Introduction to Level Measurement with Differential Pressure Transmitters ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Introduction to Level Measurement with Differential Pressure Transmitters

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Most level measurement done is achieved through inference. Inferential level measurement techniques obtain a level indication by monitoring the pressure exerted by the column of liquid.
The pressure exerted at the base of a column of liquid is given by:

Pressure = Height of column of liquid * Density of Liquid * Acceleration due to gravity

               Pressure  =  H*D*g
In terms of weight density(N/m3), S, we can organize the above formula to read:

               Pressure = H*S

Because the specific gravity of a liquid can be expressed as the ratio of the weight of the volume of a liquid to that of an equivalent volume of water, in level measurement applications, we can define level as:
Level = Height * Specific gravity of liquid = H * S.G
Where H = Height of liquid to be measured
S.G = Specific gravity of the liquid



Therefore, in order to determine liquid level, it is essential that the specific gravity of a liquid be known. Level is then determined by multiplying the liquid height above the point of measurement by the specific gravity of the liquid as indicated in the formula above.
Another point worth noting is that differential pressure transmitters are used as level transmitters to measure level. In the course of this discussion, level transmitter is used interchangeably with differential pressure transmitter.




Open Tank Level Measurement
Level measurement in open tanks is the simplest. The differential pressure transmitter for level measurement is normally mounted at or below the tank connection in order to measure the pressure of the liquid above. The pressure is sensed by the process flange and transmitted to the high side of the sensing element of the transmitter. The low pressure side is vented to the atmosphere as shown below:
Open Tank Level Measurement Installation

Here, the level transmitter is mounted at the base of the open tank corresponding to the zero level of measurement.

P(high) = P(atm) + S.G * H and P(low) = P(atm)
Therefore, Differential Pressure, ΔP = P(atm) + S.G * H – P(atm) = S.G * H



With the above scenario, the level transmitter can be calibrated to output 4 mA when the tank is at 0% level and 20 mA when the tank is at 100% level. Note that differential pressure meant for level measurement is normally expressed in inches of water column(in wc)



Please note that in all the drawings used here and other discussions in Level measurement, the level transmitter valve manifold and all forms of isolating valves have been omitted. In actual level measurement installations, you will find them. They are omitted here to make our discussion easy and simple.

Continue to the next sections on Level Measurement: