the years we used it, we have never had to terminate or cancel a dive because of a malfunction. It’s a real work horse!” Like their sister WKPP team, the MCEP exclusively uses GUE-trained RB80 divers for their exploration dives. 

GUE divers have also utilized the RB80 for cave exploration projects in China, the Nullarbor caves in Australia, caves in the south of France, cave and wrecks of Italy including the Pantelleria project, the Alviela cave project in Spain, and other karst areas around the world. In addition to cave exploration, the RB80 has been utilized by GUE divers on the west coast for the ghost net removal, and by GUE wreck divers in Canada and around the world. 

Introducing the RBK, a Sidemount RB80

In response to explorers wanting a stageable version of the RB80 as both a travel and/or bailout rebreather, Halcyon began working to develop a modified sidemount version of the RB80, called the RBK. The first version was called the RBK1 and after several years of modifications Halcyon produced two more revisions, the RBK 2 and the RBK 3, referred to simply as the RBK.

Halcyon COO Mark Messersmith diving the RBK in Mexico. Photo by Sam Meacham.
Sam Meecham with a pair of RBKs. Photo courtesy of CINDAQ.

The overall diameter of the RBK is the same as the RB80, but by reducing the height of each section, the overall length of the unit has been reduced to 50 cm.  Though the scrubber was reduced in volume to 2.4 kg/5.29 lb, the scrubber duration is rated for approximately eight hours based on user experience [See InDepth’s Rebreather Holiday Shopping Guide for add’l spec details]. Because of the smaller form factor, the RBK offers a 6-8:1 gas extension versus 8-10:1 on the full RB80.

The sidemount RBK has been used as a sidemount, travel, and bailout rebreather by both the WKPP and the MCEP, which has been testing and helping to refine various RBK prototypes since 2015. On recent long-range explorations through small passages, the RBK has proven to be an outstanding tool for shallower, long distance cave exploration. MCEP instructors are now working with the GUE board of directors and other RB80 instructors to develop a RBK sidemount training course, which should be available in the not-too-distant future.    

New Non-GUE Users

Andy Pitkin with a sidemounted RBK bailout rebreather. Photo by Kirill Egorov.

Over the last few years, Halcyon has made the RBK available to select non-GUE divers. They have sold custom versions of the RBK to militaries around the world. In addition, they have provided RBK units to exploration divers from Karst Underwater Research (KUR), who have been using the RB80 as a side mounted bailout breather for their recent long range exploration dives at Weeki Wachee Springs and other systems. The divers received their RBK training directly from Halcyon. 

As KUR project director Andy Pitkin put it, “It is undeniably true that ‘simplicity is the ultimate sophistication,’ as Leonardo Da Vinci once noted. The RBK has proved itself to be close to a perfect tool for our particular application, far exceeding my initial reserved expectations.“  

From its conception, it was quickly obvious that the RB80 would be around for a very long time. The simplicity, safety, and the robust mechanical nature of the unit, combined with rigorous training, and highly experienced users, arguably make RB80 and RBK the ultimate exploration tools.

Subscribe for the InDepth Newsletter

Dive Deeper:

InDepth’s Rebreather Holiday Shopping Guide (2020)

Halcyon: Using The RB80 As A Sidemounted Bailout Rebreather by Andy Pitkin, Karst Underwater Research (2018)

GUE: DOUX DE COLY: GUE Expedition with RB80 (2004)

Introducing the RB80 by Michael Waldbrenner and Dr. Reinhard Buchaly 

Deep Tech: Victory At Last (1998): Olivier Isler is setting penetration records with a triple-redundant semi-closed rebreather

RB80 Series on GUE.tv


David Rhea is an active GUE instructor and instructor evaluator, having been with GUE since the earliest days. An avid explorer with the WKPP since 1998, David has explored caves in China, Florida, Australia, Mexico, and France. A passion for diving started at age six, leading David to make his first dives at age nine. He became a scuba instructor at age 18. David has worked full time in the scuba industry for over 40 years, and has worked for Scubapro since 1995. David is as passionate today about exploration, teaching, and underwater photography and managing his Florida Scubapro territory as he has ever been. 

Click to comment

Leave a Reply

Your email address will not be published. Required fields are marked *

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.

Published

on

By

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

Subscribe for free

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.

Continue Reading

Thank You to Our Sponsors

Subscribe

Education, Conservation, and Exploration articles for the diving obsessed. Subscribe to our monthly blog and get our latest stories and content delivered to your inbox every Thursday.

Latest Features

Trending