Dry calibration holds true only when the liquid in the tank is at ambient temperature. When the liquid is hot or much colder than the ambient temperature, the liquid vapor or the condensate will fill the dry - leg. Under this condition, the dry calibration does not hold true. In such a situation, wet calibration method is to be adopted.
Wet Calibration
The LP leg of the DP transmitter set-up is to be filled with some buffer solution.
The buffer solution normally used in industries is diesel, glycol, glycerin or the same liquid of the tank. Now, you have to follow the dry leg calibration process, and re-ranging to complete the wet leg calibration process.
Wet Calibration Method
-
Make both the inlet pressures at HP & LP equal to the atmospheric pressure by opening both the valves V1 & V2.
-
The 0mA (Minimum span) calibration is done.
-
Now, close V1 & V2, open the isolation valve.
-
The HP inlet is subjected to maximum span and calibrates the transmitter to 20mA.
-
Now, open the dry reference leg valve, the LP side will be subjected to the minimum span.
-
Re-range the transmitter.
-
The dry method of calibration is complete.