RS Hydro > Flow Meters > Doppler

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.

We supply an extensive range of doppler flow meters for open channel measurement. Flow is measured via a sensor on the channel bed, or using a non-contacting sensor that measures the velocity of the surface water.
Doppler flow meters offer a cost effective, non-invasive solution for measuring flow rate in pipes.
Frequency
shift of signals
reflected by
air bubbles and
particles being
directly proportional
to flow rate.
Intrinsically safe doppler open channel flow meters designed for use in
hazardous environments.

Doppler Flow Measurement

Isco 4120 Submerged Probe Flow MeterThe 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.

Specifications

Although specifications are different between the various manufacturers, the following is intended as a guideline to their performance and specification.

Flow Range 0.06-10m/sec (Bi-Directional)
Accuracy ±1-5% of reading
Repeatability Typically ±0.5%
Pipe Size 12.7-5000mm
Operating Temperature
Transducers -40°C to +120°C
Transmitter -20°C to 60°C
Datalogging Typically>100,000 data points, time stamped
Output 4-20mA, RS232
Power Built-in battery - external 12v input, 8-24 hours operation
Dimensions Typically 325 x 275 x 150mm
Weight (Control Unit) Typically 3-7kg

Technical Notes

In order to obtain the optimum accuracy from a doppler sensor, the following conditions must apply:

Requirement
Location At least 10 diameters of straight-length pipework must exist upstream of the measurement point, and 5D downstream. Ideally these distances should be as large as possible.
Liquid Medium It is essential that the type of fluid is known. Also that there are no air bubbles and that the solid content does not exceed 5%.
Pipe Condition The pipe wall exterior should be free from dirt, paint or corrosion. For optimum accuracy the interior wall should also be free from deposits or corrosion.
Solid Content Solid Content ideally should be >5% or contain air bubbles.