Case geometry fault for SP D-series, G-series, Air 1, Pilot

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elan

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I've been reading about the case geometry fault in the Reg Savvy book lately. According to how it is explained the D- series has the least fault of all those regs ?

Does it reflect the real world cranking effort on all those regs ? The Air 1 and Pilot seems to have a fault closer to G series ? or I'm missing something.
 
Basically what case fault geometry is is a limiting factor in how low you can set cracking effort on a regulator and have it stay stable in all positions. As I suspect you know, its the maximum difference in depth between the diaphragm and exhaust valve. In traditional (like barrel poppet) style regs, this is usually in horizontal, face down position. Lets assume for an example you have a reg in which the exhaust valve is 1.5" shallower than the diaphragm in that position. Since the exhaust valve limits the air pressure in the reg case, it will only hold shut when the pressure in the case is equal to or less than the ambient pressure at it's depth. If that's 1.5" less than where the diaphragm meets the lever, you have to set the cracking effort to 1.5" of water (or more) or else the pressure drop in the reg case will allow the diaphragm to collapse and the reg starts flowing. In real world applications, I've been able to set my balanced/adjustables or G250s to very low cracking pressures out of water, only to have them start to flow a bit when diving face down.

The idea behind the coaxial diaphragm/exhaust valve is that the difference in depth is never more than a fraction of an inch, so the regs are more stable at very low cracking effort. I definitely have found this to be true. The difference between the pilot/air 1 and D series is that on the pilot/air 1, the edge of the diaphragm is the exhaust valve, so I guess the maximum difference would be the radius of the diaphragm, while with the D series the exhaust valve is centered, so it's slightly closer to the center of the diaphragm.

The thing about the reg savvy book is that the concept is definitely explained, and you can see how it was part of the coaxial design of the pilot/air 1/D series. But there are other factors that I'm sure contribute to stability. These would be friction in the exhaust valve, friction in the lever arrangement, venturi assist aerodynamics, and probably the orifice/seat/spring consistency. I tend to use my pilot for most of my dives, and it's been very stable, but it does breathe wetter when upside down than do my D series regs. This is true with all the pilots and air 1s that I've tried. The only explanation I can think of is that the diaphragm must let a bit more water in when it does double-duty as an exhaust valve than does the smaller coaxial exhaust valve on the D series.

Anyhow, I bet you'll be able to test these ideas out for yourself in a very short time!
 
Thanks halocline for the great explanation. How you explained it aligns well with what I have read. I seem to be getting the ideas behind conventional and D series designs but I need to think about the air1 and the pilot.
I realize that the definition of the CFG is the distance from the center of the diaphragm and the exhaust edge but in these designs as I can see from the pics there is a pocket created by the casing and the purge cover which will trapping air and I suspect that in the normal horisontal position it would be the edge of the purge buton that should be considered the exhaust edge as it will be the highest point that the air will start escaping.
I cannot wait to get the real thing to be able to experiment.
 
The exhaust point would be the point on the edge of the diaphragm that's the shallowest at the time of exhalation, although the clip holds the bottom 1.5" (roughly) so I assume that segment of the diaphragm edge wouldn't allow exhaust even if it were at the top, basically diving upside down. That might be why they breathe a little wet in that position; air has to escape out the side of the diaphragm and that might let a little more water in.
 
I checked the pics again and the cover does have holes that let the air out. You are right - the edge of the diaphragm will be the edge of the exhaust
 
https://www.shearwater.com/products/teric/

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