John Ackerman
Joined: 19 Jun 2006
Posts: 130
Location: Prescott, AZ
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 Posted: Wed Feb 08, 2012 8:37 am Post subject: Fuel tank leak testing |
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...or you could make a simple u-tube manometer by mounting a long (several feet) loop of transparent plastic tubing on a piece of board having some sort of stand to hold it close to vertical and putting some (food-dyed) water in the loop. Connect one end of the loop to the tank, pressurize the tank to a foot or so of water (distance between meniscii - the surfaces of the water in the tube), then measure and record the distance between the meniscii to get the pressure. Leave a thermometer in contact with the tank to get the temperature and record that, too. Make a series of measurements every few hours through a couple of temperature cycles (days, usually).
Temperature must be absolute temperature - add 273.15 to a Celsius temperature to get absolute temperature in Kelvins or 491.7 to Fahrenheit temperature to get absolute temperature in degrees Rankine). The pressure can be corrected using the local altimeter setting, but this is usually of less concern.
Pressure divided by temperature (P/T) "should" be constant if there are no leaks. Measure and record every few hours to get a feeling for the uncertainty in P/T. For me it was negligible (and I was lucky - Van's tanks did not leak -yet  ). Any decrease in P/T gives you a quantitative measure of the leak rate (plus error). Any increase is due to measurement error. If you are doing reasonably precise measurements (measuring the distance between meniscii as precisely as you eyeballs allow), you will see that P/T will NOT be perfectly constant. No biggie.
The biggest source of error comes if the temperature swings a lot during your measurement series - the tank will not be at the same temperature everywhere, and it's hard -no, impossible -to put your thermometer where the temperature you measure is always the same as the temperature of the air in the tank. Leave the thermometer in one location. With a set of measurements over two or more temperature cycles, you can plot P/T vs time and draw a line through the points to get the leak rate. "Eyeballing" the line should be more than adequate, or you can do a least-square fit using Excel or similar tool if you are truly compulsive.
If you do have a leak, the bubble test will find it for you - just hope that it's in one of your connections and not in the tank itself!
Hope this is useful.
John Ackerman
40458
On Feb 7, 2012, at 5:49 PM, n801bh(at)netzero.com (n801bh(at)netzero.com) wrote:
[quote]
Temp will play a bigger part then ambient air pressure.. And latex balloons leak badly, so you will see a smaller inflation of the balloon anyway. Best test is the bubble route... No bubbles - no leaks..IMHO.
do not archive
Ben Haas
N801BH
www.haaspowerair.com
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