Picked up some old US Divers gear and am looking for some input

[John C. Ratliff]

Many don't realize that the U.S. Divers Company Mistral has better performance characteristics than the Royal Aquamaster, according to the U.S. Navy Experimental Diving Unit tests. This may be one of the reasons the Cousteau team used the Mistral almost exclusively for its operational regulator.

SeaRat

 

[Luis H]

According to what I have read Cousteau liked the Mistral for its reliability.

That old NEDU test data is pretty obsolete. When I was at the NEDU, I didn't get into detail, but it was pretty clear that some of the old data was not very accurate. They have changed a lot of their methods and testing equipment. It was very interesting seeing some of the old test equipment.

I have taken lot of data (using well established engineering methods) and there is no Mistral that will come close to a well tuned Royal Aqua Master. The comparison is not even close.

A good Mistral will breath pretty good when the tank pressure is relatively low, but even then the cracking pressure and even the necessary sustained vacuum is not as low as a good Royal Aqua Master.

I have tested both with new silicone diaphragms. When I change the second stage of a RAM with the new HPR, the difference is even consistent.

 

[Nemrod]

According to what I have read Cousteau liked the Mistral for its reliability.

That test data is pretty obsolete.

I have taken lot of precise data (using well established engineering methods) and there is no Mistral that will come close to a well tuned Royal Aqua Master. The comparison is not even close.

I have to agree, and while I have no where near the test data that Luis has, I have plenty of experience with both and I do have test equipment also. There is no Mistral that can compare to a RAM, all due respect to the Navy, that data is obsolete. Well, I guess a Mistral could compare in one regard, the exhaust cycle.

But both of those are dated, the new benchmark for double hose performance is the Argonaut, particularly on the exhaust cycle. However, a Mistral is a really cool and completely useful regulator and a lot of fun to dive with and I am sure will be a nice restoration to serviceable condition. The newest silicone diaphragms make a huge, huge difference even over the first generation of silicone diaphragms much less the old neoprene originals.

N

 

[simonbeans]

Many don't realize that the U.S. Divers Company Mistral has better performance characteristics than the Royal Aquamaster, according to the U.S. Navy Experimental Diving Unit tests. This may be one of the reasons the Cousteau team used the Mistral almost exclusively for its operational regulator.

SeaRat

However, although Cousteau and team may have used the USD Mistral, they primarily use the La Spirotechnique Mistral and Royal Mistral. And as far as can be determined the US Navy never tested these French regulators.

 

[jacked_72]

I've gotten my parts for the Mistral and installed them. I'm following the FSM at VDH and got to the step where you're supposed to put soapy water over the pin that opens the HP seat. I've got no leak at all from the pin, but there is a small hole near the pin that will leak if I plug the venturi tube. If I fill the whole can, I see a small stream of bubbles (similar to a Sherwood regulator) coming from the venturi tube. Is this supposed to be there? The manual only mentions that you shouldn't have bubbles coming from the pin.

Also the regulator is really screechy/honking while breathing. Is this normal? If not, what should I look for?

Thanks.

 

[John C. Ratliff]

I'll let Luis talk to the bubbles (though I believe they should not be there). By the way, thanks Luis for the information on the comparisons. Do you have and graphical data?

Concerning the "screechy" feeling while breathing, this is normal for the Mistral out-of-water. Put it in the water, with water on the diaphragm, and it disappears. I think this has to do with the venturi out the side vents of the Mistral orifice impacting the diaphragm.

SeaRat

 

[Nemrod]

The bubbles are a result of a HP leak from the seat. Either the seat is bad or the volcano orifice is damaged/worn and that is as I said earlier--BAD. There should be no leakage, no stream of bubbles, none. You might breath on it some and let the seat and volcano orifice find home. If it does not settle in and stop leaking, like I said, is that a new seat? If it is a new seat, is the volcano orifice dirty or with verdigris? Is the volcano orifice edge sharp and clean, look at it with a 10X loupe. It should be smooth with a slight radius where it mates/to the seat.

Sometimes, if a new seat does not cure the leak, you can take rouge on a pencil eraser and twirl it gently on the volcano orifice to refinish the sealing surface. There are other more aggressive and increasingly less desirable methods that can be employed but first, make sure you have a new seat and let it settle in with some pool time.

The screeching and honking result from possibly your hooking it up to 3,000 psi??? If so that could also explain the bubbles. The Mistral was never intended for much over 2,250 psi and is really more appreciative of you if you limit it to less than that.

The single stage regulators, which the Mistral is, reduce tank pressure to ambient in one step, there is no intermediate pressure. The powerful venturi action is quite noisy and is one of the dead give aways that you are not on a two stage unit. The venturi makes a jet sound when you breath.

Also, just a little warning, do not, place your lips over the outlet horn on the can and take a nice big breath. This is a good way to blow your lungs out your rear end and I am being very SERIOUS.

N

 

[herman]

To add a little than Nemrod did not cover, that little hole is an air bleed hole and it should be open, it works in conjunction with the venturi.. It should not (can not actually) leak on it's own but the small HP seat leak you appear to have will feed it. Stop the HP leak and the bleed hole will take care of itself.