Diver Daves' DC-55 Page

This is without any doubt one of the most overlooked rebreathers in the history of rebreather design. Designed in 1955, it is the direct ancestor of the Halcyon, as it is a breathing-rate keyed SCC rebreather. The original Halcyon used a variable dump ratio, which the DC-55 lacks. The latest Halcyon-80 has a single dump-ratio, making it merely an updated version of this operating system. Basically, there are two counterlungs, a smaller one inside the larger one. On each inhalation (as both sets of bellows are collapsed by the divers inspiration) the contents of the smaller bellows is vented to the water, and at the end of the breathing inspiration cycle the lost volume is replaced by a simple second-stage regulator, or automatic-add valve. It would seem that since the maker is La Spirotechnique, which was the French sister company of US Divers, that the regulator mechanism is identical to the second stage of any other US Divers 2-hose regulator. This one is in pretty good condition, and I would dive it if I were able to replace the fairly dried-out bellows counterlungs. Anyone with a source of parts for these would be greatly appreciated.

Here the rig is laid on it's back with the harness up. You can see the valves on the bottom, and the scrubber is visible through the cutout backplate. Harness is a typical 1960's era cotton strap affair.

Taking a look at the back, we can see details of the scrubber plenum, and the feed line that brings high-pressure gas to the regulator.

Flipping the rig over, we see the filler-port peering out from the fiberglass shroud, and the scrubber filling port. The heavy fiberglass bumper at the bottom helps to protect the valve assemblies when donning and handling the unit, as it is worn valves-down.

Here I've removed the scrubber filling port, which is a brass 1/4 turn bayonet fitting.

Detail of the filler port, for servicing the equipment with gas while assembled. Chain prevents loss of the cap.

The shroud and chassis are serial numbered to match each other. Shroud is removed by depressing a small button-latch and then removing it from the rest of the unit.

The shroud neatly covers the counterlung area, and also stows the breathing hoses and DSV when the rig is transported. This is a very nice feature, as the rig is essentially impervious to damage when the hoses are stowed.

The DSV shows signs of age: although the hard parts are fine, the mouthpiece is rotted and requires replacement.

Here the counterlung bellows are extended to nearly full exhalation. Bellows are dry, although they are probably still serviceable.

As you can see, the clamp for the counterlung access is broken, but that's no problem in the Shop of Horrors: A 5 minute brazing job will cure that. If the exhaust flapper looks like the duckbill flapper of a US Divers 2-hose regulator, good: That's exactly what it is.

Opening up the cover, we see the secret of the rebreather. This inner bellows is filled when the diver exhales, and when the bellows are collapsed by the diver inhaling, the gas contained in them is dumped into the water. The faster the diver breathes, the more gas is vented. This is a practical application of the VSC system of assuming that 02 metabolism is a function of breathing rate, and the system makes use of that assumption. It's only a small step from this to the PVR BASC method used by the Halcyon.

Here we can see the regulator mechanism that adds gas when the bellows are collapsed on each inhalation. The actual guts of the mechanism are interchangeable with a US Divers 2-hose regulator.

Here is the gas line running up from the supply cylinders. The seat of the regulator is changed by removing the slotted screw. The chained cap may be removed for attachment of an umbilical, thus turning the system into a surface-supplied rebreather with internal bailout capability.

There is a small hole aligned with the regulator seat, so it may be changed without any disassembly of the unit.

Removing one cylinder, we can see the curved scrubber, which is contoured to the cylinder set.

Looking up at the intake hose plenum, we see a 1/4 turn bayonet fitting allowing access to the one-way flapper valve, which on the inhale side is serviced from the downstream side of the gas-path.

On the other side we can see the high-pressure tube supplying the regulator from the cylinders.

Cylinder valves were made by Fenzy, famous for the original BC, the "Bouy Fenzy". The manifolds are O-Ring sealed and may be loosened without tools to exchange cylinders.

The cylinders are extremely interesting. They are aluminum, but with a very thick coating of fiberglass around the outside. This is apparently an early type of composite bottle, as you can see in some of the clear areas of the fiberglass that the thickness of the glass can be estimated at approximately 1/4 inch. This is obviously structural. You can see the seam where the composite material was molded to the thin aluminum cylinder.

The only markings on the cylinders are painted serial numbers, and this marking "2200" which I take to be the fill-pressure in PSI. Again, you can see the mold-mark for the composite exterior layer of the bottle.

I found this rebreather to be a fascinating example of the state of the art, circa mid-1960's. It obviously presaged other designs, and the similarity of the basic principle to the Halcyon is obvious. This one came from the collection of a deceased US Navy Diver in New London, CT, and was supposed to have been one furnished to the US Navy for testing. The PSI markings on the cylinder would tend to confirm this, as France has never, in recent history, used PSI as a measure of gas pressure. It's a jewel of engineering, and it's a pity that it's not just a 'little bit' less dried out insofar as the rubber bits are concerned.

	Here the rig is laid on it's back with the harness up. You can see the valves on the bottom, and the scrubber is visible through the cutout backplate. Harness is a typical 1960's era cotton strap affair.
	Taking a look at the back, we can see details of the scrubber plenum, and the feed line that brings high-pressure gas to the regulator.
	Flipping the rig over, we see the filler-port peering out from the fiberglass shroud, and the scrubber filling port. The heavy fiberglass bumper at the bottom helps to protect the valve assemblies when donning and handling the unit, as it is worn valves-down.
	Here I've removed the scrubber filling port, which is a brass 1/4 turn bayonet fitting.
	Detail of the filler port, for servicing the equipment with gas while assembled. Chain prevents loss of the cap.
	The shroud and chassis are serial numbered to match each other. Shroud is removed by depressing a small button-latch and then removing it from the rest of the unit.
	The shroud neatly covers the counterlung area, and also stows the breathing hoses and DSV when the rig is transported. This is a very nice feature, as the rig is essentially impervious to damage when the hoses are stowed.
	The DSV shows signs of age: although the hard parts are fine, the mouthpiece is rotted and requires replacement.
	Here the counterlung bellows are extended to nearly full exhalation. Bellows are dry, although they are probably still serviceable.
	As you can see, the clamp for the counterlung access is broken, but that's no problem in the Shop of Horrors: A 5 minute brazing job will cure that. If the exhaust flapper looks like the duckbill flapper of a US Divers 2-hose regulator, good: That's exactly what it is.
	Opening up the cover, we see the secret of the rebreather. This inner bellows is filled when the diver exhales, and when the bellows are collapsed by the diver inhaling, the gas contained in them is dumped into the water. The faster the diver breathes, the more gas is vented. This is a practical application of the VSC system of assuming that 02 metabolism is a function of breathing rate, and the system makes use of that assumption. It's only a small step from this to the PVR BASC method used by the Halcyon.
	Here we can see the regulator mechanism that adds gas when the bellows are collapsed on each inhalation. The actual guts of the mechanism are interchangeable with a US Divers 2-hose regulator.
	Here is the gas line running up from the supply cylinders. The seat of the regulator is changed by removing the slotted screw. The chained cap may be removed for attachment of an umbilical, thus turning the system into a surface-supplied rebreather with internal bailout capability.
	There is a small hole aligned with the regulator seat, so it may be changed without any disassembly of the unit.
	Removing one cylinder, we can see the curved scrubber, which is contoured to the cylinder set.
	Looking up at the intake hose plenum, we see a 1/4 turn bayonet fitting allowing access to the one-way flapper valve, which on the inhale side is serviced from the downstream side of the gas-path.
	On the other side we can see the high-pressure tube supplying the regulator from the cylinders.
	Cylinder valves were made by Fenzy, famous for the original BC, the "Bouy Fenzy". The manifolds are O-Ring sealed and may be loosened without tools to exchange cylinders.
	The cylinders are extremely interesting. They are aluminum, but with a very thick coating of fiberglass around the outside. This is apparently an early type of composite bottle, as you can see in some of the clear areas of the fiberglass that the thickness of the glass can be estimated at approximately 1/4 inch. This is obviously structural. You can see the seam where the composite material was molded to the thin aluminum cylinder.
	The only markings on the cylinders are painted serial numbers, and this marking "2200" which I take to be the fill-pressure in PSI. Again, you can see the mold-mark for the composite exterior layer of the bottle.