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.
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.
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 |
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. |