Hot vs Cold tanks

[Beau640]

I should be able to figure this out on my own with #physics, but I had an interesting shower thought that I was unable to quickly figure out on my own...

In terms of gas in a scuba tank, it seems temperature directly effects the pressure inside the tank. For instance, right after filling them when they are hot you will read one pressure, and after they cool the cooled pressure will be quite a bit lower. Or if you leave them in a cold car overnight, the pressure when you come back the next AM will be quite a bit lower.

How does this effect you while diving in different temperature water? If I take a hot tank into really cold water, besides my normal gas consumption, should I expect quite a bit of "artificial" drop in PSI above my normal consumption secondary to the tanks cooling off during the dive which would make my RMV appear artificially high? If I dive in really warm water, will I have more gas in general because there will be less of a cooling effect?

This leads me to my main question: Should we be filling our tanks/topping our tanks off when they are at a temperature that we expect the water we will dive in to be at? For example, if I'm going to dive in 40 degree water, should I make sure my tanks are at 3000psi (or whatever they will be when full) at a 40 degree temperature rather than room temp to avoid a cooling effect/maximize my gas?

Probably a simple answer to this but I'm tired today and couldn't wrap my mind around the physics easily. Just kept taking myself in circles.

Beau

 

[guruboy]

If you have the time and effort to spare, go for it.

 

[WarrenZ]

A tank is considered full when it if filled to its rated pressure at 70*F. Somewhere someone has a cheat sheet for hot fills Im sure. When you enter the water you will notice that tank pressure will adjust itself fairly rapidly in the first ten or so minutes.

A typical AL 80 will hold 77cf @3000 PSI @70*. If you fill it fast "Hot Fill" it 3000psi hot will generally cool to the 26-2800 range underfilled and could be topped off. If you topped the tank off to the point that at 40* it had 3000 you would have an over filled tank which could rupture a bust disk if heated. I have seen this happen on a tank left in the sun at a dive site you can imagine the pressure raise in a enclosed vehicle at the restaurant during lunch. It would suck to watch your gas fill leak off because the tank gets hot.

AL tanks should never be overfilled. Steel tanks I have seen pumped up to scary pressures almost hydro pressure. I dont do it with my tanks.

Remember if the fill has stretched the tank thin enough to see thorough the side into the tank it dosnt count as a Visual.

 

[WarrenZ]

By the way you are overthinking this go diving and dont worry about it.

 

[Beau640]

It's not about overthinking. It's about curiosity and further understanding more of the physics that plays into this. That's why it's posted in advanced rather than basic.

I'm thinking more about cold water diving in the winter when ambient temps will never be high enough to cause an issue with over pressurization. For us cold water divers, if a "full" tank is technically when it is full at 70 degrees, are we actually diving with less air each time than our warm water colleagues?

 

[homerdoc]

I saw this effect the other afternoon in Bonaire.

Walked down to the dock and hooked up a tank that had been in the sun, looked at my SPG and it was 3300.

Within just a minute in the water it had dropped to 2900.

I would think you hypothesis is correct, if you filled a tank inside a temperature controlled room to 3000 PSI, then dove cold - you would have drop.

PV=nRT, so with a fixed volume, pressure is going to drop in direct relationship to temperature.

 

[WarrenZ]

It is actually the same amount of molecules contained in the tank at 3000 at 70* or ~2500 at 40* besides when you breathe the air it warms back up in your lungs so how much expansion is going on there.

 

[tbone1004]

so the formula is pv=nrt. R is a constant, V and N are constant in this case, so the combined gas laws go to p1/t1=p2/t2 with temps in kelvin, for comparing the same gas under different conditions. For the sake of this, p2=p1t2/t1. Gay Lussac's law for reference

Now, because the temp is in kelvin, even if you fill the tanks and they stabilize at 100F and you go diving in 40F water, you will only lose about 8% of your total gas, or an AL80 that stabilizes in a 100F room at 3000psi, will drop to about 2750 psi. Not really enough to worry about.

Now, the bigger one is the pressure drop after the tanks are filled since they have an immense increase in temperature during filling due to the compression so if you fill too quickly, you will have a larger pressure drop when the tank cools off and depending on the tank, this could take well over an hour. If I have the choice, I like to fill at night, and then top off in the morning to account for this, but that is rarely the case. I usually just fill my steel tanks to about 3800-3900 psi and let them cool to about 3600 psi, and aluminums to about 3300 and cool to 3000 ish.

Regarding burst discs. They are supposed to burst at 90% of test pressure. That's 4500psi for an AL80. Working backwards of the formula, we want to find T2. The T2 to get the pressure up to 4500psi is ~290F. That's REAL hot. When you usually see burst discs go is when you have people who are idiots and are overfilling LP steel tanks and don't have appropriate burst discs in them. Either stock AL80 burst discs, or the stock LP burst discs *3960psi*. If you have the wrong burst disc, it doesn't take much to get them to blow

 

[dumpsterDiver]

I did the math years ago. I think it was 5 psi per degree f but my recollection is fuzzy. Not a huge issue unless the temp swings are very large.

 

[tbone1004]

I did the math years ago. I think it was 5 psi per degree f but my recollection is fuzzy. Not a huge issue unless the temp swings are very large.

that's the rough cut assumption that is close enough to ballpark. The bigger the swing, the less accurate it gets, but 5psi is a "close enough" rule, similar to 15psi=1atm instead of 14.7