USD mistral questions

[Ron]

First of all, understand that I am a DH rookie and I know very little other than what I've read about them.

I really like the USD DW Mistral single stage double hose regulator. I like the yellow hoses, and the simplicity of the design. I looked all over the board, and I'm sure my questions aren't posted because you guys already know this stuff. So help a brother out. I know that a two stage regulator reduces tank pressure to intermediate pressure at the first stage and then intermediate pressure to ambient pressure at the second stage. All the regulators I've dove have also been balanced. So having said that, here are my questions...

-How does the mistral breathe?

-Since it is single stage, isn't it harder to breathe at certain depths?

-Is the flow rate such that you can over breathe it?

-How does a single stage only regulator work?

I'm asking this because VDH has a mistral for sale, and it looks amazing. I just want to know in advance what it is all about. I've read and researched how to dive, clear, and buddy breathe with one. I'm just curious about the technical aspects of this reg. Thanks in advance.

S

[1969ivan1]

Simply put the MISTRAL breathes a little hard at high tank pressure such as at 3000 PSI. Once the tank get s down to about 2000 PSI or so it gets easier than any reg you can find. The MISTRAL was the reg of choice of Cousteau and nearly bullet proof. There are a lot of other folks out there than can give you a lot mmore detailed info, but that is the short and sweet of it. Also do a search in the past forum entries and you will find this talked about in much greater detail if you care to read about it.

If you want a MISTRAL you cannot go wrong with one from the store....JMO....

You of course will also need a new GRAY MISTRAL t SHIRT to go with it!!

[Herman]

First of all, understand that I am a DH rookie and I know very little other than what I've read about them.

All the regulators I've dove have also been balanced. So having said that, here are my questions...

-How does the mistral breathe?

-Since it is single stage, isn't it harder to breathe at certain depths?

-Is the flow rate such that you can over breathe it?

-How does a single stage only regulator work?

I'm asking this because VDH has a mistral for sale, and it looks amazing. I just want to know in advance what it is all about. I've read and researched how to dive, clear, and buddy breathe with one. I'm just curious about the technical aspects of this reg. Thanks in advance.

S

I'll give it a shot.
Balancing a first stage keeps the IP pressure pretty much constant from a full tank pressure until the tank pressure drops below the point where the balancing design can compensate...somewhere in the 150psi range. If the IP pressure stays constant, breathing resistance of the second stage stays constant. Balancing a second stage keeps the breathing resistance constant over a changing IP pressure by balancing pneumatic forces on the second stage seat.

-How does the mistral breathe?
They breath good within a given tank pressure range, because of the HP seat design, the higher the tank pressure the more force (your inhalation ) it takes to open the valve so as your tank pressure drops the reg breathes easier.

-Since it is single stage, isn't it harder to breathe at certain depths?
No, remember the ambient pressure inside the cans increases along with depth, just like in any single or double hose reg and since there is no IP to vary, breathing resistance is unaffected by depth. This is ignoring increased air density which will increase the resistance some but is not a function of the reg.

-Is the flow rate such that you can over breathe it?
Only if a great white is considering you for lunch. Time to pull out the dive knife and stab your buddy in the leg.

-How does a single stage only regulator work?
When you inhale, the diaphragm moves inward depressing a series of levers that pushes on a pin, that pin lifts the HP seat off it's orifice allowing air to enter the can air chamber and hoses. Once the pressure in the can and hoses reaches ambient pressure again (you stop inhaling ) the diaphragm returns to it's resting position allowing the levers to return the HP to it's closed position. Basically, just like any simple single hose second stage, just with a lot higher IP (tank) pressure.

[JES]

.... If you want a MISTRAL you cannot go wrong with one from the store....JMO....

My thought as well...

[Herman]

check out this link http://www.vintagedoublehose.com/downlo ... eStage.pdf

[simonbeans]

Ron,
Check your email. Hopefully this info will help.
Allan

[Ron]

Thanks all for the great info!

[Ron]

After looking at all the diagrams, it seems like the mistral functions the same as any other diaphragm two stage regulator, just minus a constant IP and with only a single stage. This design seems very simple and functional, so much so that it makes me wonder why so many other regulators (two hose included) are two stage. Why did the industry move away from this?

[Ron]

Simply put the MISTRAL breathes a little hard at high tank pressure such as at 3000 PSI. Once the tank get s down to about 2000 PSI or so it gets easier than any reg you can find. The MISTRAL was the reg of choice of Cousteau and nearly bullet proof. There are a lot of other folks out there than can give you a lot mmore detailed info, but that is the short and sweet of it. Also do a search in the past forum entries and you will find this talked about in much greater detail if you care to read about it.

If you want a MISTRAL you cannot go wrong with one from the store....JMO....

You of course will also need a new GRAY MISTRAL t SHIRT to go with it!!

Does it breath harder at higher tank pressures because of the physical limits of how much air the body of the regulator can deliver?

[simonbeans]

A two stage regulator produces a relatively constant IP. Thus the work of breathing over the range of tank pressures remains somewhat the same. A balanced 1st stage is the most consistant, but an unbalanced 1st stage (DA) at least is some what consistant. A single stage opens (in an upstream) against the entire range of tank pressures, thus has a higher work of breathing when the tank is full than with lower pressures. In a downstream single stage, Voit 50 Fathom, the work of breathing is easiest with a full tank as the increased pressure "assists" in opening the seat. However, it does breathe a little harder when the tank pressure is lowest. One factor that I love is the wosh sound that you get with the Mistral venturi nozzle. Sort of takes the "silent" out of The Silent World.

[Ron]

A two stage regulator produces a relatively constant IP. Thus the work of breathing over the range of tank pressures remains somewhat the same. A balanced 1st stage is the most consistant, but an unbalanced 1st stage (DA) at least is some what consistant. A single stage opens (in an upstream) against the entire range of tank pressures, thus has a higher work of breathing when the tank is full than with lower pressures. In a downstream single stage, Voit 50 Fathom, the work of breathing is easiest with a full tank as the increased pressure "assists" in opening the seat. However, it does breathe a little harder when the tank pressure is lowest. One factor that I love is the wosh sound that you get with the Mistral venturi nozzle. Sort of takes the "silent" out of The Silent World.

Simon,

So really the difference in whether a single stage DH has a higher WOB at high or low tank pressures depends pretty much on whether it is an upstream or downstream design. If I understand it right, that is because of the spring pressure correct?

[captain]

One model of the La Spirotechnique Royal Mistral is the only single stage that came with an HP port to mount an SPG. It is almost exactly like the USD Mistral except built to metric dimensions. They are a bit harder to find but well worth the search.

[luis]

First a few historical notes:
The first regulators used by Cousteau were all two stages. The single stage regulator didn’t actually come out until several years later.
The single stage regulators were the first ones to include a venturi flow assist. Without a good venturi flow assist a single stage demand valve would operate very poorly.


The upstream versus down stream (non-balanced) demand valve is affected by the incoming pressure basically because the valve is moving against the flow or with the flow (this is a bit simplified, but I hope it gives you an idea).


A single stage demand valve has to operate in a very large range of pressures and therefore it requires large mechanical advantage. You may notice the compound mechanical linkage that provides a large mechanical advantage.

The other means to reduce the force variations in the links is by using smaller flow orifices (than lets say a balanced two stage regulator). This small orifice becomes an issue when high air flow is needed (as in working hard) during deep dives (mostly beyond 100 ft or so). This was pointed out by Emile Gagnon (the designer).

The Mistral is know as very good breathing regulator with low tank pressures and for dives that are not too deep. Under normal working conditions is probably a decent regulator down to about 130 ft (depending on air volume demands).


Also keep in mind that very high tank pressures (above 2500 psi) tends to put a lot more wear on the HP seat. This is less of a concern now that we have replacement seats, but in general it is best to only operate a Mistral with LP tanks (below 2500 psi). And it breaths a lot better with even lower tank pressures.


Considering the simplicity and associated reliability of a Mistral it does perform extremely well. A lot of this performance is due to the large venturi assist design into the flow.

IMHO a Mistral is a great regulator that anyone with an interest in DH regulator should have one and dive it…it is a fun regulator. I am sure the one Bryan is offering is probably the best deal around.



A two stage can deal with larger orifices specially a balanced two stage regulator. A regulator like the Royal Aqua Master can be adjusted to be the best breathing regulator at any tank pressure and any depth, but that comes with a somewhat more complex mechanism. To this day most of the regulators made by Aqua Lung are mechanically very similar or identical to the RAM, they just tend to have many more parts, specially in the second stages (due to the plastic case, etc.).

[Greg Barlow]

Given a choice between an unbalanced two stage reg and a single stage model, I will take the single stage every time. IMHO, the unbalanced diaphragm models are overly complex for the performance that you get.

An unbalanced diaphragm will actually get easier to breathe from as the cylinder pressure drops. For instance, at 2,500psi the intermediate pressure is probably around 110psi. As the cylinder gets close to reserve pressure, the IP will likely be at 130psi or so. This is the reason why the second stage needs to be adjusted at low cylinder pressure. If it is set to operate at peak performance at the 2,500psi level it will very likely have a second stage free flow as the pressure gets below 700psi or so.

The change in cracking effort (amount of vacuum needed to initiate air flow) is quite dramatic with single stage regs. It is not uncommon for a properly adjusted Mistral to have a cracking effort of 1.25-1.50" of water at 2,500psi. At 700psi, the cracking effort would likely be in the 0.75" range.

My favorite DH reg, the Voit 50 Fathom with the Gagnan system (downstream flow) will typically demonstrate a cracking effort of 0.75-1.00" with a 2,500psi cylinder pressure. My own 50 Fathoms are set to 0.8", and will just barely free flow at 2,300-2,500 while vertical in the water column. With a nearly empty cylinder, the effort has increased to nearly 2.0". I use VDH silicone diaphragms in all of my Voit DH regs. I also adjust the duckbill's position so that the flapper slits are in the center of the diaphragm plate.

You simply cannot beat the robustness of a single stage reg. When you consider that a 50 Fathom does not have a single O-ring, that pretty much says it all...

During the Sea Hunt 50th Anniversary weekend, I made a dive to 135ffw with one of my 50 Fathoms. I purposely breathed deep and fast to see if I could overbreathe the reg. I only succeeded in making myself dizzy.

If you are looking for the best overall performance from your DH reg at all cylinder pressures, then the answer is a USD RAM or better yet, a Phoenix.

As for me...I make the vast majority of my vintage dives with a Voit single stage model.

Greg Barlow

[Ron]

Greg, thanks for your informative post. I don't want to sound like an idiot here, but if a single stage regulator was balanced, it would constantly free flow right? I've studied the diagrams for the single stage regs, and what causes the air to flow through the filter is that there is a difference in pressure between the inside of the can and the part of the can where the water is. Because there is a difference in pressure the diaphragm pushes against the mechanical part and allows the air to flow into the intake hose. I know this is a rudimentary explaination, but this is how I understand it. All single stage regulators are unbalanced right?