LEVEL MEASUREMENT AND METHODS

RIDHAM SHAH



LEVEL MEASUREMENT 
  • Level measurement is used to set up the quantity of a substance within a vessel. This may be a solid or liquid substance like oil, chemicals, coal, dust, etc. The level measurement devices can be used to measure this quantity.  
  • In modern manufacturing industries which use many solvents, chemicals, steam, and other liquids and in power plant which uses the vast amount of water, the accurate measurement of liquid level is very essential.  

Level Measurement – Principle

There are three main different principles for level measurement

  1. The height of the liquid surface
  2. The pressure head
  3. The weight of the material

There are two methods used to measure the level of a  liquid
  1. Direct Methods
  2. Indirect or inferential Methods 

 Direct Methods ( Visual Methods )

  • The direct method measures the height above a zero point by any of the following methods.
  • Direct methods for level measurement are mainly used where level changes are small and slow such as Sump tanks and Bulk storage tanks.
  • Direct methods are simple, reliable and low cost .
  • There are four types of direct level measurement devices:   



1.    Hook – type of level indicator

2.    Dipstick

3.    Sight glass technique
 4.    Float type - level indicator    



Sight Glass technique
  • A sight glass is used for continuous measurement of the liquid level within the tank or vessel. 
  • The sight glass consists of toughened glass which connected with the bottom of the tank or vessel
  • As the level of liquid is increased in the tank or vessel parallel sight glass level increases equally and as the liquid level decreases sight glass level also decreases. 
  • By this technique level of tank or vessel can be measured.  

Hook type indicator

  • This level indicator consists of corrosive resistance material into U shape with one long arm.
  • This longer arm is attached to a slider with Vernier scale which attached with main scale
  • Shorter arm (u shape) is pointed with 60◦ taper
  • This device is pushed below the liquid surface and then raise up gradually till it is about to break  through the level   
  • the rod is then clamped and the level is readout from the scale. 


Float or potentiometer type level indicator 


  • Float type measurement is the simplest measurement and design of level measurement.
  • In this design,float rests on the liquid, as the liquid level will be change float position also change.
  • The float balance with a counterweight which is hang another side of the tank.
  • When liquid level will decrease, the float will also move down and counterweight will move upwards direction until it balances the float.
  • Same as when the liquid level will increases, the float moves upwards direction and counterweight will move downwards until it balances the balance.
  • Due to this change in float position, the pointer which is attached with float it will also change.
  • The pointer connected with a potentiometer,  which converts it in electrical form and it displays mV or V.
  • According to that indication,we can measure level in the tank.


Indirect (inferential) Methods

  • The indirect or inferential method of measurement uses the changing position of the liquid surface to determine level with reference to a datum line.
  • It can be used for low & high levels where the use of the direct method instruments is impractical.

1. Hydrostatic Pressure Methods
2. Displacement devices
3. Capacitance Level Instrumentation
4. Radiation-Based Level Gauges


 

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