Building the manometer was actually quite simple. It requires a flat board (~30" x 8"), 10' of clear vinyl tubing (1/4" ID, 3/8" OD), a plastic barbed T, a plastic shut off valve, several anchoring clamps - all available from Lowes.

As shown, I drew a set of horizontal lines 1" apart as my scale. Then I held down the tubing with the anchoring clamps. One end of the tubing is open while the other end is plugged into the plastic barbed T. The other end of the barbed T goes to the air vent of the fuel tank. The middle of the barbed T is connected to the shut off valve. This is where I feed in the air.

Before I tied my home made manometer to the closed fuel tank, I wanted to make sure the connections of the manometer are air tight. So I plugged up the tube end that is supposed to tie to the tank. Since 18" is not a whole lot of pressure, I can easily blow on it and get my 18" of water - especially when it is not tied to the tank. Here's a picture of the shut off valve for feeding and shutting off the air to the system.

After I blew in the air and shut the feed valve off, I found a few leaks right off. They were from my sloppy assembly skills. With a bit a attention to details, I finally got the water mark to stabilize inside the tubing. However, it was still dropping at a very slow rate. Soap water at the joints produced no bubble. I decided to give up for the night, leaving the system pressurized.

The next day afternoon, I noticed the water mark actually rose up to higher location than my original mark. Evidently, changes in room temperature change the water pressure quite a bit - and was captured by this simple manometer. I was surprised how sensitive this manometer was! I decided to get a feel for the performance of the unit before using it to perform the pressure/leak test, I added more resolution (1/10") to the scale. I also clamped a digital thermometer next to it and took some pressure vs. temperature data. 

After a few days of data collection, I learned that the water mark movement is consistent with change of temperature in the room. The water mark also repeats under the same temperature. I concluded that the manometer, as constructed, is not leaking. Its ready for the pressure/leak test after the top skin is in place!

Tire Valve and Compressor

Enter my trusted compressor and a tire valve from AutoZone. I know people recommend not to use the compressor because you can delaminate the strakes if you are not careful, but between the strakes and me, the choice was clear...

I floxed a short 1/4" tube into the tire valve and replaced the blue shut off valve (shown above). Then I turned the output pressure of my compressor to a very low setting. Now I can add (or release) air to/from the manometer system easily. You will get the hang of it after a couple of trials, the higher the compressor pressure, the quicker you can fill the strake to the desired pressure - but you'll get more water level oscillation once you stop. On the other hand, if you turn the compressor pressure way down, it will take much longer to fill the strakes, but minimal water level oscillation when you reach the desired pressure. I chose the latter and it worked well for me.

Here's the final set up for my pressure/leak test. Susann added some food coloring dye to the water column such that I could see the water level better. Through the trial runs prior, I learned that change of room temperature causes dramatic changes in air pressure in the tank (as reported via the manometer). So I was careful not to put too much air into the system at the beginning of the test.

At the beginning of my test, room temperature was around 84.6^oF and the weather report was projecting 95^oF the next day. Note that we are supposed to leave the pressure in the system for 24 hours per plan. Therefore, if I pumped the system to 20" of water at start, it will be much higher later on. I decided to bring the initial pressure to 14" of water.

I monitored the water pressure in the system vs. room temperature throughout the test period. I converted the temperature to Celsius such that I can fit both data onto the same scale.

Note that I started the test on my left tank at 4:37 pm. I filled the system to 14.6" of water - I took the reading after the oscillation of water level. The room temperature at the time was 29.2^oC.

I collected the room temperature and the water pressure level periodically until ~9:00pm. Note the room temperature and pressure were dropping correspondingly. 

No data was taken through the night until I returned the next morning ~9:23am. Room temperature was around 21.9^oC and the water level was down to ~5" (it dropped below my high resolution scale). With that reading, I could have a leak in the system or room temperature has a dramatic effect to the pressure/water level. I decided to continue to collect the temperature vs. pressure data through the rest of the 24 hours. As the room temperature rose through the day, so did the pressure in the tank. The most important point was that when the room temperature repeats (as the previous day), my water pressure returned to the same (or a bit higher) than the previous day - but not less! Note that the room temperature repeats twice that day - as it rises pass 29.2^oC around 1:58pm and drops pass it again at around 6:39pm (indicated by the 2 red vertical lines). At both times, the water pressure showed higher than 15" of water (15.8 & 16.6 or translated to 0.046 & 0.077 psi respectively). The slight variation in pressure at the same temperature could be contributed to the accuracy of the thermometer, the cooling rate of the air mass inside the tank vs. room temperature, change in barometric pressure during the test period and others. Unfortunately, I did not record the barometric pressure throughout the test period, otherwise, I could have added that into my calculations. The great news is that there is no loss in air pressure in the system and no detectable air leak with my bubble soap at all seams!  I concluded the test at 9:00pm. The tank had seen pressure ranging from 5" of water to 18" of water in the past 29 hours of pressure test. I repeated the same on the right tank with similar results.