How does a regulator adjust for ambient pressure (balanced diaphragm)?

[Litefoot]

I kind of understand the mechanics of how breathing creates pressure changes to open valves within the 1st and 2nd stages to allow air flow, but I'm a little fuzzy on the physics of how ambient water pressure on 1st and 2nd stage diaphragms "regulates" gas flow and breathing effort. Can someone help me understand?

 

[Zef]

The following video offers a good primer on the subject:


-Z

 

[admikar]

I kind of understand the mechanics of how breathing creates pressure changes to open valves within the 1st and 2nd stages to allow air flow, but I'm a little fuzzy on the physics of how ambient water pressure on 1st and 2nd stage diaphragms "regulates" gas flow and breathing effort. Can someone help me understand?

Short answer: 2nd stage doesn't compensate, it's always at ambient+X that gets delivered by 1st stage.
1 stage compensates by a lever pushing on high pressure valve to let more air into IP chamber, that then pushes back on diaphragm that is pushing the lever in the first place.

 

[RyanT]

Pick up a copy of "Regulator Savvy" by Pete Wolfinger. The figures and descriptions are fantastic and will explain this (among other things).

 

[rhwestfall]

and maybe take the @rsingler regulator class....

 

[JohnN]

So a semi-idle question that has been bothering me. . . If your first stage is balanced, what is the benefit of using a balanced second?

 

[Nemrod]

So a semi-idle question that has been bothering me. . . If your first stage is balanced, what is the benefit of using a balanced second?

A tightly balanced first stage does allow for a very fine tuning of the second stage, even with both balanced. But that is a good question and Mares (was) known for their unbalanced second stages and well balanced first stages. The same with AL and the once ubiquitous 1085 second stage.

Oddly, I have become smitten with simple things and am using an unbalanced Mark 2 Evo with a set of balanced G250s . The rig breaths just fine and it has been to 140 feet (TRL) and a couple more times over 100 feet. And I have purposely run it near to an empty tank just to see how it does and I do not notice much until around 500 psi and then the G250s begin to show a difference. With an unbalanced first stage, an adjustable second stage can be tuned to the lowest expected IP (near empty tank, say 700 psi) and then use the adjustment knob to prevent leakage at full tank and highest IP (really optional and not needed to get good performance). That is about a half turn on my G250s. IP drops over the 3000+ to 600 psi from about 145 to 128 psi. And while an unbalanced second can be set up to work with an unbalanced first, it is not ideal for deeper and more ambitious diving adventures. And, importantly, even with the improvements to the Mark 2 Evo with a 15% greater flow rate and 250A certification (support two people at a minimum of 100 feet in 4 degree C temperature water), that is still half the potential flow rate of the Mark 25. Even supposedly balanced first stage can have a variance of 10 psi from full to empty. Yes, a flow through piston balanced first stage can flow more air especially as density increases, mass flow. And even compared to a balanced diaphragm first stage which have a somewhat smaller orifice and that orifice is partially blocked by the needed push wire.

 

[Litefoot]

The following video offers a good primer on the subject:


-Z

That was a good explanation. So if I understand it, the increased pressure against the diaphagm, has the effect of increasing spring pressure on the seat between the HP and IP chambers. This results in greater pressure required to close the seat between the two chambers which maintains the 150 psi difference between ambient and IP pressure. So complicatedly simple!

 

[Eric Sedletzky]

A tightly balanced first stage does allow for a very fine tuning of the second stage, even with both balanced. But that is a good question and Mares (was) known for their unbalanced second stages and well balanced first stages. The same with AL and the once ubiquitous 1085 second stage.

Oddly, I have become smitten with simple things and am using an unbalanced Mark 2 Evo with a set of balanced G250s . The rig breaths just fine and it has been to 140 feet (TRL) and a couple more times over 100 feet. And I have purposely run it near to an empty tank just to see how it does and I do not notice much until around 500 psi and then the G250s begin to show a difference. With an unbalanced first stage, an adjustable second stage can be tuned to the lowest expected IP (near empty tank, say 700 psi) and then use the adjustment knob to prevent leakage at full tank and highest IP (really optional and not needed to get good performance). That is about a half turn on my G250s. IP drops over the 3000+ to 600 psi from about 145 to 128 psi. And while an unbalanced second can be set up to work with an unbalanced first, it is not ideal for deeper and more ambitious diving adventures. And, importantly, even with the improvements to the Mark 2 Evo with a 15% greater flow rate and 250A certification (support two people at a minimum of 100 feet in 4 degree C temperature water), that is still half the potential flow rate of the Mark 25. Even supposedly balanced first stage can have a variance of 10 psi from full to empty. Yes, a flow through piston balanced first stage can flow more air especially as density increases, mass flow. And even compared to a balanced diaphragm first stage which have a somewhat smaller orifice and that orifice is partially blocked by the needed push wire.

Not only is there a push pin inside the hole of a diaphragm, the diaphragm itself has only so much flexibility and movement to allow flow. They’ve done a lot with increasing the diameter of the diaphragms combined with making them thinner with better more space age materials, but still they are limited. Something within the natural engineer in me doesn’t like the way most diaphragm HP seats are constructed. They seem prone to cracking and failure.
With a piston the movement is unrestricted and there really is no limit to how much the orifice can open to allow flow except for the spring pressure, that’s really the only governing factor. The piston slides in a straight shaft and theoretically can open up tons with out stressing a cloth reinforced rubber diaphragm. Piston regs also have a captured HP seat that really would be hard pressed to crack and fail. That’s why I love them so much and firmly believe in the design, regardless of the minor ambient chamber salt water and grit exposure issues. With top brands like Scubapro, that has proven to be a non issue.