Fuel tank pressure checks

With the fuel tanks completely sealed, it was time to pressure check them and make sure there were no leaks.

The plans say to retap the 1/8" NPT drain holes, but I decided to hold off on that. This way, if I did have a leak, I wouldn't be questioning whether it was the fuel drains or the tank.


Attempt #1:

I didn't have an old altimeter laying around to "T" off as the plans has us do, so I went to Home Depot and purchased a bunch of fittings. These fittings can get pretty expensive, so rather than test both tanks at the same time, I focused on one for now.

I connected one of the vent lines to a 15 psi gauge, capped the other vent line, and taped a glove to the tank fuel line. The 15 psi gauge had a Schrader valve threaded to it so it made it easy to pump air into it.

The plans say to pump a delta of 1500 feet. So I pulled up the standard atmospheric altitude reference tables: at sea level we are at 14.696 psi... at 1500 ft we are at 13.91 psi... a difference of 0.79 psi. I decided to pump the tanks to 1.0 psi so it would be easier to read if there was a leak.

I figured if anything, the extra 0.21 psi of pressure would burst the glove before causing any damage to the fuel tank... using the glove as a pressure relief valve...

I stayed in the hangar for another 30 minutes or so... the gauge still read 1.0 psi and I was happy to know there were no leaks. But just to be safe, I left it overnight just to see if the pressure would hold. To my surprise, when I returned the following day, the gauge read zero! Here's the wild part, the glove was still inflated! What the heck was going on here?

I then came up with a hundred questions... do I have a leak? Is it the gauge? Is it one of the cheap hardware store fittings? But if it is leaking, why is the glove still inflated?

Rather than purchase new hardware, I figured I would try the experiment again on the other fuel tank. This way, if the pressure held on the other tank, I would know it wasn't the hardware. I removed all the hardware and re-plumbed everything to the other tank.

Go ahead... make all the cow udder jokes you please... won't hurt my feelings!

So I pumped it up to 1.0 psi... waited 30 minutes and all was good...

I returned the following day and AGAIN the gauge read zero... and the glove was still inflated!

I was convinced it had to be the gauge! If it were the tanks, the gloves would have deflated! Ok, but I needed to prove it. So I uninstalled the gauge and used existing hardware to cap it on my workbench. I pumped it up to 1.5 psi... waited a day and sure enough dropped to zero the next day!

Case and point: these gauges are NOT designed to hold pressure for long periods of time. After a bit of research, turns out the manufacturer even states this on their product! (not sure why someone would want to purchase/use this if it can't give "real-time" pressure readings?) Anyways... moving on!

Attempt #2:

A hangar neighbor of mine was kind enough to lend me a "working" altimeter. I repeated the experiment, this time using an altimeter instead of the pressure gauge.

I started by setting the altimeter to +500 ft... and then pumped air into the tank until it read -1,000 ft (delta of 1500 ft)...

I waited 30 minutes and all was good. The next day, down to zero! What the heck?!?!?!?!?!

After doing a bit of research, it turns out that altimeters are not 100% leak proof. In fact, the FAR's publish a table that shows the allowable leak rate. This was new information to me and helped explain why the gauge AND altimeter eventually read zero... but the gloves were still inflated!

Attempt #3:

I finally decided to scrap any gauge and simply connect gloves to every fuel and vent line. This method doesn't tell me the actual pressure inside the tank, but at least will let me know if there is a leak or not.

Following the scientific method, I also inflated an independent glove and taped it off (sitting on top of the center section spar).. this will serve as my "control".

The next day, all gloves (including the control) were ALL fully inflated!! Woooohoooo!!! No leaks!

I kept the gloves on and noted their reaction in the weeks/months that followed. Some days the gloves would be REALLY inflated... and other days it was seem as though they were being sucked into the tank (like a vacuum). Of course, if you've taken high school physics, you would know this is perfectly normal behavior. No, it's not witchcraft, its the ideal gas law doing its thing!

Since the fuel tank is a closed system, the air molecules are expanding and contracting due to the changes in temperature. It was cool to see this play out from day to day! (nerd alert!)

And with that, I will go ahead and stamp these fuel tanks "leak tested"!