Mistral pressures

[SeaRat]

I looked at my repair manual for Aqualung, and it does indeed say to subject the Mistral to 2000-3000 psi to check for leaks. This surprised me, as I mentioned having a problem on mine at 3000 psi. I put it on, tried to take a breath, and apparently the pin drove the seat's center metal section into the teflon seat. It then started leaking. I looked at the seat a few days ago, and it does look different from the others.

But in light of this information, I decided to put my best-performing Mistral onto a tank near 3000 psi (actually, 2800 psi). It went on, breathed fine, and saw no hardship.

So I stand corrected, and the Mistral probably can be used at 3000 psi without problems. I don't know what happened the first time around.

Concerning the yolk, I would not be worried about that. The DA Aquamaster original yolk was used on 3000 psi systems by the US Navy for many years. Since it has the same basic design, but is thinner on the bore area, than the original Mistral yolk, it should be no problem on 3000 psi systems. The change did sell a lot of parts though; sometimes I think that is the main idea--sow some seeds and then harvest the fear (works in politics too).

About the ideal system, I'll still go with my twin setup. This is an 1800 psi system of twin 42 ft3 tanks, a military harness and a Sherwood valve with duel regulator setups. I have the valve mounted backwards, so the J-reserve is facing my back os the right side and the post is on the left for a second regulator (if I use it). It is a very robust system, and ideal for either my Mistral or DX Overpressure Breathing regulator. Sometimes I use my Trieste II on it, and sometimes the Sportsways Duel Air. But if I want to get minimalist, I use the DX Overpressure Breathing regulator (the one with a hose within a hose; venturi air goes down the inner hose to the metal mouthpiece). I've put scba (fire department self-contained breathing apparatus) hoses on this regulator, which are longer than normal double hoses for diving, and quite durable. This allows me to drop the regulator down on my back for ideal double hose positioning for ease of breathing.

John

[seakrakken]

SeaRat, can you post any pics of the deformed seat? I'm curious about your description of the way the pin drove the center post into the teflon seat. was this a weak HP seat? or was there something else contributing to this? Was this a standard Mistral seat or was it something different?

[John Ratliff]

I'll take some photos of the seat, and hopefully get them up within a week.

This was a standard Mistral seat. I have two more spares. I basically have one and a half Mistral regulators right now. I used the boxes of one to make an AMF Voit 50 Fathom downstream valved, single stage regulator (I had the rest of the hardware independent of the boxes). So I have the valving for one Mistral, and the other Mistral intact.

I also have a DX Overpressure Breathing regulator, which is the earliest Mistral-type single stage design.

My one Mistral, which I really love, has the best breathing characteristics of any I've used. I don't know why it's different, but it is--smoother breathing. I put an old DA Aqualung bottom box on it because I really like the older method of mounting the exhaust duckbill on the part which can be unscrewed. So it looks very old, but is really a very nice regulator.

SeaRat

[Bryan]

What about Ryan Spence's research into Cousteau's equipment where he found that 5000 psi cylinders were used with standard issue Royal Mistrals with no ill effects ? Pretty interesting stuff!

[seakrakken]

I'm certain Mr. Spence's reseach is sound. I just don't have the ca-honies to try it on my standard Mistral. Mr. Spence, what do you have to add about using Mistrals at higher pressure?

[John C Ratliff]

The seat on the Royal Mistrals was composed differently, from what I've seen on drawings. It could be that it was designed for the much higher pressures mentioned above.

While up to 3000 psi may work on a standard Mistral, based upon my experience with the one seat that had problems, I would also be very hesitant about going above 3000 psi (unless you have a seat to give away;)

SeaRat

[seakrakken]

I don't have a standard Mistral and a Royal Mistral HP Seat to look at and I'm curious about the differences. Does anyone have a good pic of them both. I ask only out of academic interest about the higher pressure capabilities of both regulators and the Standard Mistrals abilities to do the same. I'm also curious about what made the Royal Mistral a Royal other than the case. Anyone?

[Bryan]

The seats are almost identical in all three regulators. I have no way of knowing the hardness of the seats but all are the usual white nylon. the HP spring in the Spiro Royals is thinner in diameter and a bit longer that in the U.S. version. The 1st stage body of the Spiro is slightly longer so that may explain the difference in length. The diaphragms are of a more shallow design than on the U.S. regulators as well. But I believe that is simply due to the slimmer cases. All three breathe the same in my opinion......The later Royal Mistral I have does have the HP port on the body for a gauge.
Ryan has one like it as well, perhaps he can tell us what he thinks.....
Next time one of the Royals needs service I will take a picture of the seat. I have several brand new and used seats to compare them to .......But as you all know, there is rarely any need to take these regulators apart!

[sea.explorer]

I'll bite. I think that there are two key pieces of information to shed some light on the subject. First, the Cousteau Society used 300bar (4351.13213psi) tanks for many many years. The first tank packs developed for the Undersea World of JYC housed triple 300bar tanks(1966). These were used with both the Spiro Mistral and the Spiro Royal Mistral. This was confirmed by the person who assembled the packs. The tanks were actually custom made out of the same alloys used in the diving saucer. At least early as 1963 they were diving on 3000psi tanks.

Second, the 5000psi tanks cam about in 1984. They used these with Spiro Mistrals and Royal Mistrals unmodified. This was confirmed by a former chief diver for the Cousteau Society. He maintains that there were no maintenance issues as a result of the higher pressures. He also told me that they did also use US Divers regs periodically. I do not however have any verification of usage with specific tank configurations.

Here is my opinion and experience. I have used both my US Divers DW Mistral and my Spiro Royal Mistrals on 3000psi tanks on several occasions and I do so without hesitation. Beyond the experience of using the regs at this pressure here is my theoretical rational for what it's worth.

When engineers design any piece of equipment they use a working load and an ultimate load. The working load represents the conditions for normal use and the ultimate load represents the point of failure. The margin is rairly less that a 5:1 ratio. Based on this rational I would find it highly unlikely that a regulator designed to work at 2250psi would fail at only 3000psi. After all 5000psi is only about twice the presumed recommended pressure of 2250. I think it is a conservative assumption that on a life support item there would be a significant margin of safety built into the recommendations. This is only my opinion and I am not an engineer. I do however believe in this logic. Ultimately decisions about your equipment should be made based on what you are comfortable with.

-Ryan

[seakrakken]

Thanks Ryan, Now that we have some definitive answers on the subject I' feel like my curiosity has been satisfied. I am still curious about SeaRats failed HP Seat. and if there are any construction differences between the Spiro HP Seat and the US Divers HP Seat.

[John C Ratliff]

I am not too familiar with the Royal Mistral by La Spirotechnique.

But I am almost positive that the US Divers Co. version of the Royal Mistral has a different seat configuration. It was only sold for about two years. I have somewhere seen a parts diagram of it, and I believe it is a much better design that is "balanced." I remember the seat to be significantly different from the ones in the other single stage regs they manufactured.

I actually had one in my hands in Corvallis, Oregon in about 1973, and almost bought it. But I didn't have the money then...too bad.

SeaRat

[Bryan]

I've asked Linda to post 3 pictures on this post for me. It shows HP seats for both regulators on all sides. If anyone requires any further information about the HP seats please let me know.

[SeaRat]

Check part number 111305 in the following diagram of the La Spiro Royal Mistral:

www.sea-cruise.net/royal_mistral.htm

That part number does not appear to be in the regular Mistral, and may be what I was talking about (I do remember a USD diagram, but still cannot find it).

John

[seakrakken]

What is that?

[nemrod]

I am a A&P mechanic by current trade and work for a large aircraft manufacturing concern. I build all sorts of test contraptions and it is clear to me that the seat shown for the Royal Spiro is intended for higher pressures.
When an engineer specifies an operating pressure then yes there is a margin of reserve strength/capacity and in this case it may be that the Mistral has a 50 % margin. The "margin" is not intended for normal operation. To operate beyond the design operating pressure would increase wear and likely cause other issues. Many of the pneumatic/hydrualic fixtures we test have in addition to a max pressure above normal operating pressure but may also have additional limits such as time. To overload a Mistral intended for 2475 PSI max operating pressure once or twice is one thing---to do it every use is another. I could be wrong because I am not a regulator expert by any fashion. Of course, it seems that we don't actually have available the design limits for either regulator so we are just going by antedotal evidence. N