Deadweight Tester Pressure Calculator
Calculate the pressure a deadweight tester (pressure balance) really generates, applying the corrections for local gravity, air buoyancy, piston temperature and the height difference to the instrument under test.
- F = m × g × (1 − ρair/ρmass) = 97.8492 N
- At = A₂₀ × [1 + α(t − 20)] = 80.6037 mm²
- p = F ÷ At = 12.1395 bar
- pDUT = p − (ρfluid − ρair) × g × h = 12.1227 bar
Pressure Balance Equation
pDUT = p − (ρfluid − ρair) × g × h
m is the total mass of the weights and piston, g the local gravity (use the local gravity calculator), ρm the density of the weights, A₂₀ the effective area from the calibration certificate, αp + αc the sum of the piston and cylinder expansion coefficients and h the height of the device under test above the piston reference level. Elastic distortion at high pressure is not included; use the certificate value if it is significant.
Worked example
A 10 kg load on an 80.6 mm² piston at 25 °C with local g = 9.7864 m/s² gives 12.1395 bar at the piston, and 12.1227 bar at a gauge 0.2 m higher in oil, about 0.36 % below the nominal 12.1671 bar.
Frequently Asked Questions
Why correct for local gravity?
The force of the weights is m × g. Gravity varies with latitude and altitude by up to about 0.5 %, which changes the pressure directly.
What is air buoyancy correction?
Air pushes up on the weights. For steel weights in air the force is reduced by about 0.015 %.
What is the head correction?
If the gauge is higher or lower than the piston, the fluid column between them changes the pressure the gauge sees.
Learn more on the blog
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