A Doppler ultrasonic flow meter operates on the Doppler shift principle , whereby the transmitted frequency is altered linearly by being reflected from particles and bubbles in the fluid. The doppler flow meter net result is a frequency shift between doppler transmitter and receiver frequencies that can be directly related to the flow velocity and the doppler flow meter can calulate volume if the pipe internal diameter is known. The doppler meter can calculate the volumetric flow rate but a minimum amount of solid particles or air in the line to achieve measurements. These are called reflectors.