So I’m swimming into the beach from a dive the other day. My regulator is my trusty 1958 US Diver’s single-stage Mistral. My tank pressure is down to 500psi and the reg is breathing fantastically because, being an upstream design, the lower the tank pressure, the less force needed for the main diaphragm to push down on the oversize levers, push on the pin, and provide a supply of air. I only wished I could have that same ease of breathing at the beginning of the dive on a full tank as well as at the end on a tank with 500psi left.
As I’m taking my fins off and walking toward the beach a lightbulb goes off in my brain. I am sure I’m not the first to think of this, but I hadn’t heard of it before. Anyway, why not have a first stage regulator that supplies a constant IP of 500 psi leading to the single stage Mistral? That way I get the Mistral’s famous ease of breathing at lower tank pressure throughout the dive using its giant levers. Yes, I know that this actually makes it a two-stage regulator but that’s not the point. As many of us know, even with a balanced DH regulator the 2nd stage lever is smaller than in a single stage reg and doesn’t have the advantage of a fulcrum point to make things easier as in a single stage reg.
Next I called Luis and said ok, what 1st stage single hose reg will get me an IP of 500 psi? Luis said none that he was aware of. He was correct. So next I call BT and describe my plan to him. We both actually start doing some experimenting to see how high a stable IP we can get with a scuba 1st stage. The answer is not very high. I managed to get an old Poseidon 1st stage up to 200 psi and Bill slightly less. Most were around 150-160psi max even with shims in a piston reg. However, the Mistral does not fare very well at such very low standard IP’s. The Mistral needs a minimum of 500 psi input to actually function well. Luis is always right! Back to the drawing board.
Contemporaneously Bill and I came up with the idea of using an air regulator that is used on compressors. I was thinking that many of us have bench test regs with adjustable IP. August Industries sells them. The input can be has high as 6000 psi and the IP set and locked to any pressure including 500 psi. Bill used what I believe is an unbalanced reg and I plan on using a balanced one. It really makes no difference because even on the unbalanced reg the IP swing is only about a 100 psi from full tank to a low on air tank. Easy for a Mistral to handle.
I put together a parts list as follows.
From my original design we dumped the Shrader valve as unnecessary and potentially a source of air constriction. My thought was to put a hose (that would tolerate 500 psi) between the reg and the yoke to the Mistral. Everything uses 1/4" npt threads.
Here is what Bill came up with initially using a hose. Testing showed .4 inches on the Magnahelic. Awesome!
That idea got dumped in favor of an all-in-one-unit that Bill built.
The Mistral sits slightly high and to the left but fits comfortably against the back. Bill took it 9 feet down in the pool and said it worked great! I plan on having mine on a set of twin 38's so I am working on plumbing the yoke downward and directly below the center tank valve.
Anyway, a fun project. I’ll let Bill chime in with his more knowledgeable test-bench jargon.
M
