Pressure Instrument Calibration: How to Verify Accuracy of Transmitters and Pressure Switches

Calibration consists of comparing the instrument reading against a reference standard of known and higher accuracy than the instrument, and adjusting or documenting deviations. The result can be an adjustment (AS-LEFT) or simply a verification certificate without adjustment.

1. Calibration bench elements

• Pressure standard: High-accuracy digital gauge or calibrator, with traceable calibration certificate. It should have at least 3 to 4 times better accuracy than the unit under test.
• Pressure pump or source: Manual or electric, to generate and control pressure (hand pump, screw pump, or one that generates vacuum and pressure).
• Comparator (manifold): Allows connecting simultaneously the standard and the instrument to the same pressure.
• Power supply and mA meter: To power the loop and read the 4-20 mA signal of the transmitter.

MARPATECH recommends the AMETEK Crystal portable pressure calibrators, which combine a high-accuracy standard and pressure generation in a single device with data logging and export.

2. Preparation

Before calibrating:

• Verify that the instrument is isolated from the process (block valve) and safely depressurized.
• Connect standard, instrument and pump in the comparator. Check for leaks (by applying pressure and observing stability).
• Define the calibration range (test points) based on the instrument range. Usually 5 points at 0%, 25%, 50%, 75% and 100% of range, ascending and descending.
• Allow the instrument to reach thermal equilibrium and record ambient temperature as reference.

3. Calibration procedure

1. AS-FOUND: Apply the first point (0%) and record the instrument reading without modifying anything. Repeat for ascending and descending points. This documents the initial state.
2. Compare: Calculate the error in % of output or reading for each point. A typical maximum tolerable error is ±0.1% FS for control, but it depends on the process requirement.
3. AS-LEFT (if adjusted): Perform zero and span adjustment per the manual, repeat the points and record the final reading.
4. Verify hysteresis and repeatability: The difference between ascending and descending points must not exceed defined criteria.
5. Document: Save date, standard used (with its certificate), conditions, AS-FOUND/AS-LEFT results and the calibration certificate.

4. Pressure switch calibration

To calibrate a pressure switch, pressure is applied slowly and controlled and the actuation point (energize/de-energize) is recorded in both directions:

• Set Point (actuation): The pressure at which the switch changes state when rising.
• Reset Point: The pressure at which the switch returns to the initial state when falling.
• Differential (hysteresis): The difference between set point and reset point. It must be adjusted according to the application (protection, control).

A multimeter or continuity lamp is used to detect the state change and a pressure standard to read the exact value at the moment of change.

5. Frequency and traceability

Calibration frequency depends on criticality, manufacturer, experience and plant requirements (for example, every 6 or 12 months). A good management program includes state labeling (CAL/OK) and prioritization planning. Standard traceability to national or international standards must be documented.

Conclusion

Pressure instrument calibration is a systematic process that requires a traceable standard, a clear AS-FOUND/AS-LEFT procedure and proper documentation. Having quality portable equipment and planning frequencies reduces downtime and improves measurement reliability.

MARPATECH SAC represents in Peru, Argentina and Colombia the AMETEK Crystal pressure calibrators and provides technical support to implement and optimize calibration programs in industry.