Halcyon RB80 Rebreather
The Designer's Perspective

BY DR. REINHARD BUCHALY

Introduction

Halcyon RB80

Rebreathers are not a new technology. They have evolved into what they presently are over a long period of time. As early as the 18th century, attempts were being made to remove carbon dioxide from a breathing apparatus. In the 19th century, a recirculating device was patented which used pure oxygen. Modern designs still use the same principles.

All modern rebreathers share some basic features:

1. a counterlung,
2. a carbon dioxide scrubber,
3. a mechanism to add new gas and
4. a mouthpiece, connected to an inhalation and exhalation hose.

History

Earlier version of the Halcyon Rebreather in Wakulla Springs

In 1997 the WKPP reached a point in their exploration where the use of an exploration-suitable rebreather became advantageous. The development of the Halcyon-Rebreather was the initial answer to this need. This rebreather worked primarily as a gas-extender. It did not use electronics, which may work on shorter duration dives (up to three hours), but is not the right choice for dives up to 10 hours or longer. Besides, monitoring an electronic display every minute is just not practicable on a long exploration cave dive. When the Halcyon-Rebreather breathes, it works; monitoring occurs naturally with sight, sound and feeling. Adequate bailout is integrated in the design from the beginning. For cave dives this means normally large cylinders, while for ocean dives cylinders can be a lot smaller.

Goals

Halcyon RB80

The designer of the Halcyon RB80 rebreather, Dr. Reinhard Buchaly, was familiar with the features of the Halcyon-Rebreather, and had owned/dived a unit since 1998. Working on rebreather concepts since 1996, with the cooperation of Halcyon and George Irvine, project director of the WKPP, he decided to incorporate all the benefits of the Halcyon-Rebreather into a new rebreather design that would provide divers with even better performance, limited size, reduced complexity and several added unique features.

• One of the first criteria was to limit the size of the rig dramatically. The idea was that two Halcyon RB 80 rebreathers should fit in the Halcyon-Rebreather frame, and that the profile should be no higher than the bailout cylinders on the left and the right. Indeed, this goal was realized.
• There are minimum requirements for the size of the scrubber canister and the volume of the counterlung. Early calculations showed that nearly the entire volume of the rebreather was needed for the scrubber canister and the counterlung, should it be possible to fit size-wise two units in the old frame. From the outset it was clear that this goal could only be realized with an in-line-design where each part serves a function.
• The minimum requirement for the RB80 scrubber was that it be even bigger than the super-scrubber of the Halcyon-Rebreather.
• The gas mileage should be better, but the supply gas should still be adequate for open circuit bailout.
• The water-dumping feature should be more effective and work automatically. Even an open mouthpiece should not flood the rebreather.
• Cleaning should be easy.
• Lastly, less parts and less complexity would be a nice feature.

Description of the Halcyon RB80

Halcyon RB80 Mouthpiece

The cylindrical design with the economy of parts allows maximum efficiency. The scrubber canister of the Halcyon RB80 holds 30% more CO2-absorber material than the super-scrubber of the earlier Halcyon-Rebreather. Use of Sofnolime 4-8 mesh (2.5 - 5.0mm) mesh is recommended. For the canister, polypropylene was chosen as the suitable material. The design of the canister is axial flow. Due to the gas flow and positioning of the canister, the absorber material itself is not prone to coming into contact with free water from condensation.

Filling the Scrubber

The counterlung is the most design-critical part of the RB80. A simple bag will not work as a counterlung in this position when an optimum work of breathing is desired. The counterlung has a structure that will not allow it to collapse on itself. Besides this, the right amount of elasticity is required to correspond to the behavior of the human lung and to hydrostatic conditions. To match the required properties, the counterlung bellows is custom-built for the RB 80 out of a Teflon-coated compound. While the counterlung itself could be inflated to more than eight liters volume, it is mechanically restricted to inflate only to about four liters. In case the diver's tidal breathing volume increases above four liters, the unit starts to dump the excess gas additionally and more fresh supply gas is injected. This is an added safety feature for high workload situations.

RB80 Counterlung

The venting of this constant-volume, semi-closed rebreather is on a ratio of 1:10. An inner bellows that fills on each exhalation and is vented overboard on each inhalation achieves this. Venting is through a valve that is set to 10 millibars. This valve concurrently serves as an overpressure relief valve for the whole loop. The inner bellows is constructed in similar fashion to the counterlung. Dr. Reinhard Buchaly and George Irvine also discussed a higher ratio beyond 1:10 and agreed that, even for world-record level WKPP dives, a higher ratio like 1:15, 1:20 or even 1:25 would not be desirable in the foreseeable future, because the supply gas should still be breathable open circuit.

For several reasons, no depth compensation was designed for this rebreather. A depth compensator allows different venting ratios near the surface and deep, and gives a higher venting in shallow water. This can also be achieved by choosing the right gas with an appropriate fO2 in shallow water. The needed discipline of the diver to do this task right is taken care of by the correct training program. Besides, as shown above, the counterlung layout takes care of high workload situations.

The real gas-extension is about 1:8. Due to purging the loop, up-and-down profiles like those found in caves, and mask clearing, the ideal ratio cannot be achieved.

Gas injection and automatic water removal takes place between the scrubber and the counterlung. Two injection valves replace the vented gas. Each of these valves can do the injection alone, and the same gas switch block that was used on the big Halcyon-Rebreather can control each valve on the Halcyon RB80. The gas switch block features two quick disconnect couplings, an intermediate pressure safety valve, and the two valves controlling the gas injection.

On dives longer than three hours, condensation may accumulate in rebreathers. In the Halcyon RB80, the condensation is removed automatically. The counterlung stays very dry, but even water in the counterlung, if it could ever get there, can be easily removed. Any water in the rebreather is moved to get dumped through the inner bellows without any action by the diver. Indeed, it is so effective that the mouthpiece can be removed underwater and not be closed. The water is directly routed through the rebreather without getting into contact with the counterlung or the absorber. The mouthpiece with integrated open circuit bailout of the Halcyon RB80 is the same as on the Halcyon-Rebreather.

Taking the complete rebreather apart is very simple and straightforward. Two latches have to be opened to get to the absorber and the gas injection and water removal parts. Four large screws can easily be removed to grant direct access to the counterlung and inner bellows. Cleaning and inspection is a straightforward procedure because there is direct access to the surface of each part, even to the inner surfaces of the counterlung and the inner bellows. The name of the Halcyon RB80 reflects the name of the manufacturing company, the initials of the designer and the size of an Aluminium 80 dive cylinder.

Due to its size, the RB80 can be used as a redundant rebreather.

The empty weight of the Halcyon RB80 core is 9.5 kg; add the switch block at 1.5 kg, mouthpiece with open-circuit bailout regulator and hoses at 1.5 kg, and the absorber, to arrive at a total of about 16 kg. The bailout cylinders with the frame must be chosen according to the kind of dive. The rebreather was extensively tested and used in different environments by Dr. Reinhard Buchaly and Michael Waldbrenner, not only in open water conditions, but also in cold, deep alpine caves like the Chaudanne in Switzerland, in Trou Madame, a shallow water cave in France, and in Florida caves. George Irvine and Jarrod Jablonski recently dived the unit in Wakulla.

Dr. Reinhard Buchaly and Michael Waldbrenner

Training on the Halcyon RB80 is conducted exclusively by GUE, Global Underwater Explorers. But remember, a world record is not included in the package.