An often utilized technique of indirectly measuring a liquid level is to measure the hydrostatic pressure at the bottom of the container. The level can then be extrapolated from the pressure and the specific weight of the liquid. This is the principle that a bubbler device operation is based on.
Pressure is commonly used as an indirect method of measuring liquid levels. Pressure increases as the depth increases in a fluid. The pressure is given by:
p = ϒh
Where p is the pressure, ϒ is the specific weight of the liquid (alternatively we can use density (ρ) of the liquid), and h is the depth. The specific weight is temperature dependent, hence temperature correction is required.
A bubbler device for measuring liquid level is illustrated below:
Bubbler devices require a supply of clean air or inert gas to prevent interaction with the liquid as shown in Figure 1 above. Gas from a pressure regulator is forced through a tube via a flow regulator and the open end of the tube is close to the bottom of the tank. The specific weight of the gas is negligible compared to the specific weight of the liquid and can be ignored. The pressure required to force the liquid out of the tube is equal to the pressure at the end of the tube due to the liquid, which is the depth of the liquid multiplied by the specific weight of the liquid (requiring temperature correction).
Since pressure, p = ϒh, we can calculate the height, h = p/ϒ
Therefore if we measure the pressure required to force the liquid, and we know the specific weight or the density of the liquid, we can easily calculate the height which is the level of the liquid. This technique of level measurement can be used with corrosive liquids, because the material of the tube can be selected to be corrosion resistance. Additionally, electrical power is not needed however variations in specific weight due to temperature changes will affect the results of the measurement thus temperature correction should be considered.
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