IP adjustment question

[JB]

Is there a way to stop an excessive IP fall (25psi) on purging, or is it inherent in the design of the reg. I have 2 balanced diaphragm regs (different models from the same manufacturer) on 1 there is hardly any drop <5 psi on full purge, while on the other there is this excessive drop, which does make taking a slow breath harder, a big suck of air is OK on both. The bad one has been in for a service and still manifests this huge IP drop. The only adjustment I can think of that could alter this is to back off the tension on the spring closing the HP seat, but have not seen this referred to in any service manual, for any make of reg.
I hope one of the regulator guru's has some advice on this.

 

[Zung]

Maybe the air passage on the HP side is clogged. Have you replaced the air filter? Use a schematic to trace the path the air has to go through, it can be pretty convoluted in a diaphragm reg; check if there's any crud.

 

[JB]

zung, as I said it has been service in the last week, by reputable shop. No crud anywhere, was the same pre and post service, and in fact when brand new.

 

[victor]

I am not a regulator tech so take everything I say with a pinch of salt.
Just thinking logically.
If there was no resistance in the second stage or between the guage and the second stage then a purge would result in the IP preasure dropping to 0.
Thus the difference between the 2 second stages is the ability of the regulator to shift air when it is purged, the more air the lower the IP preasure.
The difference could be the internal bore, any tight corners, constrictions, etc.
Have you tried this test with another reg of the same type and third type of reg.

Again these are just my thoughts.

 

[LeadTurn_SD]

Is there a way to stop an excessive IP fall (25psi) on purging, or is it inherent in the design of the reg. I have 2 balanced diaphragm regs (different models from the same manufacturer) on 1 there is hardly any drop <5 psi on full purge, while on the other there is this excessive drop, which does make taking a slow breath harder, a big suck of air is OK on both. The bad one has been in for a service and still manifests this huge IP drop. The only adjustment I can think of that could alter this is to back off the tension on the spring closing the HP seat, but have not seen this referred to in any service manual, for any make of reg.
I hope one of the regulator guru's has some advice on this.

Can you tell us the brand and model of the regulators?

The most meaningful test of IP drop is on inhalation, not so much when purging. A regulator with a powerful purge may show a higher drop than one with a less powerful purge, but the drop on inhalation should be similar between two decent first stages. A 25 psi drop on inhalation would be a "doggy" reg, but might be ok when purging the reg.... You also want to look at recovery speed of the IP after each inhalation (and purge); it should be very quick.

As I think about it, < 5 psi on full purge actually sounds LOW to me.... I'd worry that the 2nd stage lever height was possibly to low, and the demand valve was not opening fully? I'd expect more drop with a decent purge... I guess it depends on the reg (2nd stage design), some regs purge more powerfully then others.

5-10 psi or so IP drop sounds about right for the drop with inhalation on a good reg, followed by a rapid recovery and stable "lockup".

The more info you can give us, the more likely one of the gurus can help.

Best wishes.

Edit: Do both regs in question have the same model 2nd stage? You do need to track down and correct any problems with IP drop that may exist, but I've found that the 2nd stage generally has the greatest effect on ease of breathing (assuming a correctly-performing 1st stage)

 

[JB]

Thanks for the info Leadturn. The one that has the low pressure drop is a Beuchat VX10, the one with the huge drop is the Beuchat VR200. Recovery speed is brisk and locked onto 130 psi, no creep. On initiating a breath the needle flicks down about about 18 psi and stabilises at around -12-15psi for the rest of the breath. I'm just trying to understand this. I feel much more comfortable with the VX10 underwater, just easier to breath (same 2nd stages), and I have been trying to understand why. Honestly no matter how hard I suck air, or purge the VX10 the IP never deviated more than 4 - 5 psi from set pressure, really is a joy to breath from. The internals of both are pretty much the same, except for the rotating turret, but in terms if seat balance chamber and crown, there's not a huge difference. I never thought a 1st stage made much of a difference as they can all supply huge flow rates, I guess it's the force required to open the valve ie. the relative negative pressure to initiate flow in the 1st stage that counts, but then again I'm an amateur and may be way off base. I look forward to explanations/advice.

 

[LeadTurn_SD]

I'm not familiar with the Beuchat regs... but a quick question: Is the larger IP drop with the VR200 a new problem, or has it always been like this?

The 18 psi drop might be about par for a low-end, entry level reg, but still seems like a lot of drop... The 5 psi drop sounds like a high-performance reg. But I'd be curious about the difference in 2nd stages between the two regs. If you are noticing a large difference in how the regs "feel" when you breath them, that might be the reason.

If the 2nd stage on the VX10 is tuned a bit better, has a lower cracking effort, has a better venturi effect due to better design, etc., it might breath better..... an experiment to test this would be to switch the 2nd stages between the two regs, and see if this makes a difference...

Sorry I'm not familiar with your particular regs, so I can only offer general thoughts about what you are observing.

Best wishes.

 

[JB]

Thanks LeadTurn I change the second stages over so I've kept that as a constant only the first is on trial. My VX10 is around 15 years old and has served me well, as you say I think it is truly a top end reg, but recently I thought I'd get the latest top of the range reg by the same manufacturer. When it breathed like a dog I initially put it down to the miniature 2nd stage that it came with, and switched to a normal size 2nd stage, still it was not as good as the old VX10, that's when I started measuring pressures to try and understand why a top of the range reg performed badly. In most service manuals they say check the filter, the hose etc for a restriction to airflow, but I figure there has to be another less obvious solution like changing spring tensions. The art of reg tuning?? It's not mentioned in Vance Harlows book either.

 

[vladimir]

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[redacted]

I do not believe there is any direct adjustment. But you could maximize the venturi effect at the 2nd stage which would tend to cancel any loss of performance due to IP drop.