Gas flow compensation
Correct a gas flow meter reading for actual pressure, temperature, molecular weight and compressibility — for DP and volumetric meters.
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How this calculator works
A differential-pressure meter measures ΔP, and the flow depends on the gas density: mass flow ∝ √(ΔP·ρ). The meter (or its transmitter) was set up for one design density. When the pressure, temperature, molecular weight or Z change, the reading is off by the square root of the density ratio, with ρ = P·MW/(Z·R·T). For a standard-volume reading the factor becomes √(PaTdZdMWd/(PdTaZaMWa)). A volumetric meter measures actual volume directly; turning it into standard volume needs P, T and Z only, so its factor is PaTdZd/(PdTaZa).
The example: a DP meter set up for 20 bar a, 30 °C, Z 0.96 and MW 18 now runs at 18 bar a, 40 °C, Z 0.97 and MW 19. The 1,000 reading is really 903.8 — the uncorrected meter reads 10.6 % high.
Limits: square-root meters with a fixed coefficient; expansibility and discharge-coefficient changes with flow are not included (use the orifice calculator for those). Use absolute pressures. References: ISO 5167-1 (flow and density relationship); Miller, Flow Measurement Engineering Handbook; AGA Report No. 3 and API MPMS Ch. 14.3 (basis for natural gas metering).