Differential Pressure Transmitter Calibration: Step by Step Procedure

Differential pressure transmitter calibration verifies that the transmitter accurately measures the applied differential pressure and produces the expected output across its configured range. A typical calibration checks the Lower Range Value, Upper Range Value, zero point, span, 4 to 20 mA output and intermediate test points.

This procedure covers the preparation, safety checks, calibration setup, zero and span adjustment, five point linearity verification, HART configuration, as found and as left records, and return of the DP transmitter to service.

What is a DP transmitter?

A differential pressure transmitter measures the pressure difference between its high pressure and low pressure connections and converts the measured differential pressure into an output signal such as 4 to 20 mA or a digital communication value.

DP transmitters are commonly used for differential pressure measurement, flow measurement with primary elements such as orifice plates, and level measurement in suitable applications. The transmitter range, LRV, URV, process connections and installation arrangement must be verified before calibration.

What is DP transmitter used for?

Industrial applications include process control, flow measurement, level measurement, and pressure monitoring often employ DP transmitters. For measuring complicated processes, they are often used in combination with other instruments like flowmeters or level sensors.

A differential pressure (DP) transmitter is most often used in industrial applications to determine and monitor the flow, level, and pressure of liquids, gasses, and vapors.

A DP transmitter is used to measure the difference in pressure between the liquid or gas at the bottom of a tank or vessel and the pressure at the top of the tank or vessel. This is done in level measurement applications. The amount of the liquid or gas in the tank or vessel determines how much of a pressure differential there is.

A DP transmitter is used to detect the pressure differential over an obstruction, such as an orifice plate, venturi meter, or flow nozzle, in flow measurement applications. This pressure differential is proportional to the liquid or gas flow rate passing through the obstruction.

A DP transmitter is used to monitor the pressure differential between two points in a pipeline or vessel for pressure measurement applications. You may determine the absolute or gauge pressure at either place using this pressure differential.

Overall, DP transmitters are a popular option for a variety of industrial applications due to their flexibility and precision.

What Is Differential Pressure Transmitter Calibration?

Differential pressure transmitter calibration is the process of applying known differential pressure values to a DP transmitter and comparing its measured value and output with a calibrated reference standard.

For a standard linear 4 to 20 mA transmitter, the Lower Range Value normally corresponds to 4 mA and the Upper Range Value corresponds to 20 mA. Intermediate test points are used to verify the transmitter response across its calibrated range.

LRV, URV and Span in DP Transmitter Calibration

LRV

LRV means Lower Range Value. It is the lower endpoint of the configured transmitter range.

URV

URV means Upper Range Value. It is the upper endpoint of the configured transmitter range.

Span

Span = URV − LRV

For example, for a transmitter ranged from 0 to 100 kPa, the LRV is 0 kPa, the URV is 100 kPa and the span is 100 kPa.

LRV and URV should always be verified from the approved instrument datasheet, calibration record, transmitter configuration or manufacturer’s documentation rather than assumed.

How do you calibrate a DP transmitter?

7 Steps Calibration Procedure for DP Transmitter

This below Step by Step calibration procedure provides a thorough explanation of how to calibrate a DP Transmitter using reference standards in workshop

Step 1: Prepare Tools required for DP transmitter Calibration

1. Necessary hand tools.
2. 24 V power supply source.
3. Standard Pressure Calibrator.
4. Standard Multimeter.
5. Test leads and probes.
6. Tubes and standard fittings 
7. Soft Cloth for cleaning.
8. Communication tool is necessary, such as a Hart communicator if it is a smart transmitter.

Step 2: Safety Precautions Before DP Transmitter Calibration

1. For specifics on basic safety, general recommendations, and calibrating operations in process industries, please click the link provided below. Basic Safety and General Considerations for Process Industries Calibration Process
2. Ask the panel operator to set the controller in manual mode for the control loop and MOS for the ESD loop.
3. Locate the DP transmitter you want to turn off. Make sure that the DP transmitter is the right one and record any key information, such as the Tag number (e.g., the manufacturer, model number, pressure range, etc.).
4. Be sure there is no pressure or fluid flowing through the instrument before removing the DP transmitter by isolating the process.
5. The pipe connecting the DP transmitter to the process should be closed or disconnected. As a result, the transmitter will be separated from the process and no fluid will seep out.
6. By releasing the bleed valves or vent valves on the transmitter, you may release any pressure that could have been trapped within the transmitter.
7. Disconnect the power supply from the DP transmitter. Ensure that any nearby junction boxes or marshalling panels adjacent to the control room have their power sources switched off. (For instance, turn it off or disconnect the cable).
8. Remove the DP transmitter from its mounting bracket after removing the cabling. Remove the transmitter by unscrewing the bolts or nuts that hold it to the bracket.
9. To stop any fluid from seeping out once the transmitter has been removed, plug the lines.
10. Keep the DP transmitter in a secure place where it will not be harmed.
11. After removing the DP transmitter, label it with any relevant info (such as the date, the reason for removal, and so on) and store it safely. In order to prevent contamination or damage, keep the DP transmitter carefully if it has to be reinstalled.
12. While removing a DP transmitter from the field, it’s crucial to follow the manufacturer’s instructions and any necessary safety precautions. To ensure that the removal is done safely and properly, it may also be beneficial to discuss with a skilled technician or engineer.
13. Follow all relevant lockout/tagout procedures to prevent an unintended start. Ensure that the DP transmitter is isolated from the process.

Step 3: Prepare the Calibration Setup for DP transmitter Calibration

1. The calibration equipment needs to be put in a place that doesn’t have any vibrations or electromagnetic interference. In addition, the place must be well-ventilated and light.
2. The test bench has to be clean, solid, and level.
3. Install the transmitter on the test bench with stand.
4. One end of the tube should be connected to the high pressure side (HP) side of the DP transmitter, and the other end should be attached to the output of the pressure calibrator. Check the connections to make sure they are tight and leak-free.
5. The low-pressure side (LP) of the transmitter is always maintained with an open vent to the surrounding atmosphere.
6. The transmitter and the 24-volt power supply are connected in series with a digital multimeter equipped to measure milliampere current. Also connect the 250 ohms resistance series with the transmitter for impedance matching.
7. Attach the HART communicator to the DP transmitter’s terminal.

Equipment Required for DP Transmitter Calibration

Equipment	Purpose
Pressure calibrator	Generates calibrated differential pressure
Digital multimeter	Measures transmitter current output
24 V DC power supply	Powers a loop powered transmitter
HART communicator	Configures and trims smart transmitters
Pressure hoses and fittings	Connects pressure source
Test manifold	Provides safe pressure isolation and equalization
Reference pressure standard	Provides traceable calibration reference
Calibration test leads	Provides electrical connection
Calibration sheet	Records as found and as left results

Step 4: Calibration of Differential Pressure Transmitter

1. Make sure that the output wire connections and input tubing connections on the DP transmitter are secure.
2. Set the power supply unit to 24 v dc to turn on the DP transmitter. Before turning on the circuit, use a multimeter to verify the voltage level in the power supply output.
3. The pressure calibrator should be turned on now.
4. Consult the datasheet for the DP transmitter and make sure in the pressure calibrator’s menu is set to the correct unit.
5. Set the transmitter’s 0% Lower Range Value (LRV) to the LRV of the calibration range.
6. Set the upper range value (URV) of the calibration range for the transmitters’ span to 100%.
7. As an example, the transmitter’s 0% LRV is 4mA and it is calibrated to 0 psi if you are using a differential pressure transmitter with a power output of 4-20mA to monitor pressure in the range 0-50 psi. The 100% URV is 20mA and will be calibrated to 50 psi similarly.
8. Consult the instruction manual to find the transmitter’s ZERO and SPAN/RANGE adjustments in the display menu.
9. Apply the LRV 0% on the high side of the transmitter by pressure calibrator, and 0% on the low side that is vented. 
10. While focusing on the 4mA reading on the multimeter, adjust the ZERO adjustment menu in display or zero adjustment potentiometer of the transmitter. This is your transmitter’s LRV output.
11. Then, apply pressure to the high side of the transmitter to raise the reading to the calibration range’s 100% higher value (URV).
12. While focusing on the 20mA reading on the multimeter, adjust the SPAN adjustment menu in display or SPAN adjustment potentiometer of the transmitter. This is your transmitter’s URV output.
13. For SMART transmitters, we must use a HART communicator. After connecting the communicator and the transmitter, choose the lower range value trim and higher range value trim options from the HART Communicator Menu.

DP Transmitter 4 to 20 mA Output Formula

For a linear transmitter:

mA = 4 + [16 × (Applied DP − LRV) ÷ (URV − LRV)]

The expected output at common calibration points is:

Calibration Point	Expected Output
0%	4 mA
25%	8 mA
50%	12 mA
75%	16 mA
100%	20 mA

1. You will need to perform the method of calibration as many times as necessary until the DP transmitter is calibrated to the appropriate tolerance.
2. Depending on the specific pressure calibrator and DP transmitter being used, the calibration process may change. As a result, please make sure you follow the instructions provided by the manufacturer before starting up.

Step 5: Recording Calibration with Linearity Checks

1. Do linearity tests in the upscale and downscale directions at 0%, 25%, 50%, 75%, and 100% to make that the DP transmitter is producing the correct output values.
2. If the output value does not fall within an acceptable range, calibration is required. Once again, if the output values have deviated from the allowed range, a DP transmitter must be serviced or replaced.
3. No extra calibration is required if every output value (+/- %) falls within acceptable bounds.
4. The output data should be entered into the as found/as left column of the blank calibration report.

Note: Consistent linearity testing and dedicated collection of calibration data are required for maintaining the DP Transmitter’s accuracy and reliability over time.

Step 6: Completion of Calibration

1. When the calibration has been successfully completed, attach the calibration label to the DP transmitter.
2. When the calibration is complete, clean the instrument, store it somewhere safe, and record the calibration data for later use.
3. Disconnect the DP transmitter, pressure calibrators, and other setup.
4. The DP transmitter should be fixed back in the processing area.
5. Make sure the workplace is clean.
6. De-isolate the equipment.
7. Return the bypassed or inhibited signal to its original level.
8. Make that the DP transmitter is working correctly before resuming use of it.

Step 7: Calibration Report Preparation

The following image demonstrates how the calibration of the DP transmitter sample report was completed in a workshop using a pressure calibrator and multimeter as the reference.

The link below gives you access to the Excel template that was used to create the DP transmitter calibration report.

Sensor Trim and Output Trim in Smart DP Transmitters

Smart DP transmitters can provide separate functions for correcting the sensor measurement and correcting the analog output. The terminology and procedure vary by manufacturer, so the transmitter manual should be followed.

Sensor trim corrects the relationship between applied reference pressure and the transmitter’s measured pressure. Output trim verifies or adjusts the relationship between the measured value and the analog 4 to 20 mA output.

Calibration Procedure: Aligned with NIST and IEC 61508 Standards for DP Transmitter

Follow our differential pressure (DP) transmitter calibration procedure to ensure accurate and reliable signal transmission, as well as to improve industrial performance. This method was carefully designed in accordance with the National Institute of requirements and Technology (NIST) requirements.

The International Electrotechnical Commission (IEC) 61508 specifies safety requirements for electrical systems in essential industries. Conforming to its calibration methods is critical for safe and accurate differential pressure measurement. To ensure safety in DP transmitters, the methodical approach offered by IEC 61508 throughout the system’s lifecycle is essential.