Level Measurement Part 1

Level Measurement,

(A part of Instrumentation Basics...)

We will be discussing about level measurement which is very vital part in plant automation. Under the industrial category of instrumentation, it is very important to measure levels of fluid for open and closed loop systems.

There are two ways to measure level, the Direct Method and the Indirect Method.

For direct level measurement, we can use the Bob and tape and Sight glass methods

A bob weight and measuring tape provide the most simple and direct method of measuring liquid level. See picture at the right for the graphical presentation.

One can also use a sight glass. This consists of a graduated glass tube mounted on the side of the vessel. As the level of the liquid in the vessel change, so does the level of the liquid in the glass tube.

For the indirect level measurement, One can use pressure gauges.

This is the simplest method, for pressure gauge is located at the zero level of the liquid in the vessel. Any rise in the level causes an increase of pressure which can be measured by any gauge.

Another way is through the use of a purge system. In this method a pipe is installed vertically with the open end at zero level. The other end of the pipe is connected to a regulated air supply and to a pressure gauge. To make a level measurement the air supply is adjusted so that pressure is slightly higher than the pressure due to height of the liquid. This is accomplished by regulating the air pressure until bubbles cab be seen slowly leaving the open end of the pipe.

The air pressure to the bubbler pipe is minutely in Excess of the liquid pressure in the vessel, so that Air pressure indicated is a measure of the level in The tank.

The method above is suitable for open tank applications. When a liquid is in a pressure vessel, the liquid column pressure can't be used unless the vessel pressure is balanced out. This is done through the use of different pressure meters.

Through differential pressure meters, one can also measure the level of liquid inside the tank.Connections are made at the vessel top and bottom, and to the two columns of the D.P. meter. The top connection is made to the L.P. column of the transmitter and the bottom to H.P. column of the transmitter. The difference in pressure in the vessel is balanced out, since it is fed to both the column of the meter. The difference in pressure deducted by the meter will be due only to the changing, level of the liquid.

Another way is through the displacer type level measurement.The leveltrol is one of the most common instruments used measuring level in closed tanks. This instrument works of Archimedes principle. The displacer in immersed in the liquid due to which there is loss of weight depending on the specified gravity of the liquid. This displacer hangs freely on a knife transmitted to the pneumatic or electronic counterpart at the other end.