Leaving tanks in hot car - safe?

[jejton]

There are times I may want to go diving after going somewhere, without driving home to get my gear (i.e. after work). Is it safe to leave an aluminum tank in a hot car for hours at time? For example sunny 90 degree weather.

 

[JahJahwarrior]

We can't say perfectly safe, but it's safe enough.

Remember that PV=nRT but the T is kelvin, not farenheit. Going from 70 to 110F is a small change in absolute so it won't result in a huge pressure change.

But, it can result in a few hundred PSI and an old or weak burst disk can blow. There is plenty of anecdotal evidence, but my opinion is that proper valve maintenance will avoid this issue sufficiently to make it a non issue.

Do cover them if you can, to avoid radiative heat gain, and don't worry about the convection or conduction heat gain.

 

[Searcaigh]

I have left tanks in my car where the air temps have been 45C and above with no issues.

As a rule in these temperatures I generally crack the driver and passenger windows open a few cm to allow hot air to exit the vehicle.

 

[broncobowsher]

They will still be cooler than having them sit in the sun.
Tanks will be fine. The other gear I would worry about a little more. Here in AZ every dive shop has a warped fin on the wall to warn customers what heat can do to destroy gear, and it isn't a warranty claim.

 

[kelemvor]

When I go to Key Largo or the east coast area for a weekend, I bring enough tanks for at least two divers for the weekend in my minivan. The tanks aren't in direct sun, but it gets damn hot in the vehicle. I'm sure well above 100f inside. Has never been a problem for me. Like @Searcaigh says, crack the windows. As a fellow Floridian, I'm sure you already do this anyway.

 

[Southside]

As @JahJahwarrior said the governing equation is PV=nRT. Volume, number ofmoles, and the gas constant do not change so we are left with P1/T1=P2/T2. Where temperature is absolute.

Solving for P2 we get P2=(P1*T2)/T1

Let's say we go from 70°F(529.67°R) to 120°F(579.67°R). Putting the absolute temps in we get that P2=1.094*P1 or a 9.4% increase in pressure. A 3600 psi fill would become 3939.8psi.

Going from 70°F to 140°F gives us P2=1.132*P1 or a 13.2% increase. We go from 3600psi to 4075.8psi.

I used a starting pressure of 3600psi but you could apply the ratio from the temperatures to any starting pressure regardless of units.

 

[wnissen]

As @JahJahwarrior said the governing equation is PV=nRT. Volume, number ofmoles, and the gas constant do not change so we are left with P1/T1=P2/T2. Where temperature is absolute.

Solving for P2 we get P2=(P1*T2)/T1

Let's say we go from 70°F(529.67°R) to 120°F(579.67°R). Putting the absolute temps in we get that P2=1.094*P1 or a 9.4% increase in pressure. A 3600 psi fill would become 3939.8psi.

Going from 70°F to 140°F gives us P2=1.132*P1 or a 13.2% increase. We go from 3600psi to 4075.8psi.

I used a starting pressure of 3600psi but you could apply the ratio from the temperatures to any starting pressure regardless of units.

That may be the first time I've seen someone make that calculation not in Kelvin. However, given that the tank is going to heat from the inside out, I've always wondered if you have to take the coefficient of thermal expansion (CTE) of the aluminum into account as well. The pressure might actually drop at first!

Looks like it depends on the alloy, but the value is around 20*10^-6 per ºC.
Thermal Expansion Coefficient of Aluminum
So a spherical tank (possibly used to supply air to a spherical cow) heated 30ºC would expand in volume (30*20e-6)**3 or 2.2*10^-10. On second thought, I guess you can ignore the CTE! Basically, I would try to keep the tanks from getting excessively overheated.

 

[Bob DBF]

That may be the first time I've seen someone make that calculation not in Kelvin.

Kelvin is absolute tempature for Celsius, Rankine is absolute tempature when using Fahrenheit which is not used so much anymore. Rankine is a US customary unit and was used, and still is to a lesser degree, before Kelvin became the standard for scientific calculations.

 

[wnissen]

Kelvin is absolute tempature for Celsius, Rankine is absolute tempature when using Fahrenheit which is not used so much anymore. Rankine is a US customary unit and was used, and still is to a lesser degree, before Kelvin became the standard for scientific calculations.

Oh, I know what the unit is. It's that the Venn diagram of people who make gas law calculations and those who use Fahrenheit for them is almost disjoint.

Also, I realize that another effect that might be taken into account is that at 200 bar the ideal gas law starts to break down somewhat. I would guess that as it gets heated there might be increasingly nonlinear effects. So southside's calculation may be a lower bound.

 

[Southside]

Oh, I know what the unit is. It's that the Venn diagram of people who make gas law calculations and those who use Fahrenheit for them is almost disjoint.

Also, I realize that another effect that might be taken into account is that at 200 bar the ideal gas law starts to break down somewhat. I would guess that as it gets heated there might be increasingly nonlinear effects. So southside's calculation may be a lower bound.

In school I always had to solve problems in both metric and imperial. Even though I use metric most of the time and prefer it my brain still thinks in Imperial. I do the conversions in my head.

Converting from °F to °C to °K and back is marginally slower than going from °F to °R and back when doing the calculations manually. It would have been just as quick using Wolfram Alpha but I figured the OP was in Florida so keep it Imperial.

Now if we were talking a problem involving mass I would avoid using slugs at all costs.