Pressure Transmitter & Switch Calibration

Equipment

Pressure source: hand pump (pneumatic up to ~40 bar, hydraulic above), or a dead-weight tester for the highest accuracy.
Reference gauge or calibrator with a valid calibration certificate. It should be at least 4 times more accurate than the instrument under test (a 4:1 test uncertainty ratio).
mA meter (usually inside a documenting calibrator) and a 24 V loop supply.
HART communicator for smart transmitters (to read the PV and to trim).
Hoses and fittings rated for the test pressure, and the right fluid: never use oil-filled equipment on oxygen service.

Set-up

Pressure from the pump goes to the reference gauge and the transmitter at the same time; the calibrator powers the loop and reads the mA.

Isolate the transmitter from the process (manifold sequence for DP: see DP Transmitters), vent, and drain if liquid-filled.
Connect the pump, reference and transmitter; leak-test at the top pressure (the reading must hold steady).
For DP transmitters, apply pressure to the H side with L vented to atmosphere.
Mount the transmitter in its normal orientation, or zero it after mounting: position affects the zero.
Exercise the instrument: go to 100 % and back once before recording.

The 5-point up/down test

Apply 0, 25, 50, 75 and 100 % of the calibrated range rising, then 75, 50, 25, 0 % falling. Approach each point from one direction only (don't overshoot and come back), and let the reading settle.

%	Applied (0–10 bar)	Expected mA	Rising mA	Falling mA	Error % span
0	0.000	4.000	4.012	4.016	+0.10
25	2.500	8.000	8.010	8.020	+0.13
50	5.000	12.000	12.008	12.018	+0.11
75	7.500	16.000	16.006	16.014	+0.09
100	10.000	20.000	20.004	–	+0.03

The difference between rising and falling readings at the same point is hysteresis. This example passes a ±0.25 % tolerance.

Calculating the error

Error (% of span) = (measured mA − expected mA) / 16 mA × 100

Try it:

Expected mA Measured mA Tolerance ±%

Compare the worst error against the tolerance on the calibration sheet (often ±0.25 % or ±0.5 % of span; check your site's standard). Expected currents for any range: 4–20 mA Signal Scaler.

Adjusting: which trim?

Re-range (LRV/URV) changes what pressure the 4 and 20 mA represent. It is a configuration change, not a calibration. See Transmitter Re-range.
Sensor trim (zero and span, also called lower/upper sensor trim) corrects the digital pressure reading against your reference. Do it when the HART PV is wrong.
Analog output (D/A) trim corrects the 4 and 20 mA output against your meter. Do it when the PV is right but the mA is off.
Zero trim at line pressure (DP) removes static-pressure and mounting offsets after installation.

After any adjustment, repeat the full 5-point test and record both as-found and as-left results.

Pressure switches: set point, reset and deadband

A high-pressure switch trips at the set point on rising pressure and resets at a lower pressure. The gap is the deadband (differential).

Connect a continuity tester or meter across the contacts (NO or NC as used in service).
Raise the pressure slowly near the set point and record when the contact changes.
Lower slowly and record the reset point. Deadband = set − reset (for a high switch).
Adjust the set point screw if needed and repeat three times for repeatability.
For a low switch, the set point is on falling pressure and it resets on rising.

Records

A calibration certificate or sheet should show: tag, serial, range, tolerance, reference equipment and its certificate number and due date, as-found and as-left data, pass/fail, date, technician and next due date. A documenting calibrator can store and upload these automatically.