Doppler Flow Meters

Doppler flow meters are most often used in flow measurement applications where transit time flow meters are not a viable option. Typical applications that utilise doppler technology include flow measurement of wastewater, slurries, acids, chemicals and viscous liquids.

Doppler Flow Measurement

The doppler flowmeter sensor is installed in partially full and surcharged pipes, rectangular, trapezoid and egg-shaped channels. The flow velocity is measured using an ultrasonic doppler signal (c.0.5MHz) which is continuously emitted into the flow path. Liquid flowing over the doppler flowmeter must contain sonically reflective materials such as solid particles or entrained air bubbles. The movement of these materials alters the frequency of the beam reflected onto a second, receiving transducer. The frequency shift is linearly proportional to the rate of flow of materials in the pipe or channel and therefore can be used to develop an analog or digital signal proportional to flow rate.

To measure the water level the doppler flow meter can either (1) transmit an ultrasonic pulse through the water overhead to the liquid's surface (the instrument can interpret the level of water (head) to within ±0.25%); or (2) utilise a down-looking ultrasonic sensor for highly aerated or turbulent flows; or (3) utilise an internal/external pressure transducer to measure the head.

It is widely accepted that doppler flowmeters can measure flow velocity to within ±2%. However, reality dictates that inaccuracies are greater due to flow instabilities, cross-sectional mis-calculations. However, one device is able to interpret the velocity profile by measuring the velocity at various cross-sectional intervals.