Differential Pressure Transmitter Calibration

Differential Pressure Transmitter

General

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:

General

Dry Calibration or Dry Leg Calibration method of a Differential Pressure Transmitter is performed in a closed, pressurized tank.

Mounting

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

1. Make both the inlet pressures at HP & LP equal to the atmospheric pressure by opening both the valves V1 & V2.
2. The 0mA (Minimum span) calibration is done.
3. Now, close V1 & V2, open the isolation valve.
4. The HP inlet is subjected to maximum span and calibrates the transmitter to 20mA.
5. Now, open the dry reference leg valve, the LP side will be subjected to the minimum span.
6. Re-range the transmitter.
7. The dry method of calibration is complete.

Wet Calibration of Differential Pressure Transmitter:

General

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

1. Make both the inlet pressures at HP & LP equal to the atmospheric pressure by opening both the valves V1 & V2.
2. The 0mA (Minimum span) calibration is done.
3. Now, close V1 & V2, open the isolation valve.
4. The HP inlet is subjected to maximum span and calibrates the transmitter to 20mA.
5. Now, open the dry reference leg valve, the LP side will be subjected to the minimum span.
6. Re-range the transmitter.
7. The dry method of calibration is complete.

Differential Pressure Transmitter Capillary Tubes

When you use capillaries you need to do the calibration completely differently from normal, so be careful when using capillaries in level application.

Information Needed

Data sheet

Equipment Needed

Pressure calibrator (std.)
Multimeter (std.)
Hart communicator (used in this example).

Steps to Calibrate

1. Ensure that the control valve is isolated from the process if working on live system or newly installed system. “IMPORTANT USE LOCK OUT TAG OUT Procedures”
2. Make sure the controller is in manual mode for the control loop.
3. Verify steps 1 and 2 are done a second time to insure safety of yourself and others that may not be involved in the process being worked on.
4. Open vent valve at drip ring and open plug at the top of the drip ring (if available) to release the process pressure.
5. Clean the liquid inside the drip ring and put back the plug for high side only (if available).
6. Expose the low side to atmosphere.
7. Hook up a multimeter in series with the signal to the DCS to measure current signal.
8. In this condition both capillary flange will be atmospheric pressure at difference high.
9. Multimeter should show 4mA and PV at HART communicator should shows 0 (LRV).
10. If not, do zero adjustment at transmitter using HART Communicator.
11. Connect pressure calibrator to high side flange (drip ring).
12. Apply pressure depend data sheet span (URV).
13. Multimeter should show 20 mA and PV at HART communicator should shows same with URV.
14. If not, do span adjustment at transmitter using HART Communicator.
15. Verify the linearity by increasing and decreasing the pressure (0%, 25%, 50%, 75%, 100%, 75%, 50%, 25% and 0%of range).
16. After the calibration and it has been verified put loops back in normal mode or normalize the MOS.
17. Document all work having been done on standard calibration form and properly log in system used and update any job books required by company.

4-20 mA & Pneumatic Differential Pressure Transmitter Calibration.

These transmitters cannot measure in the negative so you need to change the HP and LP sides around so that the HP side goes to the top of the vessel and the LP side goes to the bottom tap off point on the vessel. You now need to do your calibration in the reverse.

Steps to Calibrate

1. Find the ATM value first, in other words max positive differential (HP wet leg filled and LP open to atmosphere) on the transmitter this will now be your ATM value will say (+1350mmH2o).
2. Actual zero will now be 20mA and not 4mA and will be determined by making use of the ATM value minus the actual zero measured value, multiplied by the liquid density.
3. The actual 100% value will be determined by making use of the ATM value minus the actual 100% measured value, multiplied by the density. You should end up with something like this, zero = +1250mmH3o = 20 mA and 100% = +150mmH2o = 4 mA.
4. Finally the display on your remote level indicator, this needs to be changed as well otherwise it will read in the reverse.
5. If you use a pneumatic Differential Pressure Transmitter just substitute the 4 and 20 mA with the 20 to 100Kps or 3 - 15PSI the principle stays the same.

The above are to help in calibration but may not be what you will need to do for your specific installation. Please always refer to the manufacturer's recommendations on how to calibrate the Transmitter(s) you are using and always refer to the project specifications and data sheets.

Due to the fact I am writing several methods I do make errors so if I have missed something or have made an error please send me a message and I will correct.