I agree with Luis, the new Mistral lacks a proper design to achieve an effective venturi action, much like the Trieste, you can get a low cracking force but the performance still is mediocre.
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The problem with the new Mistral is not the cracking suction. I have one that I have adjusted the cracking suction to about 0.5 inWC. The problem is that it doesnt have any venturi effect.
Most any decent regulator can be adjusted to have low cracking suction. Any of my RAM and Phoenix RAM are adjusted to between 0.5 in WC and 0.7 inWC, but as soon as I start sucking the inhalation suction goes down often close to zero and if I tune them too sensitive they will easily go beyond zero to a light positive pressure.
After any dive when I am out of the water, I suck on my regulator and immediately take it out of my mouth to induce a strong free flow and dry the hoses (the free flow will sustain itself until I block the mouthpiece). You will never be able to do that with a regulator that doesnt have a strong venturi like the new Mistral.
I can probably modify the second stage of the new Mistral to induce a venturi, but I have too many other projects in front of it. A quick inspection of the second stage and I was able to see a couple of design flaws.
Luis your absolutely right! The new Mistral is a horrendous design (and I use the term design loosely) the guys at Aqualung did not pay any attention to the Flo pattern. The biggest problem is the propensity to fill the air box first, therefore creating a positive presser against the diaphragm. Not not go in to detail here but I have been working on a modified second that creates plenty of venturi effect. The main problem is tuning the effect, so the next step is to modify the unit to allow for a variable bypass and give the diaphragm a little lift, not a lot.
Luis is right about these two defects in the design of the Trieste. I have a Trieste II, and bought it new. But I was unhappy when it did not breath as well as my USD Mistral at low tank pressure. I actually measured the suction pressure of the regulator, using a home-made water manometer, and it was something around five inches of water--horrible. The diaphragm contributed about 2-3 inches of water inhalation resistance.I am not sure how bad the venturi in the Trieste is. It is hard to tell because I have found that it has an overshadowing flaw. The diaphragm shape is like a structural dome. You can set the diaphragm in an empty can (no lever or mechanism behind it) and start adding weight on the diaphragm until it moves and it takes relatively a lot weight to move it. I can’t remember the exact numbers (I have it on my notes), but it was not good. It didn’t mater if it was a very soft diaphragm. It just has a bad geometry.
I have some ideas for my Trieste, but again it is in the list of things to do.
Many regulators can be adjusted to very low cracking suction and are very stable. The limiting factor is often the exhaust location to avoid a free flow. A regulator with a cracking effort of less than 0.5 inWC is going to free flow in some position even with the exhaust right in the middle of the diaphragm. When you get vertical just the width of the exhaust (even with a perfectly position duckbill) will be close to 1/2 inch from the center.
For that reason there is no point on get the cracking effort any less than about 0.6 inWC. Then it is the design of the venturi that makes the difference.
When I inhale on a good regulator you will normally see the suction effort dropping after the initial cracking effort. Like I said it often goes into positive flow if adjusted carefully.
I have fine tuned the new Mistral to very low cracking and (in my experience) even if I suck very suddenly I can not initiate any venturi flow. The meter never dips below the initial cracking effort. Note: different flow rates and flow build up will affect venturi effects.
It is kind of like an old Broxton (Navy Type DA, etc.). They can also be adjusted with very low cracking effort, but they have no venturi effect.
The Phoenix as others have noted is the ticket if you want to use all the current accouterments while enjoying a double hose regulator. About mine.
Pete
.....................And it has brought all of us to a better place in double hose diving, but it is an add on not a complete replacement like the MK3 which functions and feels like a modern regulator.
I really do not understand this statement. I have no diving experience with the MK-3 but many many hours with modern regs and the Phoenix. I do however understand how both work and have studied the design of both (as much as I can get hold of)....as well as that of the DH regs they modify. The MK3 and the Phoenix both function in exactly the same and they both accomplish the same ends; they provide a modern HP seat and orifice (the exact same ones) and add HP and LP ports to the DH reg, just in a slightly different way. Both in their own way are complete replacements, the MK3 replaces the entire first and second stage assembly of the regulator as opposed to the Phoenix replacing the entire first stage. Both designs have their advantages. The MK3 may be a little better suited for DIN use but adds a good bit more weight to the entire reg, a down side for the traveling diver. The MK3 eliminates the gasket between the first and second stages but at the cost of hose alignment. The second stage of both use the same USD design and parts so there is no difference in the operation of the second stages. Since they are both using the same IP, HP components and second stage components there is going to be no difference in the final result. A double hose reg will never "feel like a modern regulator" which I assume means a single hose reg of modern design; physics simply will not allow that. The MK3 is a well designed and thought out piece of gear but in the end with it on your back and the mouthpiece in your mouth, the diver will never know the difference in the 2. Both have greatly improved the old DH design for use in todays world and both have their strong points but to say one is superior to the other is misleading in my opinion.