Differential Pressure Transmitters are 2-wire transmitters. These are loop powered devices with 4-20mA current loop protocol as the standard. The lower range is 4mA, and the upper range is 20mA.
The difference between lower range and upper range is called span of the transmitter. The span requirement of the application pressure range is to be matched to the transmitter span. The transmitter reading is most accurate when it uses most of its span during measurement.
Re-ranging calibration method is required for all types of Differential Pressure Transmitter applications.
Dry leg Calibration is used only for level measurement in a closed tank with liquids at ambient temperature.
Wet leg calibration method is required for level measurement in a closed tank with hot and intensely cold liquids.
Before you can do this calibration you need to know the ATM value for the installation. The atmospheric value (ATM) can be read directly from the transmitter by disconnecting the HP side (Bottom) and open it up to atmosphere, so the only pressure on the transmitter is on the LP side and this will obviously push the transmitter into the negative. Maximum negative differential pressure for a installation = ATM pressure.
Make sure the LP line is filled to the position where it will start to run back into the vessel, then read off the displayed value on the transmitter. This is your ATM value. In this example it might be something like -1350mmH2o. This value is determined by, where you have installed the transmitter and what you use for a buffer solution. To calculate the actual zero and 100% positions on the vessel you do the same as before and just measure from the transmitter to you zero and 100% positions on the vessel, multiply them with the density of the liquid you are measuring and add them to the ATM value. You can then input these values to this transmitter’s LRV and URV and the calibration is done.
So assuming you have installed the transmitter slightly below the lower tap off point the above LRV and URV is about right in relation to the ATM value in this example. Be sure to understand the difference between the ATM value and the LRV it will in most cases not be the same. The more accurately you can determine your ATM value the more accurate the calibration will be.
Calibration of DPT
Some engineers like to reverse calibrate the transmitter so it is at 4 ma on maximum differential pressure (drum empty) and 20 ma on minimum differential pressure (drum full). So always refer to Data Sheets and any Project Specifications in reference to the transmitter you are going to calibrate.
Dry Calibration of a Differential Pressure Transmitter:
Dry Calibration or Dry Leg Calibration method of a Differential Pressure Transmitter is performed in a closed, pressurized tank.
In a closed tank, the bottom most point is the HP or High Pressure point and the top most point is the LP or the Low Pressure Point.
The transmitter is mounted near the HP or bottom most position, and the HP inlet of the transmitter is connected to the bottom most position of the vessel through an Isolation valve. The LP inlet of the transmitter is connected through a pipe to the Top most point.
Dry 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.