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Leaving tanks in hot car - safe?

Discussion in 'Tanks, Valves and Bands' started by jejton, Oct 13, 2020.

  1. jejton

    jejton Nassau Grouper

    # of Dives: 50 - 99
    Location: Florida
    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.
  2. JahJahwarrior

    JahJahwarrior ScubaBoard Supporter Staff Member ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: West Palm Beach, Fl
    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.
    D_Fresh and couv like this.
  3. Searcaigh

    Searcaigh Chromodoris gordonii Staff Member ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Dubai, UAE
    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.
  4. broncobowsher

    broncobowsher Solo Diver

    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.
    rjack321 and D_Fresh like this.
  5. kelemvor

    kelemvor Big Fleshy Monster ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Largo, FL USA
    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.
  6. Southside

    Southside Rebreather Pilot

    # of Dives: 25 - 49
    Location: Chicago
    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 likes this.
  7. wnissen

    wnissen ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 50 - 99
    Location: Livermore, Calif.
    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.
  8. Bob DBF

    Bob DBF Solo Diver

    # of Dives: I just don't log dives
    Location: NorCal
    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.
  9. wnissen

    wnissen ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 50 - 99
    Location: Livermore, Calif.
    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.
    Bob DBF likes this.
  10. Southside

    Southside Rebreather Pilot

    # of Dives: 25 - 49
    Location: Chicago
    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.
    wnissen and Bob DBF like this.

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