 David W. Spitzer

How is an orifice-plate flowmeter measurement in a flow control loop affected by an increase in fluid density?

A. Decreases flow measurement linearly with fluid density
B. Decreases flow measurement as square of fluid density
C. Decreases flow measurement as the inverse square root of fluid density
D. None of the above

Commentary
The output of a differential-pressure transmitter (Q) is proportional to the square root of the ratio of the differential pressure (ΔP) to the fluid density (ρ).

Q α ( ΔP / ρ ) ½

The flow control loop maintains the output of the differential-pressure transmitter constant, so ΔP remains constant. Fluid density is in the denominator and within the square root, so increasing fluid density will decrease the flow measurement as the inverse of the square root of fluid density. Answer C is correct. Many differential-pressure flowmeters (especially measuring gases) are compensated for fluid density by using transmitters that measure the actual operating conditions, such as pressure and temperature. Calculations are then performed to compensate the raw differential-pressure measurement for the measured operating conditions.