Pressure explainer: head correction, the metre that moves your pascal

Two pressure instruments connected by a metre of tubing can disagree by more than their combined specifications — not because either is wrong, but because of the fluid column between them. Head correction is the unglamorous term for accounting for it.

The physics is one line: Δp = ρ·g·h, the density of the pressurizing medium times local gravity times the height difference between the instrument's reference level and the point where pressure is defined. For a water-filled line, a metre of height is nearly 10 kPa; for dry nitrogen it is roughly 12 Pa — small, but decisive at the uncertainties a deadweight tester works at. Gauge, absolute, and differential modes each move the correction's sign and reference in their own way.

The metrological habit is to declare a reference level on every certificate and budget the correction's own uncertainty (density, temperature, the height measurement itself) per the GUM. EURAMET's freely downloadable calibration-guide series covers the practice for pressure balances and transducers in detail.