Doppler Flow Meter
Portable Doppler flowmeters work in a similar way to transit time meters and open channel doppler flow meters in that they emit ultrasonic signals into the flow path. Permanent and portable doppler flowmeters are generally used where no other flowmeter can be used - primarily in the process and wastewater industries. Although fixed and portable doppler flowmeters do not share the same accuracies as transit time meters, they are reknowned for establishing flowrates in very difficult measurement conditions. However, it is recommended that where conditions allow, alternative technologies should be used.
Some of the pipe doppler flowmeters available from RS Hydro:
The Doppler effect, named after Austrian physicist J. C. Doppler who first described it for sound in 1842, states that waves emitted from a source moving toward an observer are squeezed; i.e. the wave's wavelength is decreased and frequency is increased, as shown in the schematic below. Conversely, waves emitted from a source moving away from an observer are stretched; i.e. the wave's wavelength is increased and frequency is decreased. To use the Doppler effect to measure flow in a pipe, one transducer transmits an ultrasonic beam of c.0.5 MHz into the flow path. Liquid flowing through the pipe 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 and therefore can be used to develop an analog or digital signal proportional to flow rate.
Advantages of Doppler Flowmeters
- Easy to install on existing pipelines (wrap around versions)
- Up to 24 hours battery operation (battery versions)
- non-invasive, no moving parts, no wear
- No sensor fouling
- Integrated datalogger
- Simple and easy to use
Drawbacks and Limitations of the Doppler Flowmeter
- Requires significant number of reflecting particles in the medium.
- The particles must be large enough to provide sufficiently good reflections.
- The particulate sound velocity should be different to that of the liquid.
- Doppler flow measurement usually only extends into the peripheral flow. Hence measurement is only ideal for fully developed flow profiles.
- The velocity needs to be far higher than the critical velocity at which particles settle.
Which Portable Doppler Flowmeter ?
You can obtain a quotation for the following doppler flowmeters using our online instrumentation store.
| Manufacturer | Model | Verdict |
| Greyline | PDFM-IV | 70% |
| EESIFLO | EASZ-10P | 50% |
| Polysonics | SX30/40/50 | 80% |
The above verdicts are based RS Hydro's own experience and that of its customers.
Applications
Doppler flow meters can measure almost all fluids containing significant entrained deposits or air bubbles. The basic requirement is that the fluid can conduct ultrasound and have an acceptable axis-symmetrical flow.
This is a test
Typical applications include:
- Slurries
- Primary Sludge
- Dredging
- Blood Flow
- Activated Sludge
- Ideal for fluids with high solid content (>5%)
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 with 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. |
