Basics of Level Measurement

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Level Measurement Basics 

Very simple systems employ external sight glasses or tubes to view the height and hence the volume of the fluid. Others utilize floats connected to variable potentiometers or rheostats that will change the resistance according to the amount of motion of the float. This signal is then inputted to transmitters that send a signal to an instrument calibrated to read out the height or volume. 

In this module, we will examine the more challenging situations that require inferential level measurement. This technique obtains a level indication indirectly by monitoring the pressure exerted by the height of the liquid in the vessel. 

The pressure at the base of a vessel containing liquid is directly proportional to the height of the liquid in the vessel. This is termed hydrostatic pressure. As the level in the vessel rises, the pressure exerted by the liquid at the base of the vessel will increase linearly. Mathematically, we have: 

P = S.H

where 

P = Pressure (Pa) 

S = Weight density of the liquid (N/M³) = ρg 

H = Height of liquid column (m) 

ρ = Density (kg/m³) 

g = acceleration due to gravity (9.81 m/s²) 

The level of liquid inside a tank can be determined from the pressure reading if the weight density of the liquid is constant. 

Differential Pressure (DP) capsules are the most commonly used devices to measure the pressure at the base of a tank.

When a DP transmitter is used for the purpose of measuring a level, it will be called a level transmitter. 

To obtain maximum sensitivity, a pressure capsule has to be used, that has a sensitivity range that closely matches the anticipated pressure of the measured liquid. However, system pressures are often much higher than the actual hydrostatic pressure that is to be measured. If the process pressure is accidentally applied to only one side of the DP capsule during installation or removal of the DP cell from service, over ranging of the capsule would occur and the capsule could be damaged causing erroneous indications.

  1. Three Valve Manifold
  2. Open Tank Measurement
  3. Closed Tank Measurement
  4. Bubbler Level Measurement System
  5. Effect of Temperature on Level Measurement
  6. Effect of Pressure on Level Measurement
  7. Level Measurement System Errors 

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