If the calibration equipment and the gauge to be calibrated are at a different height, the hydrostatic pressure of the pressure media in the piping can cause errors. This normally is not an issue when gas is used as the media, as gas is light compared to liquid. But when liquid is used as media, the liquid in the piping will have a weight due hydrostatic pressure and can cause errors. The magnitude of the error depends on the density of the liquid and the difference in height, as the gravity is pulling the liquid inside the tubing. If it is not possible to have the calibrator and gauge at the same height, then the effect of height difference should be calculated and taken into account during the calibration.
An example of effect of hydrostatic pressure: Hydrostatic pressure is calculated as follows: Ph = ρ g h Where: Ph = hydrostatic pressure ρ = density of liquid (kg/m3) g = local gravity (m/s2) h = height difference (m) As on example: if water is the media (density 997.56 kg/m3), local gravity is 9.8 m/s2 and there is a 1 meter (3.3 feet) difference between the DUT and the reference equipment, this will cause an error of 9.8 kPa (98 mbar or 1.42 psi).
Note that depending on the pressure to be measured, the error caused by the height difference can be significant.