How To Calibrate Differential Pressure Transmitter

First, use a three-valve group or a five-valve group to safely isolate the transmitter from the process flow. When operating, be sure to first open the balance valve, close the high and low pressure side cut-off valves, and then open the open valve to completely drain the pressure. Then, a high-precision pressure source is connected to the high-pressure side, and a digital multimeter is connected in series in the loop to monitor the 4-20 mA signal. Confirm the upper and lower limits of range using a HART hand drill. First do a “zero-point fine adjustment” at normal pressure, then press down according to the range of 0%, 25%, 50%, 75%, and 100% in sequence, focusing on the output current. If the current reading is found to exceed the error allowed by the accuracy, it must be recalibrated by “sensor trim” using a hand drill. Before the meter can be returned to the operator, it must be confirmed that both its linearity and hysteresis meet industrial specifications.

Isolation Operation And Valve Group Management

Before you move the wrench, safety always comes first. The operation of the three-valve group or the five-valve group is the most prone to errors in the entire process, and it is the key to protecting the transmitter membrane box from damage caused by one-way overpressure.

Specific operation: Make it a habit to open the balance valve first, which can make the pressure on both sides of the DP unit equal in an instant. The isolation valves for high pressure and low pressure are then closed.

Pressure relief: Unscrew the exhaust valve or pressure relief screw to release the residual pressure. This step is so critical that the transmitter is not considered to be in a true “zero point” state until it is ensured that it is fully exposed to atmospheric pressure.

Construction Of Precision Calibration Instruments

The accuracy of the calibration work depends entirely on the guy in your hand. The accuracy of the check instrument is at least 4 times higher than that of the transmitter under test.

Pressure source: Connect the high-precision hand pump to the HP end. The joints must be carefully checked and there must not be any slight leakage, otherwise the pressure will not be stable during the five-point test and the data will be all messed up.

Signal measurement: The multimeter is connected to the output. If you can’t find the device using the HART hand aerator, it’s probably because the 250 ohm resistor is missing from the return line. Without it, the HART signal is too weak in the 24V DC power supply and cannot be connected at all.

Configuration Of HART Hand Aerobics

Take out the Rosemount TREX or 475 standard tools and connect them to this smart transmitter.

Parameter check: Go to the device settings menu and stare to see if LRV (lower range) and URV (upper range) are compatible with the process card. For example, if the range is 0 to 100 mbar, then we need to confirm that these two numbers are correct.

Zero Trim: While the valve group is still in equilibrium and emptied into the atmosphere, tap it directly “Zero Trim”. This tells the transmitter: “The current state is absolute zero.”

Sensor Fine-Tuning And Correction

If the current reading on the multimeter is extremely poor, then it is necessary to do “sensor fine-tuning”.

Full-range fine-tuning: Do fine-tuning at 0% “lower limit fine-tuning” and fine-tuning at 100% “upper limit fine-tuning” on the hand aerator. This is actually a re-correction of the analog-to-digital conversion logic inside the transmitter.

Recovery And Commissioning

Once the transmitter’s linearity and hysteresis are perfect, it can be cut back into the process. In order: turn off the bleed valve, turn off the balance valve, and then slowly open the high and low pressure isolation valves to resume normal flow or level measurement.

As long as you strictly follow this SOP (Standard Operating Procedure), you can ensure that the differential pressure transmitter provides the most accurate data to the control system.

Author: Robert Miller

Having calibrated thousands of smart transmitters in challenging field environments, I specialize in transforming complex technical manuals into practical, safety-first operating procedures. My mission is to share field-tested insights that help fellow technicians and engineers achieve maximum precision in industrial automation.