The short answer
How to Calibrate a Pressure Transmitter
To calibrate a pressure transmitter, isolate it from the process and vent it, apply zero and a series of known pressures from a calibrator while reading the output current, and compare each to the expected 4-20 mA value. Record the as-found data. If the error exceeds the tolerance, perform a sensor trim against the reference pressures and, separately, a current output trim against a reference meter, then repeat the five points as-left and record both sets.
Key points
- As-found before any adjustment. That record is the value of the calibration.
- Sensor trim and output trim are different adjustments. Do not use one to correct the other.
- A zero check in place is not a calibration. It catches drift but not span error.
- Tolerance comes from the loop, not the transmitter data sheet. Decide it before you start.
- Return the transmitter to service in the right valve sequence, or the first reading after calibration is wrong.
What you need
- A pressure calibrator or hand pump with a reference gauge at least four times more accurate than the tolerance
- A loop calibrator or precision milliammeter to read the output
- A HART communicator or the platform software, for trims on a smart transmitter
- The transmitter calibrated range and the loop tolerance from the instrument list or loop sheet
- The calibration form, paper or electronic
- Fittings and a bleed for the impulse line or manifold
Procedure
- 1
Gather the facts
Calibrated range, for example 0 to 100 psig. Expected output at each test point: 4 mA at 0, 8 at 25, 12 at 50, 16 at 75, 20 at 100 psig. Tolerance for the loop, typically 0.25 to 1 percent of span for utility service. Check the calibrator range and accuracy against these.
- 2
Isolate and vent
Close the process isolation valve. Open the vent or the manifold drain to release the line pressure. On a differential transmitter, open the equalizer first, then the drains. Confirm zero pressure at the transmitter before connecting anything.
- 3
Connect the calibrator and the meter
Pressure source to the transmitter high side. Milliammeter in series with the loop, or the loop calibrator in measure mode across the test jacks. Let the transmitter and calibrator sit for a few minutes to reach the same temperature.
- 4
Record as-found at five points
Apply 0, 25, 50, 75, and 100 percent of range going up, then 75, 50, 25, and 0 coming down. Record the applied pressure from the reference and the output current at each point. The down points show hysteresis. Do not adjust anything yet.
- 5
Decide whether to adjust
Compare each point with the expected current. If every point is within tolerance, the transmitter passes as-found and nothing is adjusted: record it and return to service. Adjusting a transmitter that is in tolerance adds error rather than removing it.
- 6
Trim the sensor if needed
On a smart transmitter, the sensor trim corrects what the transmitter measures. Use the communicator: a zero trim at true zero pressure, then a lower and upper sensor trim at reference pressures near the ends of the range. On an analog transmitter, adjust the zero and span potentiometers in turn, rechecking each after adjusting the other.
- 7
Trim the output if needed
The output trim corrects the digital-to-analog conversion: the transmitter is told to send exactly 4 mA and 20 mA, the meter reading is entered, and the transmitter corrects itself. This is separate from the sensor trim. If the sensor reads right in the communicator but the current is off, the output trim is the adjustment, not the sensor.
- 8
Record as-left at five points
Repeat the up-and-down series and record every point. Both as-found and as-left go on the form. Confirm every as-left point is within tolerance.
- 9
Return to service
Remove the calibrator, close the vent, and open the isolation valve slowly. On a differential transmitter, open the high side isolation, then close the equalizer, then open the low side. Check for leaks. Confirm the reading agrees with a local gauge or the process expectation and that the loop in the control system reads correctly.
- 10
Complete the record
Date, technician, transmitter tag and serial, calibrator identification and its calibration due date, as-found and as-left data, tolerance, pass or fail, and any adjustment made. Return the loop to auto and tell operations.
Sensor trim versus output trim
| Symptom | Adjustment | Not this |
|---|---|---|
| Communicator shows the transmitter measuring the wrong pressure | Sensor trim | Output trim, which would leave the digital value wrong for HART readers |
| Communicator shows the right pressure but the loop current is off | Output trim at 4 and 20 mA | Sensor trim, which would corrupt a correct measurement |
| Right pressure and right current, but the PLC reads wrong | PLC scaling or the input card | Any transmitter adjustment |
| Zero is off, span is right | Zero trim only | A full re-trim that moves the span too |
Verification
- As-found and as-left data at five points, up and down, both on the form.
- Every as-left point within the loop tolerance.
- The control system displays the correct value with the transmitter back in service.
- The valve sequence was completed and there are no leaks.
- The calibrator used was itself in calibration.
Frequently asked questions
- How often should a pressure transmitter be calibrated?
- On the schedule the utility sets, based on the criticality of the loop and the history of the transmitter. Annually is common; compliance measurements may require more. The as-found records tell you: a transmitter that is always in tolerance at twelve months can go longer, one that drifts needs a shorter interval or replacement.
- Can I calibrate in place without isolating?
- A zero check can sometimes be done by equalizing a differential transmitter or, with the process at a known state, comparing to a reference gauge. It catches zero drift. It does not check span, and it is not a calibration for the record.
- What accuracy does the calibrator need?
- The reference should be at least four times more accurate than the tolerance being checked, and ten times is better. For a 0.5 percent loop tolerance, a 0.1 percent reference is adequate and a 0.05 percent reference is comfortable. A calibrator that has not itself been calibrated is not a reference.
- The transmitter passed calibration but the PLC still reads wrong. Why?
- The problem is downstream: the input card range, the PLC scaling, or the calibrated range in the PLC not matching the one in the transmitter. Inject a known current at the panel terminals and follow the value through the scaling.
Related topics
- Pressure TransmittersGauge, absolute, and differential pressure transmitters: how they sense, how range, turndown, and accuracy specifications work, the process connection and its accessories, and the installation errors that show up as calibration problems.
- How to Test a 4-20 mA LoopProve a current loop end to end with a meter and a calibrator, without interrupting a process you were not authorized to interrupt.
- HARTDigital data riding on the 4-20 mA loop: how HART signaling works, what a handheld or a HART-enabled input can read, multidrop and burst modes, and the wiring conditions that make HART fail while the current loop keeps working.
- How to Scale a 4-20 mA Input in a PLCTurn raw analog counts into engineering units, verify at three points, and add the validity checks that stop a failed transmitter from producing a believable number.
- 4-20 mA Signals from the Instrument SideHow a 4-20 mA current loop is powered and wired at the transmitter, why the loop resistance budget matters, and how to check a loop with a meter.
- How to Diagnose a Ground LoopProve that a noisy or offset analog signal is a ground loop, find the second ground, and fix it the right way: remove the extra ground, isolate the signal, or correct the shield termination. With the measurements that make the diagnosis certain.
Direct contact
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