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Update 09-01-2012

This is information about this old rig was sent to me by Bob Nicholson. The described rebreather was the pre-halcyon rebreather designed by Jack Kellon. Thanks to Bob for sharing this information!

Below is info sent out when first the first units were made. In total there were 25 made. Mine is number 19. Designed by Jack Kellon who went on to design the first Halcyon and the new K2 scr. It was used as an escape rebreather for commercial pipe workers but most ended up as rebreathers for Florida cave divers.
The rebreather came to me from a rebreather instructor in Aberdean. It was just gathering dust a lot of the parts had been removed and the Swagelock plumbing had been renewed ( original parts still owned ). I have found it difficult to obtain fills as the tanks are composite (X4) and I do not own a compressor and cannot spend the time on the unit it deserves. However I have still learned a lot about passive addition.
The unit is old but very interesting. A great rebreather to develop as it is quite unique and may well be collectable.

Bob Nicholson

The BMD/SCR-4 specifications

Method of Operation

Semi-closed circuit recirculation using an all new Oxygen addition method that is keyed to both respiratory minute volume and depth, thus making decompression calculation similar to open circuit and just as accurate. Existing tables for various standard mixes can be used with very minor calculation adjustments. A custom Orca Phoenix decompression computer is provided with each unit for either air or EAN-32, customer's choice. Gas consumption is approximately 1/5 of open circuit at any depth due to the addition method, lower work of breathing and conservation of body heat. Additions are accomplished through a single first stage supplying a base flow metering valve and redundant (2) second stages. The manual bypass is high pressure in case of a first stage stoppage (highly unlikely). One second stage is adjustable by the diver and can be used as a bailout in case of a catastrophic counterlung breach (also highly unlikely). The same second stage can be buddy breathed by a second diver while the primary diver is still breathing off the recirculating loop. Most gas venting to ambient is done through a sizable diffuser, allowing very silent operation. A counterlung overpressure relief will expel gasses independently if too rapid an ascent is made.

Safety features

It is impossible to become unconscious as a result of hypercapnia with this unit, although a considerable degree of discomfort will occur if the scrubber is completely expended.

Gas flow obstruction due to scrubber flooding is automatically compensated for by the addition mechanism only to the degree necessary for normal breathing until the scrubber can be cleared of water. The scrubber has a power water purge that can be operated by the diver during the dive Scrubber floods are instantly detectable.

Automatic gas addition failure is detectable within 2 breaths. Manually operating the high pressure bypass does not shorten the gas duration because the control mechanism is on the exhalation side of the breathing loop. As long as the diver is able to get a breath, all is well.

Hyperoxia cannot occur as long as the supply mix is not breathed deeper than would be acceptable on open circuit equipment.

Hypoxia cannot occur as long as the supply mix is not breathed shallower than would be acceptable on open circuit equipment, even on rapid ascents. Provision is made at the valving in the full face mask (provided with the unit) to dump "caustic cocktails" overboard.


Two toroidal low profile bags, one encircling each arm at the shoulder. Breathing dynamics are virtually unchanged by varying positions. Each bag can be drained both after maintenance rinses and during the dive after inadvertent flooding. The pressure relief is adjustable by the diver during the dive. Total volume is 6.5 liters.

Gas Supply

Four composite cylinders of 18.3 cubic feet each at 300 bar for a total of 73.2 cubic feet (48.8 cubic feet at 200 bar). The cylinders are manifolded to a single valve and must be both hydrostatically and ultrasonically tested every three years by BMD. The cylinder life is 10,000 cycles over 15 years. Filling is accomplished through an exterior whip without removing the case cover. Regulated (140 PSI over ambient) alternate gas sources can be plugged into an exterior quick disconnect fitting and breathed through the recirculating system for efficiency. Disconnection of the alternate supply automatically restores the use of the internal gas supply.

Carbon dioxide removal

Twenty-five percent of all carbon dioxide production is dumped overboard before reaching the scrubber. The remaining 75% is passed transversely through a 4 lb. bed of 8-12 mesh Sofnolyme. The scrubber is constructed of heat formed and welded ABS and is double wall insulated on the exterior. It is also extensively baffled to be highly tolerant of loose packs. The scrubber is not removable. All filling and removal is accomplished from the outside of the rebreather without removing the case cover.

Buoyancy Compensation

Integrated into the system using a completely separate bag. Can be used as an optional bailout.

Exterior Case

Molded flat finish black UHMW polyethylene in 2 pieces. All components are mounted on one piece and the other serves as a removable cover for access. Dimensions are 28.8 inches long by16.5 inches wide by 4.75 inches deep.

Unit Weight

Approximately 48 lbs. dry, depending on size and type of harness.


Approximately 20 minutes per dive, including scrubber repack and rinse. Add gas refill times.

Approximately 30 minutes per week to disinfect breathing loop.

One day (24 hr. turnaround) plus shipping time every 18 months for BMD factory rehab. The first 2, included in the purchase price, are covered exclusive of shipping charges by the BMD 3 year product warranty.


A revolutionary new semi-closed diver rebreather system is now available to the scientific, military, and public safety diving communities.

Originally developed to provide additional safety within the commercial diving industry, the SCR-4 offers a dive duration of up to 4 hours in the air diving range, allowing for greatly extended operations not possible with conventional open circuit scuba systems. Breathing function is linked to diver activity level using a special proportional valve, making electronic control unnecessary. The elegantly simple design incorporates a highly efficient, low resistance scrubber unit and hybrid face mask and helmet.

The standard production unit uses compressed air as the single supply gas and can maintain safe oxygen partial pressures even at the surface. Utilizing automatic and redundant breathing loop inlet gas flows, including two linked to diver work level, the SCR-4 design prevents the development of the classical low oxygen conditions inherent to other designs. More stable inhaled oxygen levels also allow decompression to be predicted more accurately than is possible with traditional semi-closed rebreathers. The SCR-4 comes equipped with a specially programmed ORCA Phoenix dive computer built for the system.

The Minihat 1600-R Hybrid Hat, supplied with the unit, provides corrected panoramic vision, high-impact head protection, is extremely light weight, and incorporates a new concept speaker/microphone compatible with most through-water communication systems. The unique "hard hat like" enclosure system allows for easy donning and doffing while preventing inadvertent removal.

Keeping Advanced Technology Simple...

The SCR-4 system was developed in order to resolve the historical problems and limitations found in conventional rebreather designs. As an example, its unique approach utilizes torodal, pressure-balanced counterlungs that ensure minimal breathing resistance in all positions. A totally new "interactive" breathing circuit permits the safe use of air as the single supply gas. A proportional valve recirculates most of each breath and exhausts the remainder overboard through a diffuser, producing very small bubbles that are unubtrusive to marine life. This exhaust rate is linked to the diver's inhalation volume which is delivered using up to 4 independant gas addition systems. This feature permits the SCR-4 to maintain a stable oxygen level over a wide range of diver workloads. A special ORCA Phoenix dive computer takes advantage of the stable oxygen levels, making decompression efficient and easily managed. The scrubber is designed for up to four hours of use and is protected from flooding and insulated from cold. The BMD "hat" is not only light weight and low volume, but combines the airway protection of a full face mask with the protection and communications of a helmet. A special panoramic lens is perfect for close work in low visibility and ideal for photographers. This rounds out the most advanced personal diving system available today.

The Facts...

* The unit weighs less than 45 pounds and sits comfortably close to the diver's back utilizing a soft harness
* The SCR-4 has no electronic control systems requiring batteries, but included an oxygen partial pressure monitor for added safety
* The SCR-4 contains 73 cubic feet of gas in 4 composite cylinders at 4500psig. This is comparable to 292 cubic feet in an open circuit unit. (the cylinders contain 48 cubic feet of gas at 3000 psig, which is comparable to 192 cubic feet with an open circuit system)
* An interactive relationship of the components in the breathing circuit provides for a safe partial pressure of oxygen, even when utilizing air at or near the surface where PO2 is lowest.
* Standard SCR-4 units are charged with air, but special units can be factory modified for oxygen enriched air or pure oxygen missions.
* The unit has the same depth restrictions as open circuit equipment utilizing the same gas mixture and equivilent supply volume.
* The unit is designed to maintain function despite substantial flooding and is (less? more??) likely than open circuit scuba to fail to supply breathing gas.
* The SCR-4 system takes advantage of hydrodynamic differentials between the counterlungs and diver lung centroid to maximize breathing performance. Breathing resistance is lower that U.S. Navy requirements in all diver orientations.
* A partially or totally occluded breathing circuit will simply force the system to shift to proportionately greater open circuit operation in order to maintain breathing gas supply as long as primary pressure is available.
* Four in dependant gas addition devices are included in the circuit, three of which operate independantly of diver control.
* Two in dependant and redundant addition systems are keyed to diver RMV (breathing rate) with individual selection and activation depending on diver position within the water column, or use of manual overrides.
* An adjustable base flow addition system, similar to other semi-closed addition systems, provides up to 10 scf/h of gas flow independent pf diver ventilation demands.
* The fourth addition system is a diver actuated high pressure bypass which is in dependant of the regulation system. This eliminates the classic concern of first stage regulator failure.
* Buddy breathing can be safely effected utilizing the mouthpiece attached to the counterlung manual addition control.
* The SCR-4's unique circuit dynamics prevent debilitating CO2 concentrations regardless of scrubber absorbent condition.
* Scrubber design provides extremely low breathing resistance with efficiency uneffected by loose CO2 absorbent packing.
* CO2 scrubber performance is optimized by thermally insulating the unit in order to maintain the reaction in cold water environments and by dumping a portion of the diver's exhalation gas prior to passage through the scrubber.

Update 09-01-2013
Recently I received the original documents that came with this rebreather. Please enjoy, Scott thanks!!

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