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Gas Tank Question

A

Adam W

Well Known Member
well guys i'm back asking for more advice on this incredible forum

i have completed my second tank. rather than try and seal a balloon around the bulkhead vent fitting i flared a piece of tubing and extended it out so that i could more easily secure and tape around the balloon opening. i have blown up the tank three times now. each time i was quite happy to see the balloon had not changed in size at all after hours and hours. and then in each case over a short amount of time the balloon deflated substantially. it remained fully inflated (i marked it with a sharpie) each time and then after 5 hours or 8 hours it would drop down to quarter of what it was. the barometric pressure didn't change that much in that time period. i was thinking the kids were playing a joke on me but no.
i made sure no one opened the door to the garage which is heated today so there was no significant temperature change.
i guess the next step is soap and bubbles, and i would go to this if the balloon didn't remain full so long each time. it seems strange that the deflation doesn't occur for many hours and then it happens over 20 minutes. any thoughts? thanks in advance. i suspect ill be told to start looking for leaks with soap and bubbles but figured id take a shot at some wisdom here saving me a step
 
Remember that a balloon does not inflate or deflate in a linear fashion. When you blow it up, it inflates slowly then all of a sudden expands quickly once the pressure overcomes the tension in the material and it stretches. Likewise when you deflate, it stays round pretty long then loses it's shape quickly once it reaches a certain point. When I did my tanks, I noticed no change then an abrupt deflation many hours later and it was due to a small leak.
 
Pressure

Adam W said:
well guys i'm back asking for more advice on this incredible forum

i have completed my second tank. rather than try and seal a balloon around the bulkhead vent fitting i flared a piece of tubing and extended it out so that i could more easily secure and tape around the balloon opening. i have blown up the tank three times now. each time i was quite happy to see the balloon had not changed in size at all after hours and hours. and then in each case over a short amount of time the balloon deflated substantially. it remained fully inflated (i marked it with a sharpie) each time and then after 5 hours or 8 hours it would drop down to quarter of what it was. the barometric pressure didn't change that much in that time period. i was thinking the kids were playing a joke on me but no.
i made sure no one opened the door to the garage which is heated today so there was no significant temperature change.
i guess the next step is soap and bubbles, and i would go to this if the balloon didn't remain full so long each time. it seems strange that the deflation doesn't occur for many hours and then it happens over 20 minutes. any thoughts? thanks in advance. i suspect ill be told to start looking for leaks with soap and bubbles but figured id take a shot at some wisdom here saving me a step
Click to expand...

If the temperature changes, the pressure inside changes. Maybe?
Use bubbles to be certain.
 
My understanding is the soapy solution is the PRIMARY means of testing for leaks. The balloon will fluctuate with changes in atmospheric pressure, so kinda hard to draw any conclusions from that.

When I tested mine, after the balloon inflated, i sprayed the soapy solution (kid bubbles) all over the tank MULTIPLE times...no bubbles anywhere, and balloon stayed inflated for few hours after as a bonus....so I called that 'good'.
 
RV74ME said:
My understanding is the soapy solution is the PRIMARY means of testing for leaks. The balloon will fluctuate with changes in atmospheric pressure, so kinda hard to draw any conclusions from that.

When I tested mine, after the balloon inflated, i sprayed the soapy solution (kid bubbles) all over the tank MULTIPLE times...no bubbles anywhere, and balloon stayed inflated for few hours after as a bonus....so I called that 'good'.
Click to expand...

This is what I did too. P.S., I'm not flying yet.....
 
Try again

Use a different one. some of those party types are not that well made and cheep from over seas. If a good one stays up far 4-6 hours you should be able to use the type of soap that is used to fine gas leaks, like for a gas hot water heater. If you don't see any bubbles and the balloon stays up for a good time that it about all you can do, short or holding it down under water with a little pressure in the tank. Most builders don't have the means to do the under water test. Most small leaks are not that hard to seal if you find one are one shows up down the road in a few years or even right away. Hope this helps. Yours, R.E.A. III #80888
 
Or just make a water manometer. No risk of leaky connections or balloon, because you use plumbing fittings. Easy to see pressure changes exactly in inches of water. You can actually watch pressure changes with temperature (assuming you don't have a leak causing a steady drop). You still get the overpressure protection; the water will blow out of the tubing.

And manometers have lots of other uses, too.

Charlie
 
Manometer

rv7charlie said:
Or just make a water manometer. No risk of leaky connections or balloon, because you use plumbing fittings. Easy to see pressure changes exactly in inches of water. You can actually watch pressure changes with temperature (assuming you don't have a leak causing a steady drop). You still get the overpressure protection; the water will blow out of the tubing.

And manometers have lots of other uses, too.

Charlie
Click to expand...

That's how I did it. Mine held 27" for a month. Minor fluctuations due to temp and pressure.
I actually had two on one board so both tanks were tested at the same time.
 
100 low lead

I've taken mine to the airport and filled them with blue 100 LL and look for blue spots. Tilt the tank in such a way that all the inside surfaces are wetted then leave for a few days. After finding no little blue stains I was good to go. 1300 hours later and no leaks.

Steve Barnes
 
Steve Barnes said:
I've taken mine to the airport and filled them with blue 100 LL and look for blue spots. Tilt the tank in such a way that all the inside surfaces are wetted then leave for a few days. After finding no little blue stains I was good to go. 1300 hours later and no leaks.

Steve Barnes
Click to expand...

Did the same for testing my tanks. Happy (lucky?) to report no leaks but not flying yet either, so time will tell.
 
thanks for the advice everyone

found my leak very slow leak at a screw on round cover holding the float sender

thanks to all.
 
wirejock said:
That's how I did it. Mine held 27" for a month. Minor fluctuations due to temp and pressure.
I actually had two on one board so both tanks were tested at the same time.
Click to expand...

That's exactly how I did mine. It was kinda neat to see the fluctuations from day to day as the pressure and temp changed.

Bevan
 
The tank expands and also hold a lot of air. It takes a while for the air to leak down in the tank enough to drop the pressure and let the tank contract, which will happen before the balloon starts to contract.
Are you spraying with soapy water to look for the leak?
Also, inflate two balloons to keep one as a control.
 
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