I'm trying to measure water level in a 3.5m tank by using a 3mm airline connected to an I2C gauge air pressure sensor. Sure enough the water head above the airline opening at the tank bottom pushes some water into the tube until the pressures equalise.
Problem is the trapped air pressure gradually drops at ~1cm/hr. Figuring I had a leak I glued the airline to the sensor, same result. In frustration I fitted a balloon to the open airline end, now the trapped air pressure holds constant for a given depth.
I don't understand why removing the air-water interface has this effect. Air is not very soluble in water so why the continuous trapped air pressure decrease without the balloon?
Air certainly does dissolve in water, so bubblers are commonly used to avoid those sorts of problems. The air pressure required to form a minimal stream of bubbles is only slightly higher than the water pressure at the bubbler outlet.
A simpler and more accurate method of measuring the water level is with a gauge water pressure sensor at the tank outlet (as long is water is not flowing in the outlet).
I had a similar problem a few years back with a manometer. I found that the air was dissolving in the water. I put mineral oil on top of the water, that solved my problem.
The balloon is acting as a surge tank, air at 1 atmosphere the pressure is equal to 10332.56 Millimeters Of Water (mmH2O). Give it enough time and it will repeat but much slower.
Yeah air pressure dropping about 1.5cm of head/day with the balloon. Who'd have thought keeping air in a tube would be so much harder than keeping the smoke inside electronics.