# Calibration of DP type level transmitters with zero suppression

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## DP type level transmitters with zero suppression

Above is the construction diagram of Differential pressure type level transmitter and it is installed to some distance at base from level tank .
When the liquid level is up to minimum level it will exert pressure       ( SG × X ) + P atm.
When the liquid level is  up to maximum level it will exert pressure P = ( SG × X ) + ( SG × H ) + P atm

Now question arise what is zero suppression ?
If we calculate the differential pressure
Differential pressure = P (high) – P  (low)
={ ( SG × X ) + ( SG × H ) + P atm} – {  ( SG × X ) + P atm }
= SG × H
Now the transmitter output is proportional only to SG × H. DP transmitter calibration suppresses the (SG × X ) by negatively biasing the – (SG × X ). Therefore we say DP transmitter calibration with zero suppression.

Example
We consider maximum level of tank is 440 inch and specific gravity of liquid is  0.8 . If the DP level transmitter is installed below the tank 50 inch distance at base level. What will  be transmitter ranges and calibration values?
Solution:
According to above diagram and formulas
Min. Range =   ( SG × X ) + P atm
= 0.8 × 50 + 0 = 40  inch
Max. Range = ( SG × X ) + ( SG × H ) + P atm
= ( 0.8 × 50 ) + ( 0.8 × 440 ) + 0  =  392 inch

The calibration procedure will be
When 0 % tank level height ( H = 0 ), minimum level range value is 40 inch and the transmitter output will be 4 mA.
when 100 % tank level height ( H= 440 ), maximum range level value is 392 inch and the transmitter output will be 20 mA.