Tanks losing pressure

[Dnaber]

After my last dive in December I had both my tanks filled. One is a HP117 (Nitrox 32) and the other is a HP80 (regular air). Due to an injury I was unable to dive for the last 6 weeks. Well yesterday at the springs I was setting up my gear and I noticed that my HP80 had only at 2500psi and I know it was serviced to 3000psi and then I hooked up my HP117 and it was at 3000psi. and I pretty sure it was serviced to 3450psi. Will tanks lose pressure after such a short time? Plus these tanks are stored in my house. Thanks in advance.

 

[Steve_C]

If they were filled to rated pressure quickly without any cooling then they will drop some as they cool. Also if you take the tank out on a cold day it will drop some pressure as they cool. Its easy to drop a couple hundred with either effect.

 

[redacted]

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

 

[tbone1004]

Steve, not quite easy to drop a few hundred. Tanks filled to 3500psi at 100F general temp of a hot fill, will only drop to 3500 at 85F, 3400 at 65F, 3300 at 50F, hit 3200 at 35F. This is why in Cave Country, tanks are generally filled to just over 3700 and generally get around 90F, so they cool to 3600 in the water at 70. For the 3000psi tank, it would have to go from about 110F to 10F to drop to 2500psi, 110F to about 30F for 3500 to 3000. The conversion to Rankine kills the math here.

Don, not sure when your valves were last serviced, but either they are due, or you didn't have the valve closed. Also an ever so slight possibility that the tank neck o-ring is leaking. Not sure who your dive shop is, but it does sound like it's time for service on something. Also is good to get in the habit of marking the fill pressure and O2% on all bottles when they get filled, I also tend to take an IR gun with me because they're cheap and fun to have around, but just feel the bottles, if they feel warm that's ok, if they are hot, they're going to need a topoff.

 

[Bob DBF]

Read the above as it is good info, but before rebuilding your gear, make sure you know what the tanks were filled to, at home after the tanks have cooled, and then check for a loss with the same gauge over time. I found out a shop I went to had a gauge that was reading low, so my loss was from the fill cooling and a misreading gauge. The shop replaced their gauge, although it could have been my gauge that was off but, this time, it wasn't.



Bob

 

[claymore]

It seems strange that two tanks would have the same issue at the same time. I would make sure that this isn't a case of short fills and not equipment issues.

 

[Steve_C]

It seems strange that two tanks would have the same issue at the same time. I would make sure that this isn't a case of short fills and not equipment issues.

I agree. He is seeing this with both tanks. We had a local dive shop that knew its guage for testing (not for filling) was off by 300 and shows us fills were ok.

I also agree that 500 is a lot to loose just from cooling.

I have had tanks given to me that were well over 100F. Combination of a hot fill and a bad guage at the shop can do it.

I have picked up tanks more than once that were 200-300 off and their gauges worked. Just a very hot fill not topped off. Not same as the previous shop.

 

[tbone1004]

i've had the same. With as cheap as tank checkers have gotten it doesn't make sense not to have one.
https://www.divegearexpress.com/gas/cylacc.shtml#775
$40 for that is impossible to beat and if for some reason the gauge goes bad, it uses standard 1/4" NPT gauges which can be had for about $20 on amazon

 

[RickyF]

Gauges can easily be out of calibration and I have yet to see a shop or a diver that spends the money to keep them calibrated. At my work, we are required to have all gauges calibrated yearly or any time they have beeen dropped or handled roughly.

 

[tbone1004]

agreed, but the SPG's can't really be calibrated, they're either in spec or they've been mishandled and need to be replaced, and most of the time the presure gauges at the shops are stationary on the fill stations, so pretty hard to mishandle those and there are usually multiples, one on the fourth stage from the bank and one on the fill whip, so if one is out whack you can usually tell pretty quickly. That and if a shops gauges are off, they'll have lots of angry customers getting short fills. Why I'm a huge fan of having a tank checker, SPG's are expensive, decent quality NPT gauges aren't. $20 to replace them every few years as needed is pretty cheap in the grand scheme of things