Air Pressure

[Aslan]

Okay, I bought a HPA tank recently and was pretty happy about the deal. It's 68/4500 which I thought would give me alot more playing time than my 47/3000.

Anyways, I got it and had it aired up at the local pb shop to make sure there were no problems with it. I trust the guy I bought it from, just wanted to make sure because I know he switched the regs on it before selling it to me (it used to have a LP reg and he put a HP adjustable reg on it).

Anyways, they aired it up and it read about 4250psi as I brought it out to my car. Today, 2 days later...it sits in it's padded case in the garage and the pressure reading is just under 3500psi.

Now, here's where we get back into our freshman chemistry...the temperature it was at when it read 4250psi was slightly above room temperature (it had just been filled and was a little warm to the touch). In my garage, it's a brisk 29 degrees F.

Seeing that the pressure was lower made me first say, "oh crap! It's leaking!" This made me very anxious because it's the most expensive piece of paintball equipment I own...and I just got it and was all happy. I'm also anxious because if it is leaking I either have to try and resolve the problem with the seller or see if I can get it repaired (is that possible...if a tank has a small leak to have it repaired at a pb shop or something???)

But then I remembered my ideal gas equation from 15 years ago, freshman chemistry. Essentially, with the temperature decreasing, and everythig else remaining constant...the pressure should also decrease. But in order to figure out exactly what pressure drop I "should" see...I would need to know the value "n" (which I believe is the weight of the gas that was added to the bottle)...which I don't know.

Any experience with this. Is this normal that the pressure would drop like that just due to the cold weather? Is a faint leak repairable? How much am I looking at ($$$)?

I will try and bring it in the house overnight and see if the pressure increases due to the temperature change. I can also leave it in the garage and see if it empties to zero...which would be evident of a leak.

Thanks for the help...I'm panicing over this!! :ninja: :bounce: :ninja:

[FinchMan]

warm to the touch, so... maybe like 90*F ?


90*F is about 305 Kelvin, 29*F is about 271 K.

Some gas formula from chem class: P1V1/n1T1 = P2V2/n2T2

Volume hasnt changed... so P1/n1T1 = P2/n2T2 ; P1n2T2 = P2n1T1 ; n2 = n1(P2T1)/P1T2

n2 = n1(305K)(3500psi)/(271K)(4250psi)

n2 = about 92% of n1

thats with those rounded off numbers, so i don't think its leaking. I would suggest weighing the tank when its first filled, because temperature doesnt change the weight.

[Tao]

Okay, I bought a HPA tank recently and was pretty happy about the deal. It's 68/4500 which I thought would give me alot more playing time than my 47/3000.

Anyways, I got it and had it aired up at the local pb shop to make sure there were no problems with it. I trust the guy I bought it from, just wanted to make sure because I know he switched the regs on it before selling it to me (it used to have a LP reg and he put a HP adjustable reg on it).

Anyways, they aired it up and it read about 4250psi as I brought it out to my car. Today, 2 days later...it sits in it's padded case in the garage and the pressure reading is just under 3500psi.

Now, here's where we get back into our freshman chemistry...the temperature it was at when it read 4250psi was slightly above room temperature (it had just been filled and was a little warm to the touch). In my garage, it's a brisk 29 degrees F.

Seeing that the pressure was lower made me first say, "oh crap! It's leaking!" This made me very anxious because it's the most expensive piece of paintball equipment I own...and I just got it and was all happy. I'm also anxious because if it is leaking I either have to try and resolve the problem with the seller or see if I can get it repaired (is that possible...if a tank has a small leak to have it repaired at a pb shop or something???)

But then I remembered my ideal gas equation from 15 years ago, freshman chemistry. Essentially, with the temperature decreasing, and everythig else remaining constant...the pressure should also decrease. But in order to figure out exactly what pressure drop I "should" see...I would need to know the value "n" (which I believe is the weight of the gas that was added to the bottle)...which I don't know.

Any experience with this. Is this normal that the pressure would drop like that just due to the cold weather? Is a faint leak repairable? How much am I looking at ($$$)?

I will try and bring it in the house overnight and see if the pressure increases due to the temperature change. I can also leave it in the garage and see if it empties to zero...which would be evident of a leak.

Thanks for the help...I'm panicing over this!! :ninja: :bounce: :ninja:


It may not be leaking but if it is it is a very slow leak. Don't be too worried if your tank leaks a pressure over several days. It can be a really small piece of dirt holding the vavle open ever so slightly. I usually don't bother filling my tanks at the end of the day just incase it does happen where they leak everything out...just as long as it doesn't empty itself when you are on the field! :P

[rb211]

I get about a 200-250psi drop on my bottle when I have it filled, and I know that it is because it is being hot filled. I would get it filled and just watch it over a couple of days to see if it continues to drop. As mentioned before, you could have a small leak which could be caused by several things. One way you can check it is to put a few drops of oil on tp of the pin valve (if it is a screw in tank) and just look for bubbles in the oil. I have lost the o-rings in mine a couple of times due to bad air fills. If your tank isn't a screw-in, drain the pressure from the lines in your gun and wait a couple of days after re-attaching your tank to your gun and see if it re-pressurized the lines.

Shouldn't cost too much to fix it if it is leaking.

[Aslan]

warm to the touch, so... maybe like 90*F ?


90*F is about 305 Kelvin, 29*F is about 271 K.

Some gas formula from chem class: P1V1/n1T1 = P2V2/n2T2

Volume hasnt changed... so P1/n1T1 = P2/n2T2 ; P1n2T2 = P2n1T1 ; n2 = n1(P2T1)/P1T2

n2 = n1(305K)(3500psi)/(271K)(4250psi)

n2 = about 92% of n1

thats with those rounded off numbers, so i don't think its leaking. I would suggest weighing the tank when its first filled, because temperature doesnt change the weight.

I was thinking PV=nRT...the problem is n is molar weight...which I could convert to if I had the weight. Since I don't have the weight...I'd have to assume the weight is the same...which means I'd be ruling out a leak as all the gas would be assumed to still be in there. Also, don't you have to convert pressure to atm? Or as long as the units are the same it doesn't matter?

Anyways...update...after 6-7 hours at 72-74 degrees F...the guage now reads about 3950psi. :bounce: :clap: I'll check it again in the morning then put it back out in the garage in it's case. If it's at or above 4000 by tommorrow morning...I'm going to assume it's not a leak and just the result of temperature. If that's the case, maybe this post will be useful (if anyone uses the search feature!) to people in the future regarding the effects of temperature on a tank's pressure. I never would have guessed the pressure drop would be so great based on temperature alone...interesting... :wow:

[Aslan]

warm to the touch, so... maybe like 90*F ?


90*F is about 305 Kelvin, 29*F is about 271 K.

Some gas formula from chem class: P1V1/n1T1 = P2V2/n2T2

Volume hasnt changed... so P1/n1T1 = P2/n2T2 ; P1n2T2 = P2n1T1 ; n2 = n1(P2T1)/P1T2

n2 = n1(305K)(3500psi)/(271K)(4250psi)

n2 = about 92% of n1

thats with those rounded off numbers, so i don't think its leaking. I would suggest weighing the tank when its first filled, because temperature doesnt change the weight.

Nevermind...I just read your analysis again and see what you did...dropping R and V as constants and comparing 1 and 2. Very good sir...very good work. So your analysis is that the 8% loss over 2 days is either an oversetimate based on rounding or normal pressure loss. And, to verify your theoretical calculations with my test...my bringing the tank back up to room temperature...3950/4250 = 93%. Thus a 7% (vs 8%) loss over 2 days.

However, since it's not all the way back up to say 90F...the difference between 90F and say 73F would be expected to be (using the same formula); n2 = n1(305K)(3500)/(296K)(3950)...we would expect to see a 9% drop based on going from "warm to touch" after just being filled to room temperature. 7% vs 9%...pretty good given not knowing exact temperatures.

Guess the wonders of general inorganic chemistry have paid off by allowing me to sleep better tonight knowing my tank is not leaking.

[shades]

Pressure is directly proprotional to temperature. When the temp goes down your pressure goes down. A HPA tank is very warm when it is filled so the pressure is slighlty higher then when you leave it your car over night in the "cold" (down goes the pressure,Danm physics again). :mad: This is also the reason you never fill a CO2 to max. Because, the pressure will increase as the bottle reaches room temp. Leave it in your car during the summer and youll blow the burst disc.
Q.E.D.
Moral never race an three legged mutated canabilistic hamster with a head cold. Beacause Tunas own hand grenades. I hope that clears things Up. :confused: