Tanks losing pressure
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ok, so lots of misinformation on cooling and pressure, look at my first post #4 in this thread. Those numbers are calculated numbers using the actual formula to calculate p1/t1=p2/t2 it is either Gay-Lussac, or should really be Amontons Law. Temps are in Kelvin or Rankine *depending on where you live, Kelvin follows Celsius, Rankine follows Fahrenheit*, and because of this the relationship is not linear. So the temperature differential is much less important than the pressure because while going from 100F to 50F is 50% in that formula, it is really going from 560 to 510, which is only a 10% change in temperature.
so with that, let's stop and actually calculate and you'll see that the 100psi/10F is complete and utter bollocks, though note the first pressure mentioned in my post should be 3600psi not 3500psi for the temperature relations. It drops 50*F and only drops 300psi for the HP tanks, the AL80's and LP tanks are even less effected by temperature.
Regarding Geoff's claim of 4000psi down to 3300, that's just BS, I've filled those acorns plenty of times and unless you are getting the up to well over 150*F *which you can't touch without burning yourself btw* and you're house is 60, highly unlikely, the more likely answer is the gauge is off at that high of a pressure and while you may think you're getting 4 grand, you're really down at 3700-3800, physics doesn't lie, I've taken IR thermometers to fill stations and I've never seen a HP tank above about 130F, which would cool to about 3600 from 4000psi fill if the gauges are accurate. Again, not saying that you are lying about having seen a gauge read 4000, and a gauge read 3300, but if the gauge was only a 4500psi gauge, or even a 5000psi gauge, it is not going to be particularly accurate around 4000 and will likely be reading slightly high, especially if it is not the newest of gauges and hasn't been calibrated *very few fill stations replace their gauges regularly....
None of this is relevant to the OP as the pressure swing is likely due to bad gauges more than anything. OP needs to take his regulators to the dive shop and cross check them against known tank fills, i.e. get his tanks filled to whatever pressure, then put his reg on and verify the number or not the discrepancy, then check at some lower tank pressure, though busted SPG's usually read high not low due to stretched tubes, and then make sure he does a good bubble check on the tank to make sure the neck o-rings aren't leaking though this is likely a gauge issue due to the tanks being similarly off.
so with that, let's stop and actually calculate and you'll see that the 100psi/10F is complete and utter bollocks, though note the first pressure mentioned in my post should be 3600psi not 3500psi for the temperature relations. It drops 50*F and only drops 300psi for the HP tanks, the AL80's and LP tanks are even less effected by temperature.
Regarding Geoff's claim of 4000psi down to 3300, that's just BS, I've filled those acorns plenty of times and unless you are getting the up to well over 150*F *which you can't touch without burning yourself btw* and you're house is 60, highly unlikely, the more likely answer is the gauge is off at that high of a pressure and while you may think you're getting 4 grand, you're really down at 3700-3800, physics doesn't lie, I've taken IR thermometers to fill stations and I've never seen a HP tank above about 130F, which would cool to about 3600 from 4000psi fill if the gauges are accurate. Again, not saying that you are lying about having seen a gauge read 4000, and a gauge read 3300, but if the gauge was only a 4500psi gauge, or even a 5000psi gauge, it is not going to be particularly accurate around 4000 and will likely be reading slightly high, especially if it is not the newest of gauges and hasn't been calibrated *very few fill stations replace their gauges regularly....
None of this is relevant to the OP as the pressure swing is likely due to bad gauges more than anything. OP needs to take his regulators to the dive shop and cross check them against known tank fills, i.e. get his tanks filled to whatever pressure, then put his reg on and verify the number or not the discrepancy, then check at some lower tank pressure, though busted SPG's usually read high not low due to stretched tubes, and then make sure he does a good bubble check on the tank to make sure the neck o-rings aren't leaking though this is likely a gauge issue due to the tanks being similarly off.
mjrob
Contributor
I agree with tbone. I'm gonna bet it's a gauge issue. The water bath will tell you quickly. If bubbles emanate from the valve or o-ring seal (and not the walls of the cylinder, if that's the case, RUN!), it should be a pretty inexpensive fix. But I'm going with gauges. Let us know!
Does the surface temperature of the tank equal the temperature of the gas inside during filling? I've had plenty of fills for a HP80 where the outside is cool to the touch, but hours later the tank is down at least a few hundred PSI, using the same pressure gauge for all measurements
With a hot fill to 3800, when I took my tank off the fill whip the tank felt warm, but by the time I got my tank to my car the outside it felt quite a bit warmer. I never thought to bring an IR thermometer with me for fills, but that will be an interesting experiment in the future
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The tanks are usually a few degrees cooler than the gas inside, but this is why it is important to let them calm down a bit before you check pressure.
Inverse to what was mentioned above, a tank instantly filled from 700 to 3600 at room temp will hit about 2300F if it happens instantly, obviously it doesn't, and the combination of the steel being a good conductor and being at room temp and having quite a bit more thermal mass than the air does it doesn't heat up anywhere near that much, but it wouldn't surprise me if the air in the tank got well up over 200F during filling. Water bath's can help somewhat, but slow filling is really the answer and making sure the tank equalizes before you do pressure checks on them. If you're getting fills from a shop, drop it off and pick it up the next day if possible, or drop off and go get lunch/dinner etc. to make sure the tanks at least mostly equalize before you put the gauges on them. Most shops that do a lot of fills are usually a nonissue, cave country I almost never get short fills, same with coast of Carolinas, but they fill tanks all day every day so they know their equipment and how slow/fast to fill the tanks and what not, and how high to fill them so when they get hot they have controlled cooling.
Inverse to what was mentioned above, a tank instantly filled from 700 to 3600 at room temp will hit about 2300F if it happens instantly, obviously it doesn't, and the combination of the steel being a good conductor and being at room temp and having quite a bit more thermal mass than the air does it doesn't heat up anywhere near that much, but it wouldn't surprise me if the air in the tank got well up over 200F during filling. Water bath's can help somewhat, but slow filling is really the answer and making sure the tank equalizes before you do pressure checks on them. If you're getting fills from a shop, drop it off and pick it up the next day if possible, or drop off and go get lunch/dinner etc. to make sure the tanks at least mostly equalize before you put the gauges on them. Most shops that do a lot of fills are usually a nonissue, cave country I almost never get short fills, same with coast of Carolinas, but they fill tanks all day every day so they know their equipment and how slow/fast to fill the tanks and what not, and how high to fill them so when they get hot they have controlled cooling.
halocline
Contributor
The air compressing inside gets hot, so it heats up the tank, cooling as it does so. So initially the air is much hotter than the tank during filling. The tank absorbs heat from the air and dissipates it to the outside air, so I guess there could be a point at which the tank is actually warmer than the air inside for a very brief time. It doesn't matter, it's the air temp, not the tank temp, that determines the pressure.
I'm guessing that steel would heat up a bit more slowly and cool a bit more slowly than AL, so it's possible your HP80 (aka bowling ball with valve
) could have a lag in time before it heats up from the air inside.Similar threads
