tank filling theory...e.g. getting the pressure right

[scubasean]

Hi folks:

what I know: Filling a tank heats the tank. Because the tank will later cool, the pressure will drop, due to PV ~ T , so you must fill the tank to a pressure that "looks" high, which will then be right once the tank has cooled.

what I don't know: What is the ratio between temp and pressure...For example, does the pressure drop 5psi per degree F? Also, how much will the temp of an empty tank (say....less than 200PSI in it when the fill begins) increase during filling?

So, what I'm after is really....My 80 is rated to 3300PSI, and I want to know how much it needs to be filled to in order to arrive at 3300PSI later, when it is at equilibrium, say at 70F or so...Seems like you'd need to go up 200PSI or so to get it right...

Finally, is it reasonable to expect filling stations to accommodate?

Thanks,

Sean

 

[jonnythan]

scubasean once bubbled...
Hi folks:

what I know: Filling a tank heats the tank. Because the tank will later cool, the pressure will drop, due to PV ~ T , so you must fill the tank to a pressure that "looks" high, which will then be right once the tank has cooled.

what I don't know: What is the ratio between temp and pressure...For example, does the pressure drop 5psi per degree F? Also, how much will the temp of an empty tank (say....less than 200PSI in it when the fill begins) increase during filling?

So, what I'm after is really....My 80 is rated to 3300PSI, and I want to know how much it needs to be filled to in order to arrive at 3300PSI later, when it is at equilibrium, say at 70F or so...Seems like you'd need to go up 200PSI or so to get it right...

Finally, is it reasonable to expect filling stations to accommodate?

Thanks,

Sean

It's a direct relationship using absolute temperature (ie Kelvin).

However, it's hard to know exactly how much the air in the tank will heat up.

I imagine it's something gained from mostly experience... fill to 3200, let it cool to 2800, top it off to 3000 or 3100...

But then, I've never filled a tank. I'm sure someone who knows what they're talking about will be along shortly

 

[Uncle Pug]

there was a product called honest phil... inventor was on SB at one time... a color changing sticker that indicated how much a tank would end up shrinking after cooling down.

here is the link to their URL.

Now I know this doesn't answer your question... but the answer to your question was on their website.

 

[Uncle Pug]

jonnythan once bubbled...
*

....after a while you just get a feel for how it will turn out... or at least close enough.

 

[LUBOLD8431]

Technically, you are not supposed to fill a cylinder above its rated service pressure.

So, if your cylinder has a service pressure of 3300 psi, if i was filling your cylinder, I would fill it at a rate of 600 psi per minute up to the rated service pressure of 3300, then let it cool off, and then top it off again. Now, if you didnt mind waiting, I would let it cool off again, then top it off one more time to the rated service pressure.

There is also the debate of using a water tub to cool the cylinders... With steel tanks, its almost a necessity. We have them and use them. The whip and the valve never go below the surface of the water. ALuminum tanks usually dont need it, but sometimes get it anyways...

 

[drbill]

I have been fillling tanks recently and have been told to overfill the tank by 200 psi. Although LUBOLD8431 is correct re: fill rate being important and ideally topping it off after it cools, whenb filling tanks in high numbers like we do it is more practical to slightly overfill by the 200 psi and let them cool down afterwards.

I dive with steel HP (120 or 100) and this generally results in a correct fill pressure of 3500 psi when I'm ready to dive.

Dr. Bill

 

[Uncle Pug]

LUBOLD8431 once bubbled...
I would fill it at a rate of 600 psi per minute

I'll go slower if I can. I usually aim for 300 psi/min but sometimes I get excited.

 

[Cave Diver]

Uncle Pug once bubbled...
I'll go slower if I can. I usually aim for 300 psi/min but sometimes I get excited.

Just sometimes?

 

[sexyscubadiver]

5 psi for every 1 degree F.

 

[Dyno Bill]

Well I hope I get this correct
http://www.flash.net/~table/gasses/laws.htm
This is a site I found that has some formulas that might remove the guess work from the pressure dilemma.