USD mistral questions

[luis]

The Us Divers Royal Mistral is a balanced single stage regulator.

In concept a balanced single stage could work very well. But, from what I have read and heard (I haven’t actually worked on any Royal Mistrals) the design of the seat had some major flaws. They didn’t seem to last and because it is a cone shape seat, they are close to impossible to resurface.

[Ron]

Luis,

Great info. Are there any other balanced single stage regs?

[captain]

The USD Royal Mistral ( no relation to the La Spiro Royal Mistral) was an attempt at a balanced single stage. It was not very successful I believe because of issues with longevity of the seat material.
I worked on one a year or so ago to try make a new seat. Although I could get it to work the seat did not last for I believe two reasons. One, the seat was made of a softer material than the typical hard nylon used in the regular Mistral and two, I believe the seat could not stand up to the high velocity flow and would rapidly erode it.

[Ron]

This is all starting ot make sense now. The DW mistral has an upstream valve seat, so as the tank pressure gets lower, the mistral gets easier to breathe. So the single stage downstream regs gets harder to breathe as you get a lower tank pressure. Does this mean that the mistral is prone to free flowing at lower cylinder pressures?

[luis]

During the early Royal Aqua Masters they also used soft seats for the balanced first stage and they didn’t seem to last long either. The difference is that a small IP leak in a two stage regulator could go unnoticed for a while if the IP pressure doesn’t build up fast enough between breaths.

Back in the early 70’s I ended up converting my RAM into an Aqua Master due to its higher reliability at the time. That is not the case anymore with the new Teflon seats.

My round label RAM is back into its full glory of a RAM and in the same original configuration (but with a silicone diaphragm and some other material improvements). Its cracking suction is never higher than about 0.6 to 0.7 inWC.


I don’t know of any other attempts at a balanced single stage regulator, but that doesn’t surprise me since a large attraction of a single stage is its simplicity. A balancing chamber will normally add at least one O-ring.


Greg makes a great point about the single stage is very attractive as compared to an unbalanced two stage. An Aqua Master can be adjusted to breathe very well, but the performance is not necessarily proportionate to the increase in complexity.

An Aqua Master does have an intermediate pressure if needed for any accessory, were the Mistral doesn’t.

On the other hand a Royal Aqua Master can be adjusted to consistently have a cracking suction as low as 0.5 inWC (independent of tank pressure or depth). The primary limiting factor in adjusting the cracking suction less than 0.5 inWC is that it could free flow in some positions because the exhaust opening (the duckbill slits) are large enough that at some point the exhaust will be higher than the center of the diaphragm by the ½”.


In case you are not aware, the primary issue with breathing performance of a double hose is the vertical position (and changing position) in the water column of the regulator diaphragm in reference to the lungs (and the inner ear were pressure is sensed by humans). This is overcome mostly by technique, breathing technique, etc.


BTW, Greg’s deep test of a single stage regulator to see if he can over breathe it is not significant IMHO. The guy has such a low SAC that even if he is working hard, he could be breathing from a Mistral using a straw instead of hoses and would not notice the difference.

[luis]

Does this mean that the mistral is prone to free flowing at lower cylinder pressures?

With a good seat, a good volcano orifice, and a good stiff spring they don’t seem to leak at any pressure. Even at low pressure the stiff spring provides enough force to seat the seal.

[Ron]

... "In case you are not aware, the primary issue with breathing performance of a double hose is the vertical position (and changing position) in the water column of the regulator diaphragm in reference to the lungs (and the inner ear were pressure is sensed by humans). This is overcome mostly by technique, breathing technique, etc."

Why is that?

[simonbeans]

Use this to help understand position, pressure, breathing variations:

[captain]

What is not obvious to those who haven't seen the internals of the USD Royal Mistral and it is not very well seen in the diagrams of it here on the site is that it is reversed what what is normally seen in first stages and the Mistral. The moving poppet is all metal and is cone shaped. The seat that it seats against is removable and is made from metal with a rubber center orifice bonded to it that seats in the body similar to the the volcano orifice used in the Phoenix and the single hose first stages. It is this rubber part of the orifice that gets eroded by the high velocity flow.

[luis]

What is not obvious to those who haven't seen the internals of the USD Royal Mistral and it is not very well seen in the diagrams of it here on the site is that it is reversed what what is normally seen in first stages and the Mistral. The moving poppet is all metal and is cone shaped. The seat that it seats against is removable and is made from metal with a rubber center orifice bonded to it that seats in the body similar to the the volcano orifice used in the Phoenix and the single hose first stages. It is this rubber part of the orifice that gets eroded by the high velocity flow.

The old Poseidon Cyklon 300 first stage also has a metal cone poppet and a fixed soft ring for the seat. The donut soft ring is not that soft do. It was made out of what seems as white nylon. Just about the right stiffness to be long lasting, but not so stiff that it didn’t seal well.

I wonder if it would be possible to modify a Royal Mistral with a nylon or Teflon donut ring for the soft seat.

[Greg Barlow]

During the early Royal Aqua Masters they also used soft seats for the balanced first stage and they didn’t seem to last long either. The difference is that a small IP leak in a two stage regulator could go unnoticed for a while if the IP pressure doesn’t build up fast enough between breaths.

Back in the early 70’s I ended up converting my RAM into an Aqua Master due to its higher reliability at the time. That is not the case anymore with the new Teflon seats.

My round label RAM is back into its full glory of a RAM and in the same original configuration (but with a silicone diaphragm and some other material improvements). Its cracking suction is never higher than about 0.6 to 0.7 inWC.


I don’t know of any other attempts at a balanced single stage regulator, but that doesn’t surprise me since a large attraction of a single stage is its simplicity. A balancing chamber will normally add at least one O-ring.


Greg makes a great point about the single stage is very attractive as compared to an unbalanced two stage. An Aqua Master can be adjusted to breathe very well, but the performance is not necessarily proportionate to the increase in complexity.

An Aqua Master does have an intermediate pressure if needed for any accessory, were the Mistral doesn’t.

On the other hand a Royal Aqua Master can be adjusted to consistently have a cracking suction as low as 0.5 inWC (independent of tank pressure or depth). The primary limiting factor in adjusting the cracking suction less than 0.5 inWC is that it could free flow in some positions because the exhaust opening (the duckbill slits) are large enough that at some point the exhaust will be higher than the center of the diaphragm by the ½”.


In case you are not aware, the primary issue with breathing performance of a double hose is the vertical position (and changing position) in the water column of the regulator diaphragm in reference to the lungs (and the inner ear were pressure is sensed by humans). This is overcome mostly by technique, breathing technique, etc.


BTW, Greg’s deep test of a single stage regulator to see if he can over breathe it is not significant IMHO. The guy has such a low SAC that even if he is working hard, he could be breathing from a Mistral using a straw instead of hoses and would not notice the difference.

Luis,

Thanks for the compliment about my SAC rate. I once breathed my 50 Fathom down to a nearly empty cylinder at 25ffw just to see what it was like to do so (I did have a pony tank for safety). I really couldn't tell a big difference until a dozen breaths or so from the time that I pulled the J-valve reserve. At that point, I was needing to use my diaphragm muscles to draw a breath. I ascended without going to the pony and later found that the pressure gauge needle barely moved. My guess is that I was down close to 100-150psi. Had it been a reg with intermediate pressure, I might not have been able to make the last few inhalations.

The Voit 50 Fathom used a clear nylon seat, and despite being in contact with the hard seat they often last for close to half a century. The design's beauty is its utter simplicity. No wonder it was a major inspiration for the modern single hose's downstream second stage.

Greg

[pescador775]

There is that goofy illustration from Bill Barada, again. Will it ever go away?

Barada copied that stuff from US Divers literature published in the early 1950's. It was based on some conjecture from J. Cousteau.

Does anybody actually believe that a double hose regulator inhales easier in the head down position as opposed to the head up position? It is just the opposite.

Cousteau thought that the inhalation resistance was determined by the relationship between the diver's lungs and the regulator box when in fact it is determined by the relationship between the relative positions of the mouthpiece and the regulator.

Some cling to the notion that the drawings must be true because the hoses form a closed loop. That is only true if the mouthpiece is blocked. The loop is intersected by the mouthpiece which produces a so called differential pressure based on the varying positions in the water column of the mouthpiece and box. When the diver is vertical, the regulator produces a pressure equivalent to its greater depth, thus at a higher value than water pressure at the diver's mouth. This is the very definition of differential pressure. The result is that there is positive pressure felt at the mouthpiece. If the mouthpiece were removed from the diver's mouth air would blast out. The opposite occurs when the diver is inverted. When the diver is horizontal, face down, the mouthpiece and the lungs are at about the same depth, and below the regulator. This is a coincidence and is central to what Cousteau misunderstood. Clearly, the result is harder breathing relative to the bench number for his regulators which is about 1 inch water column. Swimming horizontal, this number is more like 5 or six inches. Cousteau thought that this effect derived from the position of the diver's chest, but it is actually due to the mouthpiece which is deeper than the regulator causing it to produce a differential pressure less than that needed to expand the diver's lungs. So, additional effort is needed to inhale. In any case, a solution was sought and in 1955, US Divers introduced the Venturi assisted single stage regulator, the DX.

The single stage regulators have a deserved reputation for producing huge volumes of air at deep depths. This is to be expected since thousands of psi are directly available on demand as opposed to 120 psi from the two stage design. However, the bench value for inhalation effort can never be as low as for the two stage. This is because certain compromises have to be made in all unbalanced regulators. For the single stage in particular there are rather severe constraints in producing an easy breathing regulator throughout the range of supply pressures, more particularly at high tank pressures.

Much additional engineering work was applied to the two stage regulator making is a superior breather overall. Improvements to the single stage were mostly toward making it safer with non return valves and adding a long yoke for SPG, but there was not much additional work to improve reliability which was already at a high level. Attempts to produce bench test WOB equivalent to the latest two stage regulator models were terminated prematurely, IMO. I'm referring to the Royal Mistral.

[Ron]

There is that goofy illustration from Bill Barada, again. Will it ever go away?

Barada copied that stuff from US Divers literature published in the early 1950's. It was based on some conjecture from J. Cousteau.

Does anybody actually believe that a double hose regulator inhales easier in the head down position as opposed to the head up position? It is just the opposite.

Cousteau thought that the inhalation resistance was determined by the relationship between the diver's lungs and the regulator box when in fact it is determined by the relationship between the relative positions of the mouthpiece and the regulator.

Some cling to the notion that the drawings must be true because the hoses form a closed loop. That is only true if the mouthpiece is blocked. The loop is intersected by the mouthpiece which produces a so called differential pressure based on the varying positions in the water column of the mouthpiece and box. When the diver is vertical, the regulator produces a pressure equivalent to its greater depth, thus at a higher value than water pressure at the diver's mouth. This is the very definition of differential pressure. The result is that there is positive pressure felt at the mouthpiece. If the mouthpiece were removed from the diver's mouth air would blast out. The opposite occurs when the diver is inverted. When the diver is horizontal, face down, the mouthpiece and the lungs are at about the same depth, and below the regulator. This is a coincidence and is central to what Cousteau misunderstood. Clearly, the result is harder breathing relative to the bench number for his regulators which is about 1 inch water column. Swimming horizontal, this number is more like 5 or six inches. Cousteau thought that this effect derived from the position of the diver's chest, but it is actually due to the mouthpiece which is deeper than the regulator causing it to produce a differential pressure less than that needed to expand the diver's lungs. So, additional effort is needed to inhale. In any case, a solution was sought and in 1955, US Divers introduced the Venturi assisted single stage regulator, the DX.

The single stage regulators have a deserved reputation for producing huge volumes of air at deep depths. This is to be expected since thousands of psi are directly available on demand as opposed to 120 psi from the two stage design. However, the bench value for inhalation effort can never be as low as for the two stage. This is because certain compromises have to be made in all unbalanced regulators. For the single stage in particular there are rather severe constraints in producing an easy breathing regulator throughout the range of supply pressures, more particularly at high tank pressures.

Much additional engineering work was applied to the two stage regulator making is a superior breather overall. Improvements to the single stage were mostly toward making it safer with non return valves and adding a long yoke for SPG, but there was not much additional work to improve reliability which was already at a high level. Attempts to produce bench test WOB equivalent to the latest two stage regulator models were terminated prematurely, IMO. I'm referring to the Royal Mistral.

Wow
You are obviously an incredibly educated dude....and you just confused my tiny brain. Can you break down why the mouthpiece regulator relationship changes breathing effort? Does this apply to single hose as well? I've dove single hose since I've gotten certified (Atomic Z1). I've never noticed any difference in how hard the reg breathes dependent on body position. Is there any?

[Fibber McGee]

In theory, yes the single hose regulator would have a different inhalation effort depending on position...however, in practice it is negligible because the distance from the second stage diaphragm and mouthpiece is very short.....if you were to put a long hose and mouthpiece on a second stage you could mimick some of the effects noticed in a double hose regulator...depending on position and how long of a hose you had, you could actually make it impossible to pull air through it. (ever tried breathing through a 12 ft hose from the bottom of the pool...you can't)

It all comes down to the differential pressure between the ambient water pressure at the regulator's diaphragm and the pressure at the lungs....that's why you try to position the double hose regs as close on the back as you can to minimize this effect.

I maybe didn't explain it well but I hope it makes sense.

[Nemrod]

I find the diagram fairly accurate, where I have decided to go rebel is on this thing of wearing the tank down so low on the back, if the diver where to swim in a vertical postion that might be OK but I don't. I find that to be counter productive to good trim and a good horizontal swimming postion. If you are horizontal then the postion of the tank (and reg) on your back would not matter be it fore or aft as to breathing ease. As well, I tend to roll my body slightly to one side or the other and use a scissor kick with a flutter thrown in between thus reducing the differential between the regulator diaphram and my lungs.

N