Temperature compensation for level sensing technique

[praveenkesavan]

Dear Friends,

I am trying a simple method to sense liquid level in an open tank using hydro-static principle. Though it resembles bubbler/purge method of level sensing, it avoids the need of external purging thus making it simpler. The setup consist of a hollow SS tube of particular diameter which is running till the bottom of tank. The top end of tube is attached to a U-tube manometer via a narrow pipe. Manometer show zero deflection until the liquid level in the tank touches the bottom tip of SS tube. But once the liquid level crossed the bottom tip, a particular volume of air is trapped between the liquid in the SS tube and the manometric fluid.The pressure of this trapped air increases proportionally with the rise in liquid level of tank, which shows a deflection in manometer. The Δh of manometer can be calibrated to liquid level H and the level can be measured.Please find the attachment to get a clear idea about the measurement technique.

Now the issue is temperature compensation. The trapped air expands and contracts w.r.t ambient temperature. For a particular liquid level H, the deflection in manometer is constant provided a constant ambient temperature. If the temperature is increased the pressure also increased showing a positive deflection in Δh and vice verse. This variation Δh vs temperature is linear, but the slope is different for different liquid level H, so a direct compensation is not possible. With the change in temperature the volume, pressure and density of trapped air is changing. I am stuck up with this problem, how to compensate level w.r.t. change in temperature and I am not getting the right way to continue.

 

[CWatters]

Just a thought but suppose you connected a similar size SS pipe (but with a closed bottom) to the other side of the manometer. Put this tube in the liquid as well. Both tubes would have roughly same volume of air in so the variation in pressure due to temperature would be roughly the same.

Edit: Would need some valves to allow the set up to be zeroed.

 

[praveenkesavan]

Thanks for your suggestion.
"suppose you connected a similar size SS pipe (but with a closed bottom) to the other side of the manometer. Put this tube in the liquid as well". As of now the other end of manometer is opened to atmosphere, but if I close this end using a similar size SS pipe, the Δh in manometer per unit change in liquid level will be reduced thus reducing the overall sensitivity in measurement.
"Both tubes would have roughly same volume of air in so the variation in pressure due to temperature would be roughly the same." The major part of the trapped volume of air is contributed by the bigger dia. SS pipe inserted to the tank. As the level increases the pressure increases and volume decreases. This change in volume may not exactly match with the volume of air trapped in the other end of manometer. The effect of temperature on trapped air at different pressure and volume varies.