Volume of air in tank at a given pressure/temperature

[jwalko]

Hey all:

I was wondering if there is a formula to use to figure out the volume of gas in your tank at a given pressure. And, is the math linear (in the sense that, is the volume always directly related to pressure assuming the same temperature).

The reason I am asking is because I have a set of steel LP 85's. They are + rated, so they "can" be filled to 2640. At 2640, they are supposed to be 85 CF (or are they 85 CF at 2400 lbs, which is their standard rating?).

Does this mean that at 2000 lbs, they are roughly filled to 64 CF (85/2640*2000)? I ask because one dive shop I regularly go to almost always underfills my tanks...usually in the 2000-2100 lb range. The last time I was there, I asked for the fill to be at 2640 (or at least close). I got two answers...one that the lower pressure is easier on the burst disk, meaning longer life (???) and two that their gauge is off and shows higher pressure than is actually in the tanks. In other words, the "underfill" isn't intentional.

I am tired of getting partial fills, but that is besides the point. It got me thinking about how much gas is really in my tanks, and about my air consumption compared to some of my diving partners. Of course, I seem to be running low on air sooner, but this would make sense if they are diving 80's or 100's that are full compared to my diving what is essentially a 65 CF tank (65 cf fill).

Is my figuring above correct? Is there some standard formula to figure out the volume of gas in a tank at a given pressure and temperature?

Thanks;

John

 

[icestac]

Your math looks correct. Basically it is the ratio of the full rated pressure to your current pressure (again assuming a constant temperature).

The LDS is full of it. Get the fill that you want and even more should that be the case. Those tanks will be fine well over 3000psi. You're burst disc should be replaced periodically anyways and it is a couple of bucks. Not rocket science.

Hope it helps.

~Jeff

 

[khacken]

See
Boyle's Law and Combined_gas_law for more info.

Basically P1V1 = P2V2. So, given a constant temperature ( 85/2640*2000 ) would be on the money. To check the pressure, just bring your own pressure gauge and check it yourself. Maybe they are right. Also, remember that after a tank is filled, it is hotter, so the pressure would be higher. Once it is cooled down, the pressure will go down. P1V1/T1 = P2V2/T2 (The temps are at absolute temps, so be sure to convert). You can see a decrease of 200 lbs easy from fill time until you dive.

 

[Divingguy]

Kevin: fun to actually put those formulas to use from time to time isn't it. I really enjoy the buoyant force and lift bag / required air volume problems.
Tom

 

[Rhone Man]

Yes, straight Boyles law. Nice to put some chemistry to work.

I also had the chance to show my son a "severe endothermic change caused by decrease in pressure" when a burst disk blew on the boat, and the whole K valve iced over in the Caribbean sun.

 

[Sideband]

Since you asked about temperature there is a combined gas law that covers this. P1/T1=P2/T2 where P is pressure and T is temp. in degrees Kelvin.

 

[sheck33]

Since you asked about temperature there is a combined gas law that covers this. P1/T1=P2/T2 where P is pressure and T is temp. in [-]degrees[/-] Kelvin.

Just to be picky because I can, since 1968 temperatures are measured in kelvin.
'degrees kelvin' is no longer used.

The combined (complete) gaslaw is P1V1/T1 = P2V2/T2.

 

[jwalko]

See
Boyle's Law and Combined_gas_law for more info.

Basically P1V1 = P2V2. So, given a constant temperature ( 85/2640*2000 ) would be on the money. To check the pressure, just bring your own pressure gauge and check it yourself. Maybe they are right. Also, remember that after a tank is filled, it is hotter, so the pressure would be higher. Once it is cooled down, the pressure will go down. P1V1/T1 = P2V2/T2 (The temps are at absolute temps, so be sure to convert). You can see a decrease of 200 lbs easy from fill time until you dive.

I've heard that steel tanks heat up more than aluminum tanks during filling. I guess this has to do with the heat disapation (sp) properties of steel vs. aluminum. But 200 lbs difference... wow...I didn't realize it would be that much.

I've had my tanks filled by these people four times and each time the fill was 1900 lbs to 2100 lbs. The last time I was there, I mentioned it to them and I think they actually got the fill to between 2200 and 2300 lbs. Should I ask for a slight "overfill" given that the tanks are warm, and will cool down.

This troubles me a bit, because I normally dive with guys who have larger tanks (up to 120's). I don't think I hoover air, but am usually the one to turn the dive. When I dive with my daughter (and she doesn't use air...she makes air if you know what i mean), we usually have about the same bottom time, so I actually think my air consumption is pretty good.

But, if I am really only getting about 65 cf at 2000lbs in an 85 cf tank, then this makes all sense. Shoot, I am anywhere from 15-55 less CF. That sucks. Time to convince my wife to let me buy larger tanks (lol). Or better yet, doubles...

John

 

[BarryNL]

Since you asked about temperature there is a combined gas law that covers this. P1/T1=P2/T2 where P is pressure and T is temp. in degrees Kelvin.

Of course, we could get picky and point out that these are ideal gas laws - real gas doesn't quite behave in a linear manner, though for diving purposes it pretty much does (with the possible exception of people with 300 bar tanks).

 

[Blackwood]

Of course, we could get picky and point out that these are ideal gas laws - real gas doesn't quite behave in a linear manner, though for diving purposes it pretty much does (with the possible exception of people with 300 bar tanks).

correct. But it's close enough for government use.