IP difference with lower tank pressure

[porbeagle]

I have just started to service my own regs and I have a question for the usual suspects.
I am under the impression that 1st stage IP is checked initially with a full tank (3000psi).
I have lots of old 72s but no al80s.Will the roughly 700psi difference affect how I should
set IP? If factory spec. should be,let's say,between 125psi and 145psi must I compensate
When I set IP because I use a lower pressure tank ? How does this work?
Thanks in advance
PORBEAGLE

 

[redacted]

I don't think you will have to worry about it with your balanced diaphragm 1sts. It will make a noticeable difference with an unbalanced piston 1st and a little difference with many balanced piston 1sts. But the balancing on a BD should be quite precise so 3000 vs 2300 (or even 3442) should not matter. So just check at 2300 and at some low pressure to made sure nothing odd is going on.

 

[herman]

Depends on the reg...... don't you just love a straight answer.
With balanced first stages, it doesn't matter that much....the whole purpose of balancing is to keep IP constant across the usable range of tank pressure (and thereby keep the cracking pressure of the second stage constant), as long are you are within reason, the IP should not change with tank pressure more than a couple of PSI, not enough to worry about.
Unbalanced regs are a different matter, with piston regs the IP drops as tank pressure drops so you want to use the max tank pressure you normally use (2250 if you normally use steel 72s or 3000 if you use mostly AL-80s) although the difference is not going to be much. Unbalanced diaphragm regs (DA Aquamaster and some of the very early USD, Dacor and other early single hose regs) are just the opposite, their IP increases as tank pressure decreases so you actually want to set them up at the minimum tank pressure, usually 300 PSI is used. Truth is, unless you set your second stages as light as you can a few PSI of IP change is not going to be noticeable so being dead on the "ideal" tank pressure is not really a big deal. If your second stage is balanced, it's even less of an issue since the purpose of balancing a second stage is to compensate for a varying IP which again keeps cracking pressure constant. Unbalanced but adjustable regs like the SP-109 allow you to compensate for a little IP shift/cracking pressure on the fly so again being a little off does not really cause a real problem. Not the text book answer I know but a more realistic view IMO.

 

[DA Aquamaster]

Way back in the day the shop I used for annual service work would tune my Mk 3 High Performance (an unbalanced first, unbalanced second stage reg) for peak performance on a steel 72. That worked great when I dove my steel 72s but when I'd borrow a dive club AL 80, I'd get a slight free flow as the 750 psi difference in service pressure made a difference as they were that finely tuned.

That's really the worst case scenario. As Herman indicates with a balanced first stage the IP is essentially stable (although you can see 4-6 psi shifts over a 300-3300 psi range on some "balanced" first stages as they are not perfectly balanced) so the net effect on the downstream force acting on the second stage is minimal as tank pressure changes. And, with a balanced second stage, there is only a slight downstream bias to the system so changes in supply pressure to the second stage make comparatively little difference. So if you have either of the above, tank pressure won't have much effect on the tuning of the second stage.

As an aside unbalanced piston first stages will show a decrease in IP as tank pressure falls, while unbalanced diaphragm first stages will show an increase in IP as tank pressure falls. With that said, you have to find a really old first stage to find an unbalanced diaphragm first stage as diaphragm first stages are incredibly easy to balance and no one makes unbalanced diaphragm first stages anymore.

 

[porbeagle]

Thanks much for the responses,Awap,Herman,DA. I think I have it.
I must say,though,it's not easy.

 

[bractime]

If you want a good read and better understanding of regulator design and service go to www.scubatools.com and get "Regulator Savvy" it is worth the $32. This is also a good place to get tools.