Tue Sep 11 2007
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Tue Sep 11 2007

Tank Testing - Success!

These are the self-sealing screws from McMaster-Carr. They have a silicone o-ring built in under the head of the screw. Perfect for this application. The part number is 90825A816 for a package of 10. I bought three packages...enough to do both tanks (plus have a few left over).

I was wondering what the relationship between temperature change and pressure change in the tank would be, so I cleared out some of the cobwebs in my brain from college physics. The ideal gas law is:

pV=nRT where:

  • p is the absolute pressure [Pascal]

  • V is the volume [m3]

  • n is the amount of the substance [mol]

  • R is the ideal gas constant [8.314472 m3x Pa x K-1x mol-1]

  • T is the absolute temperature [Kelvin]

However, since V, n, and R are (essentially) constants for closed volume leak testing of a fuel tank (i.e. using a manometer), we find that p is proportional to T.

p0/T0=P1/T1

If we make a few assumptions about starting temperature and pressure, we should be able to come up with a nice conversion between temperature change and water level change.

Assumptions:

  1. Ambient air temperature is somewhere around room temperature (i.e. 72 degrees F)

  2. You use roughly the recommended 1 psi (27.7 inches H2O) for leak testing the tank

  3. Ambient air pressure is somewhere around standard pressure (i.e. 29.92 inches Hg)

Plugging all this in results in a conversion that for every 1 degree F temperature increase, the change in the height of the column of water will be .821 inches.

Don't forget (like I did) that this will only measure as .410 inches of rise, because for every bit that the water rises in one tube, the water level will go down in the other tube by the same amount.

Damn I'm such a nerd. :-(

Well, with all that figured out, I got down to testing the right tank. Here's the water level and temperature at the start of the test...

...and after a few hours. The change in water level is quite close to what would be expected for the indicated change in temperature. Right tank certified leak free!

I was feeling pretty cocky as I set the right tank aside, hooked up the left tank and started pumping it up. But what the frack was going on?!? I couldn't get the water level to even budge. I felt kinda silly when I noticed that I'd left the cap off the flop tube fitting.

Anyway, here's the water level and temp at the start of the test...

...and a few hours after. Again, the change in water level is right in-line with what would be expected for the temperature change. Left tank certified leak free!

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