Measurement of liquid level is important in a variety of industrial processes. The liquid level may be expressed in terms of the pressure column exerts over a datum level or in terms of the length of the liquid column.
Liquid level may be measured by two methods:
- Direct method
- Indirect method
In direct methods the hydrostatic pressure of the liquid column is measured.
Indirect methods of liquid level measurements generally used in industries are:
- Hydrostatic pressure type
- Electrical methods.
Hydrostatic pressure type:
A liquid in a tank at rest exerts a force on the wall of the tank. This force in a liquid at rest is known as hydrostatic pressure. This is proportional to the depth (or height) of liquid in the tank.
Hydrostatic pressure methods used for liquid level measurement are listed below:
- Pressure gauge method
- Air Bellows
- Air Purge System (or) bubbler system
- Liquid purge system
- Diaphragm box type
- Force balance type
In electrical methods, liquid levels are converted into electrical signals and it can be measured by electrical or electronic means.
The types of electrical methods are:
- Resistive (or) contact point type
- Inductive method.
- Capacitive method