Pressure gauge calibration calculator: error, hysteresis and head correction

How the calculation works

1. 01Head correction: when the reference and the gauge are at different heights in a liquid, the column adds or removes ρ g h. With the gauge above the reference it sees less pressure than the reference reads, so the pressure at the gauge is P_ref − ρ g h. For water that is about 97.9 Pa per centimetre.
2. 02The error at each point is the gauge reading minus the pressure at the gauge, for the rising series and, if supplied, the falling series. Hysteresis is the difference between the falling and rising readings at the same reference pressure.
3. 03The largest error and the largest hysteresis are reported in the gauge's units and as a percentage of its span, and checked against a tolerance you enter. No tolerance is built in, because the acceptable error belongs to your procedure or the gauge's accuracy class.

Limitations

• A pass compares the gauge's errors with your tolerance and ignores the uncertainty of the calibration itself. If the margin is small, a conformity statement needs an agreed decision rule.
• The head correction is for a liquid medium. For a gas the column is negligible and is normally ignored, and the fluid presets are typical figures rather than a measured density.
• It does not handle absolute, gauge or differential reference conversion, temperature effects on the gauge, or the reference's own uncertainty.
• Readings must be in one unit, the one selected. Convert first with the pressure converter if the reference and the gauge differ.

Frequently asked questions

What is head correction in pressure calibration?

The pressure difference caused by the height difference between the reference and the gauge when the medium is a liquid. It is ρ g h: about 97.9 Pa per centimetre of water, 8.5 kPa for a metre of oil. On a low-range gauge it can be larger than the gauge's own accuracy, so it is a correction rather than a detail.

What is hysteresis in a pressure gauge?

The difference between the reading on the way up and on the way down at the same pressure. A Bourdon tube or diaphragm does not return along the path it took, and on a mechanical gauge hysteresis is often the largest single error, which is why a calibration with only a rising series flatters the gauge.

How many points should a pressure gauge calibration have?

At least five across the span, including zero and full scale, in both directions, is the usual minimum. More points are worth it if the gauge is used over a narrow band you care about.

Which sign does the head correction have?

If the gauge is above the reference, the gauge sees less pressure than the reference reads, so the correction is negative. If the gauge is below, it is positive. The calculator takes the height as positive when the gauge is higher.

Does this tell me whether the gauge passes?

It tells you whether the largest error is inside the tolerance you entered, and nothing about whether that tolerance is the right one. Accuracy classes and acceptance limits come from the gauge's standard or your own quality system, and a result close to the limit needs the uncertainty of the calibration taken into account.