Finally, someone understands me!
luis wrote: ↑Fri Jun 12, 2020 2:09 pmCorrect.
The "cracking effort" is the vacuum (or pressure differential) required to initiate the opening of the demand valve. At that point there is zero flow, therefore there is no venturi assistance or any other flow dynamic effect.
The water column differential between the top of the exhaust and the center of the demand valve diaphragm can be (and will be) what provides the pressure differential even if the diver is not inhaling. You can actually test this in a pool (or any comfortable calm body of water, even a big tub) with a DSV in the closed position and turn the regulator so that the exhaust is in a position higher than the demand valve diaphragm.
BTW, in single hose regulators they commonly call this issue: "case fault geometry". The "case fault geometry" describes the distance between the center of the demand valve diaphragm and the top of the exhaust valve when the diver is either up-side-down or in any position that places the exhaust valve edge at the highest possible (vertical) distance from the center of the demand valve diaphragm.
The significance of the "case fault geometry" is that it dictates the minimum cracking effort a single hose can be adjusted without getting into a free-flow condition in some diver position. Most single hose regulator cannot be adjusted to have a cracking effort to less than 1 inWC due to the "case fault geometry".
Scubapro went to a lot of trouble with the Pilot and later the Air-1 and the D series (D-300, D-350, D-400) regulators to have an exhaust valve concentric with the demand valve diaphragm (just like a double hose regulator with DBE or the Argonaut). Even when the diaphragm and exhaust are concentric you still have to deal the the radius of the valve and the distance to the top edge of the valve.
I hope this is clear and it makes sense.
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