Pakarab fertilizers limited multan

80 Field Instrumentation GEN Rev)

4.1 Level measurement

Level measurement is important for proper process operation and also for cost accounting and inventory purposes. Liquid measurement needs two reference points. Surface of liquid being measured Datum point (fixed reference point, either bottom or top of tank.
4.2 Methods of Level measurement

The common methods employed for automatic continuous liquid level measurement areas follows. Displacement (Buoyancy) Head (Bubble tube, Diaphragm box, Pressure, Differential pressure) Capacitance Radiation (Nucleonic) Ultrasonic
4.3 Displacement Buoyancy method

It is a type of force balance transmitter. It is based on Archimedes’s principle which states as A body immersed in a liquid will be buoyed upward by a force equal to the weight of liquid displaced. This method is used to measure liquid level by sensing the buoyant force exerted on a displacer by the liquid in which it is immersed. The buoyant force on an object depends on how much liquid is displaced and the density of the liquid. The buoyant force always equals the weight of the displaced liquid. If the buoyant force becomes equal to the object’s weight, the object floats.
Displacer element is a cylinder of constant cross sectional area and heavier than the liquid displaced. This method is used for both open and closed tanks. Buoyancy transmitter is normally used in vessels where lower connection is not possible/permissible, fluctuating pressures or levels and high temperature service.

81 Field Instrumentation GEN Rev)

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