Azimuth Rebeather Gas Addition System

The Azimuth uses the standard mass-flow system of SCC operation, with a few twists. First, the flow rate is user adjustable, so it can be tuned for any gas mixture and 02 use rate. Second, the system is designed so that the top cylinder is used for normal SCC supply, with the lower cylinder being bailout and utility gas (wings, drysuit, etc), but which can also be switched into the flow supply when needed by flipping a lever on the belt-block. Advanced SCC diver will realize immediately what this means: It means that a diver can use two different mixes in the cylinders, a lean mix for the bottom and a richer one for travel and decompression, while not needing to use a very high flow rate needed to keep a lean bottom gas safe near the surface. Thus, gas consumption for deeper dives can be reduced. In addition, the belt-block can be fed with ease by offboard cylinders, offering a modular system approach to far more advanced technical SCC diving that has otherwise been possible. Let's get to the good stuff:

		The gas flow starts with the very well known and excellent MR-12 first stages. These are diaphragm type regulators, so are well designed for environmental protection and are extremely easy to adjust for different intermediate pressures. Kudos to the maker for their selection of these first stages. They could not have done better.
		The gas then feeds through standard second-stage supply hoses to this belt block and dose unit device. A few words on this: First, it was an excellent idea to use standard hoses and fittings for this supply system. Blow a hose? No sweat. Just replace it with any stock second stage hose. Blow a hose on a Draeger? You better have one, or you're done diving until you find one. The gas connections are color coded: Blue = Bottom cylinder, generally the reserve and bailout system. Red is normal feed gas, and yellow is the return supply back to the counterlung. This block is held at the divers waist, over the right hip, where access is easy.
		The gas block and dose unit is normally located on the harness. Here, I've detached it so you can see how it's attached to the rig by the three hoses described below.
		First things first: The red lever is the manual addition lever, which freeflows supply gas (either blue or red gas) into the counterlung. The block is positioned so that you can just do this by pressing it in with the palm of your hand.
		The blue lever is the one that turns on the reserve gas supply to the rig. As will be seen below, using this requires a deliberate effort, and there's no chance of inadvertently selecting it by accident.
		Here I have begun lifting the reserve gas supply lever......
		...... and here it has been selected to "reserve gas on". The lever requires a 270 degree motion to select the gas, and operation is positive.
		Now for the good part: Flow setting. To set the flow, you simply remove the threaded cap on the block, and gently turn the now exposed finger handle in to increase flow, and out to decrease flow. That's all there is to it. Replace the cap and you are done. No tools, no mess. Nice and simple.
		How to measure the flow, you ask? How about with this laboratory grade flowmeter, supplied with the rig. I've laid it on it's side to show how it's connected to the belt-block while setting up the flow. This flowmeter must be seen to be believed. It's fully 12 inches tall, of chrome plated brass construction, with a glass tube protected behind a plastic guard. it weighs (no joke) 2 pounds. It must be worth $500 alone. Compare that to the 2 inch tall plastic piece of junk that Draeger supplies for flow-checking. This is the real McCoy.
		Adjusting the flow: Here the flowmeter is standing up for use, attached to the outlet port on the block (note the removed yellow hose) and ready to adjust. Turn on the gas, wait a recommended 1 minute for the regulator to stabilize, and turn the adjustment in or out as needed. This is a GREAT system.
		The yellow hose (counterlung feed) then runs back to the counterlung using a stock BC inflator hose, from which the Schraeder Valve core has been removed. The snaps to the bottom of the counterlung just like any BC hose. Once again, blow a hose and you're not in trouble. And again, no tools are needed.

OK, so now the obvious question, as was sent to me in an email within just a few hours of my posting of the teardown:

Question: But as I understand it, this is a constant mass flow rebreather. How can it be constant flow system, when it uses a normal balanced first-stage, and is not like the Dolphin first-stage with a constant absolute intermediate pressure?

Answer: The Dolphin attempts to gain constant mass flow by using a small sonic orifice and a high intermediate pressure to obtain a sonic (Mach-1) flow at the nozzle. The property of a sonic orifice is that it runs at a constant gas velocity of Mach-1 as long as the gas inlet pressure is equal to about 2 times the exit pressure. This is theoretical, from a practical perspective the inlet pressure must be about 2.5 times the exit pressure. Once sonic (Mach-1) flow exists, the mass of the exiting gas stream will be constant as long as the inlet/outlet pressure ratio stays within the 2.5:1 ratio. BUT! This is "almost" constant mass flow, but is not a true mass flow since the speed of sound (Mach-1) changes with density, so the flow *does* change with depth, since the gas density increases with depth. This is not a commonly known fact about these systems. With a sonic orifice, the diameter must be very small, and the inlet and exit geometry must remain constant (the shape of the orifice is very important in sonic flow designs), which means that the orifice is subject to clogging (salt crystals, etc), as has been a clear problem in the Dolphin. In fact, the nozzle does not need to be fully blocked by a salt crystal to inhibit sonic flow on the Dolphin, it must just impinge on the gas stream sufficiently to change the fluid dynamics of the geometry of the nozzle design.

The Azimuth (and also the Draeger Ray!) uses a standard first stage, with a larger diameter orifice, and in the case of the Azimuth, the orifice diameter is adjustable (needle valve). This, with the lower pressure, obtains near-mass flow within the reasonable bounds of the depth chosen. Yes, the flow will become slightly larger at depth. But, and this is a very important idea, the Dolphin does not provide any real advantage from a mass-flow standpoint, as the gas flow increases with the depth on that system also, as gas density and the resultant increase in velocity at constant Mach number increases. Plus, in the Dolphin, the mass flow orifice ends being sonic when the pressure of the first stage becomes less than about 2.5 X the water pressure. At the depth where the water pressure = the first stage pressure, the flow is totally ended! This is not true with the Azimuth (or the Ray), since the first stage is always increasing to the pressure of the water, plus the extra first stage overpressure (about 140 PSI). Interestingly, Draeger abandoned the more complex and more critical system used on the Dolphin when designing the Ray, and realized that any practical advantage gained by using the more critical system is really pretty negligible.

So!

For the Dolphin:

Advantage: Theoretically better CMF at moderate depths.

Disadvantage: Small diameter orifice has failure mode of clogging with salt crystals. This is a very well documented problem with the Dolphin. "CMF" is only theoretical, as the change in actual velocity for constant Mach-1 flow changes with gas density (and temperature). Limited depth for "semi constant-mass flow", with an additional absolute depth limit for any flow whatsoever. These depths are outside of any that are reasonable, so these last are really just theoretical issues. Requires a special regulator, not easily field-serviceable. Not able to accept quick-connect offboard gas supplies, unless each offboard bottle is fitted with the special Draeger first stage and unless each bottle has the special Draeger Nitrox valve.

For the Azimuth (and the Draeger Ray, as well):

Advantage: Deeper absolute depth of use. Simpler first stages, field serviceable. Larger diameter orifice, not subject to clogging and with user-adjustable flow rates. Easy to use with offboard gas supplies, as any normal first stage can supply the gas, and those regulators can be fed by any common cylinder.

Disadvantage: Slightly more flow-rate with depth increase. This tends towards a safety factor, but does slightly increase gas consumption as depth increases. Within normal depth use the flow increase is negligibly when compared to the Dolphin which is also increasing it's flow rate due to Mach-1 velocity increase with increased gas density.

BOTTOM LINE: The systems are different. The Dolphin is more critical, and when clogged it stops flowing at all. With the Dolphin, you need to buy new dose-units when you want to change the flow. The Azimuth allows user-tuning of the system, and is more flexible for gas selection and use of offboard gas supplies. The theoretical decrease in efficiency of the Azimuth is not as great as it would first appear, as the Dolphin flow also increases with depth, due to the issues of mach velocity increasing with gas density (sonic flow is NOT absolute constant mass flow unless gas density, IE: Depth, remains unchanged)

More questions:

Q: "Can I dive it as a pure 02 rebreather?"

A: Sure. Just turn out the flow needle until flow stops, and then manually add 02 as the counterlung is emptied. Result: Instant fully-closed 02 rebreather.

Q: Hey..... If I do *that*, why couldn't I just add a PP02 monitoring system, close the mass flow to zero, run air in one cylinder and pure 02 in the other, and dive it as a manually-flown CCR by using the air for the descent and then once on the bottom, flipping the "reserve" lever to the 02 cylinder, and then using that same manual-add valve to manually add 02 every now and then as I reduce the PP02 by breathing? I'd probably want to raise the PP02 manually on the surface to about 0.4 or so using that same valving management before jumping in the water, but it sure seems to me that it's a CCR in sheeps-clothing.

A: You're a pretty clever guy, aint you? I *bet* you'd add a Draeger Oxygauge port, an Oxygauge, and a VR-3. That way you'd have independant PP02 measurement, and real-time decompression, and a nice manually flown CCR for little effort. Gee, I bet we all know someone who's actually sick enough to try that, eh? ;-)