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it may drop 500 psi over the course of 10 minutes. Could this be due to water against the regulator. I turn it off so the 500 Psi is wht is in the hose only.
You have a small leak either on the 1st stage or second stage or gauge. Simply put it under the water turned on and see where the air leaks from.
If from the second stage mouthpiece it is either the 2nd stage seat or perhaps the first stage IP creep (although being turned off its probably not that).
Maybe a hose O ring leaking. You should see it leaking. Have you tried charging the reg set on the surface and seeing if it leaks away slowly when turned off. If not then it might be that water pressure on the second stage diaphragm is making the second stage open.
Maybe during a safety stop, charge the set and then shut off and watch it? If it doesn't leak down at all then, I would suspect the second stage is set light and perhaps swimming along is sufficient for it to open and dump air.
Try the air leakage test on the surface, and then also at depth and watch for leaks at depth. If you are at say 15-20 m and charge it fully for a few min, then shut it off and see if it then drops while at depth, if so you must have a leak somewhere. I suspect a small leak on perhaps the second stage O ring where it attaches to the hose. See if small bubbles are leaking away (or anywhere else on the reg set)
Does it lose 500psi total, or does it lose 500psi every 10 minutes? I kind of doubt you have a leak, because 500psi/10minutes is a substantial leak. You will see bubble in water for sure. If you lose 500psi a one time deal, most likely you have a hot fill, then once the tank cool down by water, you lose some pressure. It is not unusual for cold water dive.
a temp drop should not be able to cause that much psi loss. I say this because of 5psi per degree. it would have to be a 100 degree temp drop. If the given is ,,, you have 3000 at room temperature.
How about (I am guessing)...
At the surface having opened the valve and closed it there is 200 bar in the first stage and 10 bar in the lp hose to the second stage. At 10m the first stage will increase the lp hose pressure to 10 over ambient so 11 bar. It does this by letting some 200 bar gas past. Thus the 200 must drop a bit. How much will depend on the ratio of volumes in the HP part of the first stage and all the low pressure hoses. At some point when ascending the second stage might see a high enough pressure to let some gas past, as the ambient pressure drops and the lp host pressure remains high.
I was taught to open and close the valve on an otherwise closed deco cylinder now and again on a dive to avoid water getting back past the second stage.
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