Adjusting unbalanced diaphragm double hose regs

[Greg Barlow]

Okay, Bryan has been very helpful lately in explaining the "tricks" of working on the double hose regs, but...My science background and knowledge of working with first stage designs has got me thinking about the proper method for setting the IP of the DA and Mistral (and any other unbalanced diaphragm design).

US Divers recommended setting the DA intermediate pressure at 110psi, but many "in the know" tuners go with around 1150-125 for the better gas flow rates. Since these two regs are unbalanced diaphragms, the IP will actually increase as tank pressures fall. Getting out my formulas and making the actual calculations shows that if the IP is set for a tank pressure of 2,400psi then the IP will increase around 8-12psi when the tank has slowly dropped to below 800psi. This explains why so many divers remark that their Mistral breathes better as the dive progresses.

The Royal, on the other hand, is a balanced design. The IP remains fairly consistent with differential pressures, without much more variance than 2-4psi. It will vary somewhat, as the balancing design has to compensate for spring and lever mechanical principles.

Now, on the other hand...Unbalanced piston designs have a drop in IP as the pressure falls. This is due to the mechanical nature of incoming air pressure working directly on the soft seat of the piston stem.

The cracking effort of the reg needs to be set to the highest IP that the reg will be subjected to. If this doesn't occur then the reg will begin to free flow when the IP climbs to a point where it can push the poppet hard and soft seats apart.

So, are you double hose tuning experts setting your unbalanced double hose regs IP at around 1,000psi or is turning the adjustment back 1/8 of a turn (from no audible leakage) sufficient with a tank of around 2,400?

Of course, now one needs to consider the indentation that will occur in the HP soft seat after the hard seat has worked its magic. This in of itself will cause the IP to typically change 3-5psi.

Perhaps I am being too anal about the whole issue. I suppose the best policy would be to use a band clamp on the reg, and readjust the IP and cracking effort after a couple of dives.

Enquiring minds want to know....

Tuning hundreds of single hose regs over the last 30 years has done nothing but cause me to seek perfection with these ageless classics.

I do want to thank Bryan and Rob S. for all of their help with my double hose regs.

Greg Barlow

[captain]

On DA's I make IP and all other ajustments at 300psi tank pressure and ajust IP until free flow occures and than back off until it stops. Mistrals have no IP ajustment as they are single stage. In these the lever height is the ajustment and it is trial and error as there is no pressure to set.

[captain]

More, on Mistral the levers are ajusted on a full tank(2500). As the tank pressure drops breathing becomes harder because of the lesser differential between seat spring pressure and tank pressure.

[Nemrod]

And even more, the DW Mistral does not have an IP--nope--the only pressure it sees is the tank pressure. I set my lever even to the top of the can I think and I don't see much difference in lever height from full to empty--do you guys?
James

[duckbill]

The lever height shouldn't vary with the pressure. What will vary with pressure is the resistance to opening.
What seems to be overlooked so far is the difference of operation between an upstream vs. downstream seat. An unbalanced upstream seat (diaphragm/piston, makes no difference) will ALWAYS open with less resistance with a drop in supply pressure (i.e. Mistral and DA AquaMaster). However, a drop in supply pressure will also mean a drop in airflow. A two-stage system will cushion any IP delays to some degree.
The Mistral should, in theory, breathe more easily on a tank at lower pressure, as long as the pressure is not so low as to significantly affect air flow past the seat.
Lever height on a Mistral is critical, as the diaphragm can act as a counterspring. Set too high, the seat spring may be overcome by diaphragm pressure at low tank pressures, causing a freeflow condition. Set too low and there will be excessive freeplay and vibrations, resulting in funny, groaning sounds on inhalation. At least, these have been my observations on my Mistral during adjustment. Trial and error is about the only way to adjust the Mistral with any degree of certainty. Once the levers are in place and pre-adjustment is complete (lever top flush with the rim of the box), just hold the boxes together tightly (with the diaphragm in, of course) and check for freeflow. Then breathe off the inhalation port to check for resistance and noise. It should take very little effort to start airflow. Keep in mind that as you clamp the boxes together that the diaphragm probably will set a little lower still, so you may want to back the adjustment off just a tad. I set my Mistral at a tank pressure of about 300 psi until it just barely freeflows, then adjust the adjusting nut up (lever down) maybe an eighth of a turn more. Works for me. Finally, water test it. The regulator should not freeflow until the mouthpiece is a couple of inches above the regulator.

[captain]

About the Mistral I should have said the breathing becomes easier as tank pressure drops because there is less tank pressure forcing the valve closed. The closing pressure is tank pressure plus spring pressure. The multiplication of force by the two levers is what makes opening the valve against the high pressure possible. Next time I'll engage my brain before my fingers.

[Greg Barlow]

Guys, thanks for all of the great responses. There certainly is a wealth of knowledge on this board.

Greg Barlow

[treasureman]

IS there much of a IP pressure diferential setting it at 500 psi vs 300 psi.

I set mine at the 500 psi at 115 (da aquamaster). she breathes real good. when i put it to a 3000 psi tank pressure dropped about 10 psi not really that much, but it was not more noticeably more diffcult to breathe with

[Greg Barlow]

IS there much of a IP pressure diferential setting it at 500 psi vs 300 psi.

I set mine at the 500 psi at 115 (da aquamaster). she breathes real good. when i put it to a 3000 psi tank pressure dropped about 10 psi not really that much, but it was not more noticeably more diffcult to breathe with

There isn't a noteworthy difference between 300-500psi. The difference you quoted from 500-3,000psi is about what it should be. The extra 10psi would be "pushing" more on the second stage poppet, so it will breathe a bit easier, but you may not notice the difference.

Greg