Pressure calibration
Pressure calibration compares a device with a reference at several points, up and down the range. Head height, temperature and the choice of gauge or absolute reference all matter as soon as you want better than 0.1 %.
Instruments and typical accuracy
| Instrument | Typical range | Typical accuracy | Where it is used |
|---|---|---|---|
| Piston gauge (deadweight tester) | Around 1 kPa to over 1 GPa | 0.008 to 0.05 % of reading | Primary or secondary standard, calibrates references |
| Digital reference transducer or calibrator | Vacuum to about 100 MPa, in ranges | 0.01 to 0.05 % of span | Working standard for most labs |
| Pressure controller | Many ranges | 0.01 to 0.05 % of span | Automated calibration of gauges and transmitters |
| Test gauge, ASME grade 4A to 1A | Many ranges | 0.1 %, 0.25 %, 0.5 %, 1 % of span | Reference gauges and critical process |
| Industrial pressure transmitter | mbar to hundreds of MPa | 0.04 to 0.25 % of span | Process plant and OEM equipment |
| Liquid column manometer | Up to about 100 kPa | Limited by reading the meniscus | Low pressure and draft checks |
Types of pressure: gauge (against atmosphere), absolute (against vacuum) and differential. The accuracy figures are typical ranges to help you compare instruments. Always use the specification of the instrument in front of you.
ASME B40.100 gauge accuracy grades
| Grade | Accuracy (% of span) | Typical use |
|---|---|---|
| 4A | ±0.1 % | Test and reference gauges |
| 3A | ±0.25 % | Test gauges |
| 2A | ±0.5 % | Test gauges and critical process |
| 1A | ±1 % | Process gauges |
| A | ±2 % in the first and last quarter, ±1 % in the middle half | Industrial gauges |
| B | ±3 % in the first and last quarter, ±2 % in the middle half | General purpose |
Grades C and D are coarser still. Gauges at the ends of the scale are the least accurate, so choose a range that puts your normal pressure in the middle half.
Reference standards and traceability
- Piston gauges. Pressure from mass, local gravity and effective area, so traceable to mass, length and time. The primary working standard in many labs.
- Digital references. Calibrated against a piston gauge or higher reference, with a stated uncertainty and drift over time.
- Gauge standards. EN 837 and ASME B40.100 define accuracy grades and tests for analogue gauges.
- Quality system. ISO/IEC 17025, with local gravity and air buoyancy corrections for piston gauges.
How it is calibrated
Typical procedure
- Check for leaks and let the system settle at each point.
- Zero the device (for gauge devices, vented to atmosphere) and record the zero error.
- Connect the device to the reference at the same height, or apply a head correction.
- Exercise the device by taking it to full scale once or twice.
- Apply 5 to 10 points across the range, commonly 0, 25, 50, 75 and 100 %, going up and then down.
- Wait for stability, record the reference and the device, and compute error and hysteresis.
- Report as-found and as-left readings with the uncertainty.
Where it goes wrong
- Head height. A 250 mm height difference in oil is about 21 mbar, which is 0.2 % of a 10 bar range.
- Leaks and temperature. Gas compresses and warms when pressurised: wait before reading.
- Wrong media or orientation. Oil in an oxygen gauge is dangerous and a gauge calibrated vertically may read differently on its side.
- Gauge against absolute. Mixing them makes a barometric error of about 1 bar.
- Hysteresis ignored. Only calibrating going up hides the largest error on many gauges.
What goes in the uncertainty budget
- Reference standard uncertainty and drift.
- Head correction and fluid density.
- Temperature of the device and of the reference.
- Zero, resolution and repeatability of the device.
- Hysteresis and, for piston gauges, local gravity and air buoyancy.
Combine the contributions as in the uncertainty budget calculator, then decide pass or fail with a guard band as explained on the calculators page.
Calculators
Everything runs in your browser. Nothing you type is sent anywhere.