PLC

Calibration Of The DP (Differential Pressure) Level Transmitter :

Updated: Jan 26

In this article, we will learn calculation require in a calibration process of the Differential Pressure level transmitter with a clear explanation that makes you feel easy to apply the same in the field.


To do the calibration of the level transmitter we have to calculate the value of the LRV (lower range value) and URV (upper range value) and also you have to check that whether the tank is open or closed.


After knowing types of the tank you have to check where the transmitter is installed. You can install the transmitter in three-way which is listed below.


1. Zero elevation (Transmitter is installed at 0% or at 4mA signal)

2. Suppression (Transmitter is installed below the 0%)

3. Elevation (Transmitter installed above 0% level)


Here, we will discuss zero elevation and we will see the equation for both open tank and closed tank which is used to determine the calibration value.

OPEN TANK


Below is the diagram of the open tank with transmitter installed at zero elevation.

To calculate differential pressure due to the tank is open, the transmitter's LP side is kept open to the atmosphere so, it will compensate pressure of the atmosphere.


DP = HP - LP

= (P(atm) + P(liquid)) - P(atm)

DP = P(liquid)


where, HP= High pressure

LP= Low pressure

atm= Atmospheric pressure


Due to the open tank and DP transmitter, atmospheric pressure cancels out and you will have the pressure of the liquid only.


CLOSE TANK


If the tank is closed then, we have two possibilities,


1. DRY LEG (Empty leg)


Differential pressure is given by,


DP = (P(liquid) + P(vapour)) - P(vapour)


Due to P(vapour) cancels out,


DP = P(liquid)


Here the tank is closed so, on the HP side pressure will be the sum of filled liquid and vapour pressure. On the LP side due to dry leg, there is nothing filled inside thus the only pressure act is due to vapour.

2. WET LEG (FILLED LIQUID- MOSTLY GLYCOL)


Here, we have filled liquid present in the leg which can be mostly glycol.


Now, the Differential Pressure is given by,


DP = HP - LP

= (P(liquid) + P(vapour)) - (P(vapour + P(SF))

where, SF(sealed fluid) - specific gravity of filled liquid in a wet leg.


Due to the cancellation of Pvapour,


DP = P(liquid) - P(SF)


Above all the calculation is required to calculate LRV for the transmitter. In the next article, I will be discussing the calculation of the LRV and the URV with the help of the example.


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