US Divers Royal Mistral: chronicle of a death foretold another case of "Better is the Enemy of Good Enough" 17/08/2018 by Stephane Eyme with no comment Regulator 1/ The background Pressure from the inside In 1965 DAAM and DW MISTRAL were already best sellers in the US for almost a decade. Back in 1958, E. Gagnan introducing a strong Venturi effect to DA Navy Approved and DW STREAM AIR, turned DAAM and MISTRAL into game-changers for US Divers (see my post “1958… and then came Venturi”). 1965 was also the year when the scuba equipment company decided to go one step further and commercialised Royal Master. Based on DAAM but this time featuring a balanced first stage. This new regulator will be updated in 1967 with a renovate compensated chamber featuring a lib to retain the inner O-ring, and become the Royal Aqua Master, more known as “The regulator for those who want the best” DW Mistral was introduced simultaneously with DAAM. It was a single stage regulator also featuring a strong Venturi effect cheaper than his two-stage brother. It was also easier to maintain and yet supplying a great deal of air at any depth. Mistral was nevertheless not compensated. During its 8 years lifespan, few evolutions were brought to its original design. DW MISTRAL was somehow technically falling a bit behind its competitors. Pressure from the outside Previously, Spirotechnique in France hit the market in 1962 with a new single stage. This new regulator, called Mistral Royal, made sales skyrocketing. Mistral Royal had no major technical innovative features compared to its predecessor Mistral, other than the reshaped body cans (with same diaphragm diameter and smaller internal volume) to lower the “take-off” effort, and a redesigned nozzle to create a stronger Venturi effect to increase the air delivered and assist the in-breathing effort. These updates were perfectly combined with other minors tweaks. Mistral Royal became what many divers consider the best single stage regulator ever made. It was delivering far better performance than its Spirotechnique Mistral predecessor. A call to action So in 1966, US Divers probably felt stressed to introduce an evolution to its DW Mistral. This development should bring an innovative upgrade based on a real technical difference. It would place this new regulator on a matchless level and maintain US Divers at the edge of innovation. And of course, increases its market share like Spirotechnique did with Mistral Royal. This background probably shed some light on what happened next. US Divers engineers adapted Royal Master compensated process to DW Mistral, giving birth to Royal Mistral this very year. 2/ The technical solution A mix between DW Mistral and Royal Master/RAM It is remarkable to see how much of a Frankenstein Royal Mistral is. Made with parts of Royal Master/RAM and DW Mistral, Royal Mistral has few parts of its own. It really seems to be an in-between regulator. All in all, the technical innovation has been limited to a very few numbers of components. This is the blueprint of a USD Royal Mistral. In blue the exact same spare parts from DW Mistral. In green from Royal Master/RAM and in red the ones belonging only to Royal Mistral. As we can see, few spare parts were specially designed for Royal Mistral. The majority of them were reused from the two other regulators. In a nutshell: Royal Mistral and Royal Master/RAM share the same crown spring and crow blocker Royal Mistral and DW Mistral share among many others, the same body pin, body gasket, plate screws, primary and secondary lever, adjustment nut screw, adjustment nut, diaphragm… The singularity remains in the ad hoc designed spare parts The Top Box Same as the DW box top, except for the intake horn reducer. The reducer aims to limit the quantity of air entering the intake horn. Looks a bit like a parch added to the original design. Maybe to solve a particular issue, we will see how important this part is that later on. The Disc & Retainer This is probably (with the poppet) the most interesting part. This retainer features a soft inserted disc in its centre that will seal the poppet when closed. The disc has to be glued to or moulded in the retainer to ensure sealing, otherwise, the regulator will leak. The O-ring around the retainer will make the seal with the body. The Sleeve This spare part is meant to guide the poppet perfectly straight onto the disc of the retainer to provide sealing. The Pin It really looks like a DW Mistral’s pin that would have been straighten. The idea is to be able to reach the end of the hole in the poppet. The HP Spring Very similar function as for RAM The Poppet Here is another major difference. The poppet is conic, the seal is made between the poppet and the retainer thanks to the soft disc. The poppet has a hole in its centre that will allow the air to reach the compensated chamber. The Body While looking amazingly similar to DW Mistral’s one, Royal Mistral body is quite different. No thread to mount the Venturi nozzle, no bypass drilled in the top part. It also presents a large hole on its side to compensate the Ventury effect. Compensated first stage with a slight difference. Royal Mistral is compensated the same way Royal Master or RAM are. Royal Master/RAM have a fully compensated first stage. This means the pressure in the compensation chamber is always balanced with the intermediate pressure given by the regulator. This is regardless if the air is flowing or not through the regulator. The proper mechanism allows the intermediate pressure to go freely into the compensated chamber at any moment. Royal Mistral works the same way... Royal Mistral works that way also. There is no intermediate pressure provided. It is a single stage regulator meaning the high pressure is directly brought to surrounding pressure. The compensated chamber is in constant equalization with the low/surrounding pressure. The equalization is done since the push pin and poppet are not sealed together (it is a metal against metal contact). The low/surrounding pressure is allowed to flow into the compensated chamber at any time through any of the body exits. Though it might take some more time to do it... BUT because of the section/pressure ratio it might take some time. Taking into account the very tiny section between push pin and poppet allowing the equalization into the compensated chamber AND the fact that the pressure to compensate is only low/surrounding pressure (no additional 8 bars), the equalization of both ends of the poppet will probably not be instantaneous. That means there might be a short lapse of time before the pressure inside the compensated chamber equalizes the new low/surrounding pressure. Royal Master or RAM don’t show this lapse of time due to the fact the pressure has an additional 8 bars on top of surrounding. The air is “pushed” much stronger into the compensated chamber compared to the only surrounding pressure of Royal Mistral. This means the ratio section/pressure is clearly in favour of compensating much quicker the pressure on both ends, almost instantly. 3/ Tuning Royal Mistral The conic poppet and the disc section are generating a huge deal of air. This is to the point that Royal Mistral doesn’t even need to feature a nozzle to generate a very strong Ventury effect. Actually, the air is rushing into the admission horn very strongly. It needs to have an entry horn section reducer to somehow control the air flow! Once the two levers are tuned – which is quite straightforward to do – there is no other tuning to perform. There is no intermediate pressure to tweak, no second stage spring to squeeze, exactly like a regular DW MISTRAL. The horn section reducer is a major part This said the horn section reducer is a major part of this regulator. It is normally welded at manufacturing with the top box and you can’t tune it at all. The thing is, when I received my Royal Mistral, it was leaking and former owner said he wasn’t able to finely tune it. Curiously, when I opened it, the horn reducer was removed, I still wonder why… As a result, when the Venturi effects kicks-in at inbreathing, a huge quantity of air rushes into the admission horn and generates a very unpleasant surprising AIRBLAST!!! The airblast is somewhat softened by the hose itself. This said, the impression you get is that something exploded into your mouth and lungs… see my video about that. https://youtu.be/BIDXXzynVtk How it works I had to reproduce a new horn section reducer to control the quantity or air running through the admission horn. The tuning of this spare part is a bit of a success/failure process, reducing too little the horn admission hole and you still get the airblast, reducing it too much and the Venturi assist effect is too weak to hold the diaphragm in low position… and the difference between both is really tiny… Here are 3 small videos of the impact of the horn section reducer on the Venturi effect: https://youtu.be/6l_i9-Pgv28 https://youtu.be/GF_dg04dsh8 https://youtu.be/Scd7iaQwoAA No horn section reducer, Venturi effect creates an airblast, the air flows so strongly, it is hard to close the exit horn with my hand. This is not divable. Horn section reducer is limiting too much the quantity of air, Venturi effect is barely appreciable, actually, the diaphragm goes back to its original position before I even have time to close the exit horn with my hand. Properly set, the horn section reducer allows a sufficient amount of air passing through the horn to create a strong Venturi effect but not an airblast. I have to admit the horn section reducer looks a bit like a desperate parch to solve a resilient issue. But eventually, this tiny piece of brass will define the performance of the entire regulator!! 4/ The performance Results are there! Take-off effort is light and the regulator responds well to any kind of inbreathing solicitation, delivering the adequate quantity or air. The breathing effort seems the same during the entire dive, from 200 bars down to 40 bars, or at least I didn’t feel any remarkable difference. This is what the compensated chamber is meant to do. Of course, the airblast is gone, Venturi effect is triggered very soon and assists the breathing effort from the very beginning. Even trying to trigger it very heavily, Venturi effect responds but doesn’t generate any airblast. As well, I had the feeling the Venturi effect is smoother than the one created by DW Mistral, you feel it assists you but it stays on the second plan, in a smooth and gentle way. With a surprising good behaviour! Equally, Royal Mistral delivers a great deal of air at any depth from the surface down to 100 feet. The feeling I had was that I could have dived much deeper the result would have been the same. Royal Mistral’s inner sections are allowing the air to flow without limiting it in any way, and the feeling is just that, you have plenty of air and the regulator can deliver much more if needed. Another side I found intriguing, I felt much less difference diving in any position than compared to a DAAM, DW or RAM. Diving face-up or down, head up or down, the inbreathing effort feeling is very similar. This is a bit weird I have to admit as this feature is endogenous with double hose regulators… Last, if I had to compare the performance of Royal Mistral vs. its compensated competitor RAM, I would say both are probably very close, at least from a dive trial standpoint during shallow dive. I guess a serious benchmark would take at least several dives down to 150 feet to check it properly, though my guess is the result would presumably be the same. All in all, I’d say the performance of Royal Mistral is on the level of what was expected, delivering a great deal of air at any depth, in a smooth and comfortable way, providing this feeling you could ask him much more without any difficulty. Let’s see how it performs once properly tuned: 5/ A yet a very short life... The technical innovation is delivered but not overwhelming... Eventually, Royal Mistral is a good but not overwhelming regulator. The balanced first stage works fine but doesn’t make a huge difference with its direct competitors. It is a hard to tune regulator, leakage from HP seat and free-flowing are common, and compared to a DW Mistral on actual dives, Royal Mistral doesn’t seem to be a game changer. I would even argue, but that’s only me, that I would tend to pic DW over Royal Mistral because of simplicity, reliability and the fact that feeling a stronger Venturi effect gives you the (fake) impression of safety and potential. Marketing placement is not clear… US Divers Royal Mistral has only been commercialised for a couple of years between 1965 and 1968. I don’t have the exact number of how many were manufactured though I guess it is a low figure. The catalogue price was a bit higher than DW Mistral and a bit lower than DAAM and this was probably a misstep that confused many potential buyers. Either your budget was limited and you would go for a DW MISTRAL or in case of mandatory need for an intermediate pressure, a DAAM; or, you had a higher budget, and you would definitively go for a RAM and get “the regulator for those who want the best” So Royal Mistral’s market positioning in US Divers’ catalogue, was unclear and that probably made it difficult to sell. I guess this is another major reason divers didn’t line up to order it and as a result, the regulator didn’t have the commercial success it was intended to have. A death foretold I guess the good diving performance of Royal Mistral didn’t balance the delicate maintenance and the mistaken marketing strategy. These are perhaps the reasons why this regulator didn’t bring the expected sales and was discarded from US Divers’ catalogue only two years after its launching. My understanding is there was no real gap-to-fill between DW and DAAM. Perhaps an evolution of DW MISTRAL, the way Spirotechnique did with its Mistral Royal, improving performance of the regulator by adjusting and tweaking it, would have been enough to call the attention of the market and give DW MISTRAL a new incentive to be bought. DW MISTRAL remained in US Divers’ catalog until 1970, far beyond Royal Mistral, making happy divers as performant, simple and affordable regulator. Thing is, I guess this was somehow a predictable outcome and another illustration of NASA’s assertion: “Better is the Enemy of Good Enough”