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I am measuring water level in a 25-inch tall vase using hydrostatic pressure. Like this.

To do this I have a 1/4 OD inch nylon tube connected to a gauge pressure transducer. So far it has been very accurate. If I put the tip of the tube into 1-inch of water I read 1-inch. Same if I dip the tube down to 24-inches of water and anywhere in between.

But I noticed when I leave it sit overnight that in the morning (the tube was submerged all night) that there is a significant error. When I pull the tube out of the water I see some water remains in the tube (about the same amount as the error). If I tap the tube and get all the water out it becomes accurate again.

So my question is what is causing this. Where is the air pressure going? I've tried to seal the fitting between the sensor and tube the best I can. It is already a pretty tight fit. It is one continuous tube. Incase there was a pinhole I've tried multiple tubes and all seem to yield similar results.

Is it feasible for a system like this to work for multiple months at a time without draining the water out of the tube all the way?

I graphed the output with tube at a constant depth and here are the results. I started at 15.9" of water at ~2:00 pm and by 8:45 pm the reading is 15.1" of water.

Graph of water level change at a constant level of 15.9"

In case the details matter here is what I am using:

  • MPX5010GP - pressure gauge
  • 1/4 inch OD (0.17" ID) vinyl tubing 6' long (like this one)
  • 25" tall glass cylinder about 6" ID (see photo below) w/ 3 tubes running three tests

enter image description here

Added Info 9:43pm 5/2

Did another test to eliminate the sensor/tube junction. I put a loop in a tube and filled part of it with water (you can see it on the left side of the picture above. Purple mark is the water starting point). Then I pushed the other end of the tube into several inches of water which cause the water in the loop to rise. I've only had it there but it looks like the water in the loop is very slowly sinking which seems to yield the same result as the sensor. I am going to let this run overnight and check all measurements in the morning. Tomorrow I will buy different tubing to see if that has any effect on this.

Added Info 10:38pm 5/3

I did not lose much pressure in the loop. Maybe I did not have the tube deep enough in the water... Or worse there is a leak inside the sensor...

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  • $\begingroup$ How precise does your measurement have to be? $\endgroup$
    – David White
    May 3, 2019 at 17:38
  • $\begingroup$ Not super precise I would like it to be within .25" of reality. But it needs to works for months or better years without adjustment once installed. $\endgroup$
    – Yepher
    May 3, 2019 at 20:36
  • $\begingroup$ OK. I'll give you an alternative. There is a level device called a bubbler. If you slowly bubble air through the water column, you will need enough air pressure to overcome the static pressure of the water column that you are pushing the air through. If you measure this pressure, you will know how deep the water is. Advantages - the air flow will slowly and continuously sweep the tygon tubing; it's impossible for a liquid level to corrupt your pressure reading as long as a small amount of air flows. $\endgroup$
    – David White
    May 4, 2019 at 1:46
  • $\begingroup$ I am trying to stick with the current design until it is proved not reliable. But I love the bubbler idea. $\endgroup$
    – Yepher
    May 4, 2019 at 15:14

1 Answer 1

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Many things are possibly occurring here, as follows.

First, your tube is probably tygon, not nylon. The diffusion rate of air and water vapor through tygon tubing is nonzero, and that diffusion is driven by concentration gradients across the tube wall. This means that the amount of diffusive transport (which might make the level of water in the tube go up or down) will depend on ambient temperature and relative humidity.

Second, although water may not wet the tygon well upon first exposure, it will wet it better on timescales of order ~tens of hours, which means that capillarity effects can alter the level of water in the system and thereby affect the pressure head measurements.

Third, air is soluble in water, and will either dissolve into or exsolve out of the water depending on temperature and the initial concentration of air in the water. This will also affect the water column height and hence the pressure readings over time.

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  • $\begingroup$ Do you know how tubes are usually spec'd for diffusion or air? $\endgroup$
    – Yepher
    May 3, 2019 at 11:38
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    $\begingroup$ This is a hard question with which I struggled for most of my career, where I had to measure small pressure differences accurately. There is a company that makes peristaltic pumps (MASTERFLEX) and they sell many kinds of tubing to go with them, Check their spec lists for "GAS TRANSMISSION RATE". $\endgroup$
    – niels nielsen
    May 3, 2019 at 16:39

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