Tanks losing pressure

[Dnaber]

Thanks guys for the input and suggestions. My HP117 is brand new (second dive) born on date of 03/14. My HP80 is less then 5 yo and is coming up for it's hydro 05/10 and at that time I'll have them go through the valve as they should. In the mean time I'm going back to the LDS that did the last fill and going to have them top off the the 80 and refill the 117. But this time I'm going to leave the tanks so they should be cool when I collect them. I'll recheck the pressure at that time.

Again I also thought it strange that the pressure would drop so much after just sitting just 6 weeks. I'll report back on my findings.

 

[nimoh]

Thanks guys for the input and suggestions. My HP117 is brand new (second dive) born on date of 03/14. My HP80 is less then 5 yo and is coming up for it's hydro 05/10 and at that time I'll have them go through the valve as they should. In the mean time I'm going back to the LDS that did the last fill and going to have them top off the the 80 and refill the 117. But this time I'm going to leave the tanks so they should be cool when I collect them. I'll recheck the pressure at that time.

Again I also thought it strange that the pressure would drop so much after just sitting just 6 weeks. I'll report back on my findings.

Just for clarity, valves are not normally checked when a hydro is done, and for that matter, valves are not normally inspected but rather taken apart, cleaned and rebuilt. You will likely have to request this service, but it wouldn't be unheard of for a shop to include this service when a hydro is done.

 

[tbone1004]

shops should be doing it as part of VIP realistically since it is recommended to be done every year, same as the regulators, but at minimum the valves should be rebuilt every go at hydro, but the hydro shops typically don't touch valves, only the tanks.

 

[redacted]

shops should be doing it as part of VIP realistically since it is recommended to be done every year, same as the regulators, but at minimum the valves should be rebuilt every go at hydro, but the hydro shops typically don't touch valves, only the tanks.

I have a half dozen tank valves that have been in use for about 20 years. None have been rebuilt. I did have to replace a stem o-ring on one.

If it ain't broke, don't fix it.

 

[tbone1004]

I agree, and I don't rebuild them unless they ask for it, but the recommendation is to do the when the regs get serviced.... I do replace the main O-ring every time the valve comes off though, that's cheap insurance

 

[geoff w]

No matter how slowly I fill my HP80 tanks, when they cool down they will drop many hundred PSI. They're a small volume and high pressure, so they get really hot inside when you're filling. If I fill slowly to 3700-3800, they'll cool down to about 3400. I've gotten hot fills at 4000 that cool down to 3300. Larger HP tanks seem a little less susceptible to cooling after filling, but they're called high pressure for a reason, and you know what happens to a gas under high pressure, right? The best thing to do is a day or two after you get a HP80 filled, get it topped up and you will be amazed how much PSI you lose just due to cooling.

To confirm you don't have a leak - fill your HP80 to what you think is the ending pressure, nominally 3442. Come back a day later and be amazed at how much air you "lost". Don't top it up. Come back several days later and see if you've lost any more pressure (probably not). Now top it up to full. It will still cool off some and you'll lose some pressure, but not as much as filling up an emptier tank

 

[jwelburn]

If you have two tanks that all of a sudden seem to have dropped by almost exactly the same amount of pressure, I would bet that the issue is with the SPG, and not the pressure in the tanks. I would do a pressure check on those two tanks with a couple other regs/SPGs to see if they all agree. The pressure loss due to cooling can certainly be drastic sometimes, but I do think 500psi is a bit much, unless they just slammed the air into them in a minute or so.

 

[duckbill]

Where there is a leak, there are bubbles. Do a leak check.

The most obvious first step. All discussion past this point is tail chasing until this has been done.

 

[spectrum]

The 2 occurrences and magnitude pretty much point to the 2 culprits mentioned, probably acting together.

1) If the recalled fill pressure as on cylinders that had not cooled to ambient (~4 hours) if the location was even that temperature then checking them when colder will drop about 100 PSI for every 10F.

2) We are talking about 2 uncalibrated pressure gauges and each could be off by a few hundred PSI and in opposite directions. This can be somewhat mitigated by using your own gauge when picking up fills at the shop. Then, accurate or not you have continuity when you go home or to the dive site. You can easily compare all of your own instruments to know of any offsets.

Between the two you can easily see an apparent pressure drop of 500 PSI.

Pete

 

[redacted]

The 2 occurrences and magnitude pretty much point to the 2 culprits mentioned, probably acting together.

1) If the recalled fill pressure as on cylinders that had not cooled to ambient (~4 hours) if the location was even that temperature then checking them when colder will drop about 100 PSI for every 10F.

2) We are talking about 2 uncalibrated pressure gauges and each could be off by a few hundred PSI and in opposite directions. This can be somewhat mitigated by using your own gauge when picking up fills at the shop. Then, accurate or not you have continuity when you go home or to the dive site. You can easily compare all of your own instruments to know of any offsets.

Between the two you can easily see an apparent pressure drop of 500 PSI.

Pete

I believe the pressure change when starting with 3500psi is 7 psi per degree F or 12 psi per degree C, so cooling probably accounts for about a 300 psi drop with HP steel tanks.