If differential pressure flow measurement assumes constant density but density varies, what should be done?

• A. Ignore density changes.
• B. Keep density constant and use same equation.
• C. Change to a volumetric flow meter.
• D. Recalculate flow using updated density.

Answer: (D)

Differential pressure meters relate a pressure drop to a flow rate using a relationship that includes fluid density. If density isn’t actually constant, applying the same density value in the equation gives an incorrect flow. The way to keep the measurement accurate is to use the real, updated density in the calculation and recompute the flow accordingly. Remember that mass flow is the product of density and volumetric flow, ṁ = ρQ, so changes in density directly affect the interpreted value unless you adjust Q with the current density. If density variations are large or difficult to track, a mass-flow measurement approach may be considered, but the given best step is to recalculate using the updated density.