What are the limitations of different types of Level transmitters?


Limitations of different type of level transmitters

Differential pressure/Diaphragm seals

Transmitter measure pressure at the bottom of tank and converts that pressure to a level reading.

Limitations -
1.Assumes a specific gravity to calculate level
2.Must contact liquid to measure
3.Works on liquids but not solids
4.Capillary seals have issues with vacuum,high/low temperature,varying process temps and generally fragile.

Float Transmitter

Float transmitter have a float that moves up and down as they float on the surface level.The float position is detected via variety of methods.

Limitations -
1.Float transmitters work on variety of liquids provided the liquids SG is lower than that of the float.
2.The moving float can have issues if the product is sticky or impeds movement.
3.Doesnt work on solids
4.Most contact liquid to measure it.

Displacer Transmitter

A displacer has a large float which is connected to a torque tube.As the level rise the float pushes up on the torque tube which increasing force.This force is converted to level.

Limitations -
1.The displacer assumes a SG of the liquid.If it changes,the level reading changes.
2.Doest work on solids
3.Must contact liquid to measure it.

Weigh Cells

Weigh cells use strain gauges to measure the weight of a vessel and the material within.Weight is converted to level.

Limitation -
1.Weigh cells assume a SG of the material to determine level
2.Weigh cells are difficult to retrofit
3.Weigh cells tend to be expensive
4,Proper installation on a process vessel with piping is very difficult

Capacitance Transmitters

A rod extended down into a tank and measures the conductivity/RF admittance between the probe and tank wall.As the level changes this electrical value changes and determines level

Limitation -
1.Reading is very much affected by the electrical characteristics of product
2.If the conductivity changes the reading will change too.
3.Calibration is difficult -must be set on actual level
4.Must contact to measure