Short Fills In HP Tanks

[Leadking]

Just remember that it is roughly 7psi per degree. If the shop filled them put 3400 at 115degrees and they are then stored in 70degree air, or sit on a concrete floor that is even cooler, you're talking 40degrees or 280psi. 115 doesn't feel that warm. That is only 20 degrees warmer than us.

I would bet you the tech shop is anticipating the drop and overfilling. Burst disks are rated at 140% of rated pressure. They may fill up to 3600-3700psi anticipating the cool down and that is well within the rated pressure of the burst disk.

Burst discs are rated at hydo pressure (+0/-10%)

 

[H2O 70]

It sounded low to me so I was opening up a discussion not questioning anything else. Sounds like you have the facts. Thanks![/QUOTE

Wasn't sure if you were thinking this was low or high.

 

[H2O 70]

Burst discs are rated at hydo pressure (+0/-10%)

Hydo is what, 5/3 rated pressure or approx 150%?

140% is within your 10% margin of error.

 

[Ghanguss]

My LDS has the gear to fill HP tanks and still I always end up with 2800-2900 PSI. I can count the times that I have seen a fill over 3300 on one hand.

+1.

My HP tank rarely gets filled properly. I think most dive shops get in a routine of filling 3300 psi and need to be reminded for HP tanks.

 

[LAP57]

Thanks for all the input. Still leaning towards some new HP tanks. Most of my diving will be done with tanks filled at the LDS and if you keep an eye on them you should get a good fill.

 

[dannobee]

3900 psi at a reasonable fill rate will cool down to 3500.

I too, dive off of SoCal dive boats, and by the end of the day you're lucky to see anything over 2700 psi once the tank goes in the water.

I've filled tanks in less than 15 seconds and between the cooling through the valve and the thick tank walls, the tanks don't even get hot (not enough time for the heat to transfer to the tank wall). But in those cases, 3900 cools down to about 3400. Of course you should never fill tanks that fast.

You'll never see any dive shop fill tanks that fast, but I've been told that it's done almost that fast when fighting fires (30 second fills to 4500 psi).

 

[spectrum]

Is this really an issue? If so how often have you encountered this problem. Is this a matter of getting 3400 instead of 3500 or is it a matter of getting 3000 instead of 2500?

Dive shops that can't go to 3500 do exist. None in my area but they are out there. If it's an outside/rare chance it's not a big deal. Check the shops you will frequent. Likewise on boats. It's nice to avoid steel fills on boats anyhow as they can be prone to moist air.

As for regular shop fills it's hard to escape physics.

A conscientious fill person that lets your cylinders rest on the whip for :15 or puts them back for a top-off after a slow fill will get you settled to 70F at home with about 3300+/-50. Ideally I drop them off and they do a perfect top off at pick-up. Sometimes shops will take them over the rated pressure when filling such that they hit 70F at the correct pressure which is the real rating. This is no different than what your tank experiences when perfectly filled at 70F and then left in the car on a summer day. It can take about 4 hours for that ball of gas to equilibrate to ambient temperature so you need to wait before objectively testing.

I see plenty of short filled AL80s so dive a premium steel cylinder and forget about it.

Pete

 

[spectrum]

I've filled tanks in less than 15 seconds .......You'll never see any dive shop fill tanks that fast

Wanna bet?

 

[scubaskipper]

Drop off day before you need them and have them topped off at pick up. This way it doesn't matter which tank monkey filled the things, you leave with a full tank.

 

[DA Aquamaster]

3AA steel and aluminum tanks have a test pressure that is 5/3rds the service pressure. The test pressure for most steel special permit tanks is 3/2 the service pressure.

So, for a 3442 psi special permit tank the test pressure is 5163 psi. For a 3000 psi aluminum tan it would be 5000 psi and for a 2400 psi steel tank it would be 4000 psi.

Burst discs are supposed to fail at between 90% and 100% of the test pressure.

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The fastest tank fill I ever saw was a set of of doubles I had filled at lake Rawlings. I heard about 15 seconds worth of air moving around in various lines while two of the employees were screwing with the valves in their air bank. I figured they were just doing a really bad job of managing the cascade system prior to starting the fill. As it turned out they were in fact also filling my tanks - something I did not realize until they unhooked them. I still doubted they had been filled until I felt them and realized they were hot. It's the first and last fill I ever got there.