Tank Leak in 9A QB

avi8tor50 · #1 03-02-2007, 08:00 AM

Finishing up the wing in my 9A QB. Finally got around to sealing the end plate on my fuel tank. Pressure testing with a manometer to 27" (1 PSI) as recommended by Vans shows a VERY slow leak. The level actually remained stable for 12 hours and then dropped about 3" overnight. Have gone thru the usual soapy solution spray MANY times and can find no bubbles at the end plate, the screws, etc. Gas cap is tightly sealed with fuellube and duct tape. Have sprayed all the rivets in the skin top and bottom and see no bubbles.

Would prefer to avoid removing the tank since it would be a major pain to get to the bolts of the Z brackets.
Would appreciate thoughts of any who have had similar experience.

Peter K
9A QB

Ron Lee · #2 03-02-2007, 08:08 AM

Did a weather system come through overnight that might account for the change?

bullojm1 · #3 03-02-2007, 08:17 AM

Peter,
I had the same issue when I was testing one of my tanks. Turns out that the barometric pressure changed enough to cause me to have a slight drop. Next time you do your test, log onto weather.com and keep track of what the barometric pressure is doing. If it is going up, then the level in the manometer will fall. Since your manometer level stayed constant for 12 hours, i think the change in pressure is the culprit.

RV7Guy · #4 03-02-2007, 08:17 AM

Try to do the test on a day when the weather will be stable for at least 24 hours. I did this on a tank and had a drop after a high pressure front went through. Didn't really think about it and repeated the test. It was brought to my attention later that this could have been the cause for the drop.

I've had one of my tanks off 3 times (now completely fixed). I can do the Z bracket bolts in about 30 minutes. It isn't that bad and if you don't get a good test in stable weather you may have to do this.

Good luck.

jsherblon · #5 03-02-2007, 12:01 PM

Is your garage/shop heated? My garage cooled off overnight when I shut the heater off and went to bed. The next morning the manometer level had dropped. When the temperature was back up, it went higher than the start level.
I put a piece of masking tape next to the tube and marked the level, time, and temp.

fstringham7a · #6 03-02-2007, 12:58 PM

Peter

I think Charles Law got ya. Volume is directly proportional to a change in tempreature.
Assuming pressure is constant ....well then we would have to look at Boyles law. Man
do I sound like the old used up retired chemistry teacher that I am............

I had a very small leak in the BNC connector which is part of the capacitance fuel level sending stuff. I believe you probably have the floats so this wouldn't be a problem.

Frank @ SGU RV7A STILL NOT DONE!!!!!!!!!!!!!!!!

Ron Lee · #7 03-02-2007, 02:48 PM

PV=nRT

If T (temp) drops, P (pressure) drops since the volume is the same.

fstringham7a · #8 03-02-2007, 03:30 PM

Hi again form the chemistry corner???????So what is the "R" # and units. Also what are the units of temp/ and Units of Pressure.............I know that this isn't getting my plane done but I just couldn't help myself!!!

Frank @ SGU.............Plane Not Done......maybe I should hit the garage!!!!!!!!!!!!

Scott Will · #9 03-02-2007, 04:04 PM

My QB tank is leaking at the big washer under the main fuel outlet bulkhead fitting on the wing root. Looks like they didnt put enough proseal on the backside of the bulkhead fitting and/or under the large washer. I have to get to it one of these days!!

It took me a while to find the leak.