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What are the methods of level measurement?

There are two ways of measuring level :
1.    Direct
2.    Indirect.

1.Direct level measurement :

( a ) Bob and tape:
Heighest point reached by A bob weight and measuring tape provide the most simple and direct method of measuring liquid.                             
Bob and Tape Level Measurement

( b ) 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.

Indirect level measurement :

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

( b ) Purge system :
In this method a pipe is installed vertically with the open and at zero level. The other end of the pipe is connected  to a regulated air r 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.
Purge System Level Measurement
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 are 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.

( c ) Differential pressure meter :
Connection 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.

( d )  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
Comments (3)Add Comment
written by fredapinto, March 20, 2014
Follow the rule of (ASME) American society of mechanical engineering and get vessels more safe for future.smilies/smiley.gif
written by fredapinto, March 20, 2014
Follow the rule of ASME and get vessels more safe for future.smilies/smiley.gif
written by Mocham, June 07, 2015
Thanks for information...

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