Scubapro chrome plated brass regulators...

[Couv]

the flow from the second stage valve overcame the ability of the micro pump to suck, and the negative pressure at the mouthpiece dropped back to zero. When that happened, the valve closed in the second stage, the valve closed in the first stage, and the IP returned to baseline. With the micropump continuing to run, the process began all over again: magnehelic rises towards cracking effort; IP begins to drop; cracking effort is reached and the micropump keeps up with the initial flow and continues to suck more negative, until the valve opens fully and the micropump is overcome. And it starts again...

Yep. That is what's happening. As always @rsingler puts it into words we can all digest.

 

[rsingler]

I've gotta say, what @couv built, and @Overweighted built, and I finally built from this wonderful thread from last year by @njmorrell that (RIP) Cameron Donaldson got enthusiastic about, is the best damned way to measure cracking effort that I've EVER seen! The precision you can get when tuning a second stage is unbelievable!

Regulator test rig you don't need to suck.

Here's my little toy modelled after @njmorrell :

It was all of $12 in parts, an old tiny power supply and switch from the junk box, some garden parts and some tubing. I don't even have to look at the IP gauge for a drop any more. I keep the bleed pretty wide, and turn up the pump until it stabilizes. It consistently sucks only 0.1" harder with the bleed open than the reg cracks, and I can tune regs now exactly where I want them.

My air bleed is a plastic needle valve garden irrigation bubbler, and an irrigation t-piece.

 

[Open Ocean Diver]

Okay. I've watched this video
https://share.icloud.com/photos/0F5bO9lhXbQxWvNtN_BsopviA#Downtown_Toronto
a dozen times, now. Here is what I think is going on...

The micro pump is sucking at a very small rate from the mouthpiece of the second stage (remember that much of the pump performance is deliberately wasted by the bleed from the flowmeter, to keep changes small and slow).
As the pump sucks, the magnehelic drops from zero toward the cracking effort negative pressure (even though it's registering as a positive displacement of the Mag needle).
When the negative pressure reaches cracking effort, the second stage valve begins to open microscopically. But what we noticed here was an increasing number on the magnehelic despite a dropping IP. Whaaat?

I think that is occurring because, at this very early point in valve opening, the micro pump was able to keep up with the output from the second stage. During this brief interval then, the IP continued to fall as flow increased from the second stage valve. And the magnehelic continued to rise because the micro pump could keep up with the tiny second stage flow to develop more negative pressure, opening the valve further.

At some point, however, the flow from the second stage valve overcame the ability of the micro pump to suck, and the negative pressure at the mouthpiece dropped back to zero. When that happened, the valve closed in the second stage, the valve closed in the first stage, and the IP returned to baseline. With the micropump continuing to run, the process began all over again: magnehelic rises towards cracking effort; IP begins to drop; cracking effort is reached and the micropump keeps up with the initial flow and continues to suck more negative, until the valve opens fully and the micropump is overcome. And it starts again...

But it was a magical combination of rate of suck and rate of valve opening that allowed this to appear. "Could not duplicate" has been uttered by more than one garage manager when presented with a bizarre auto problem. Same thing here, I'm guessing.

 

[Open Ocean Diver]

Okay. I've watched this video
https://share.icloud.com/photos/0F5bO9lhXbQxWvNtN_BsopviA#Downtown_Toronto
a dozen times, now. Here is what I think is going on...

The micro pump is sucking at a very small rate from the mouthpiece of the second stage (remember that much of the pump performance is deliberately wasted by the bleed from the flowmeter, to keep changes small and slow).
As the pump sucks, the magnehelic drops from zero toward the cracking effort negative pressure (even though it's registering as a positive displacement of the Mag needle).
When the negative pressure reaches cracking effort, the second stage valve begins to open microscopically. But what we noticed here was an increasing number on the magnehelic despite a dropping IP. Whaaat?

I think that is occurring because, at this very early point in valve opening, the micro pump was able to keep up with the output from the second stage. During this brief interval then, the IP continued to fall as flow increased from the second stage valve. And the magnehelic continued to rise because the micro pump could keep up with the tiny second stage flow to develop more negative pressure, opening the valve further.

At some point, however, the flow from the second stage valve overcame the ability of the micro pump to suck, and the negative pressure at the mouthpiece dropped back to zero. When that happened, the valve closed in the second stage, the valve closed in the first stage, and the IP returned to baseline. With the micropump continuing to run, the process began all over again: magnehelic rises towards cracking effort; IP begins to drop; cracking effort is reached and the micropump keeps up with the initial flow and continues to suck more negative, until the valve opens fully and the micropump is overcome. And it starts again...

But it was a magical combination of rate of suck and rate of valve opening that allowed this to appear. "Could not duplicate" has been uttered by more than one garage manager when presented with a bizarre auto problem. Same thing here, I'm guessing.

Great explanation makes total sense, I wish I would of open the valve a bit more to see if the Oscillation would cancel it self. My mission will be to find this teeter totter point again. thanks

 

[Open Ocean Diver]

I've gotta say, what @couv built, and @Overweighted built, and I finally built from this wonderful thread from last year by @njmorrell that (RIP) Cameron Donaldson got enthusiastic about, is the best damned way to measure cracking effort that I've EVER seen! The precision you can get when tuning a second stage is unbelievable!

Regulator test rig you don't need to suck.

Here's my little toy modelled after @njmorrell :
View attachment 581916
It was all of $12 in parts, an old tiny power supply and switch from the junk box, some garden parts and some tubing. I don't even have to look at the IP gauge for a drop any more. I keep the bleed pretty wide, and turn up the pump until it stabilizes. It consistently sucks only 0.1" harder with the bleed open than the reg cracks, and I can tune regs now exactly where I want them.

My air bleed is a plastic needle valve garden irrigation bubbler, and an irrigation t-piece.

On my flow metre above 2 1/2 LPM I can hear the air flowing in the second stage when it opens. So sweet.

 

[Open Ocean Diver]

At what tank pressure do you adjust and check your regulator cracking effort?

What I find with the balanced regulators, the tank pressure makes a huge difference on cracking effort. Once the IP recovers after the demand valve opens IP will vary based on tank pressure hence cracking effort is inversely proportional to tank pressure. What’s happing with the idea of a balanced first and second stages? Or is a variance of .3 on the mag gauge not a big deal?

 

[Open Ocean Diver]

Same flow two different tank pressures... pictures below.

 

[Open Ocean Diver]

Tested with SP 108HP, the difference is only (.05) same flow on the two different tank pressures with an unbalanced SP 108HP and same Atomic 1st stage. I know at depth the balance chamber makes a difference but how do we test it here on the surface?

 

[Couv]

At what tank pressure do you adjust and check your regulator cracking effort?

Never mind the tank pressure. Adjust the cracking effort of the second stage at the highest IP of the first stage.

 

[Couv]

I've gotta say, what @couv built, and @Overweighted built, and I finally built from this wonderful thread from last year by @njmorrell that (RIP) Cameron Donaldson got enthusiastic about, is the best damned way to measure cracking effort that I've EVER seen! The precision you can get when tuning a second stage is unbelievable!

Totally agee. I really appreciate the new set up where I can superfine tune the cracking effort. Actually, that is not accurate-I'm finding the exact point where the second stage cracks and can monitor the dynamic cracking pressure. (The force required to maintain flow after the valve is open.)

The next item to plumb into the system will be a vacuum accumulator-probably a wine bottle with a t-fitting in the stopper. I think the accumulator will assist the pump under varying loads.