A very important consideration for Doppler ultrasonic flow measurement is that the calibration
of the flowmeter varies with the speed of sound through the fluid (c). This is readily apparent by
the presence of c in the above equation: as c increases, f must proportionately decrease for any
fixed volumetric flow rate Q. Since the flowmeter is designed to directly interpret flow rate in terms
of f, an increase in c causing a decrease in f will thus register as a decrease in Q. This means
the speed of sound for a fluid must be precisely known in order for a Doppler ultrasonic flowmeter
to accurately measure flow