Doppler Flowmeters for Open Channels
Doppler Open Channel Flowmeters utilise a similar technology to the versions used for pipe-flow measurement. Doppler technology has been used for open channel flow measurement invasively and non-invasively. Some doppler flowmeters are deployed on the channel-bed whilst others measure the velocity of the surface water. However, to compute the overall flowrate (Q=V*A), the head measurement is determined by utilising a level sensor. For in-channel doppler flowmeters this is normally achieved by utilising a hydrostatic sensor (pressure) although ultrasonic level devices can be used. The reverse is true for non-invasive devices ie. level can be determined using ultrasonic or hydrostatic level sensors.
Open channel flowmeters available from RS Hydro:
How it works...
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.
Advantages of Doppler Ultrasonic Flowmeters
- Permanent or Portable Versions available
- Up to 12 months battery operation
- Simple in-channel installation
- Limited sensor fouling
- Integrated Datalogger
- Simple and easy to use
- No structure required
Applications
- Open Channel Flow Measurement
- Sewer Flow Measurement/CSO Monitoring
- MCERTS
Which Portable Ultrasonic Flowmeter ?
You can obtain a quotation for the following flowmeters using our online store.
| Manufacturer | Model | Verdict |
| Nivus< | PCM3/PCM Pro | 80% |
| Unidata | Starflow | 65% |
Greyline |
70% |
The above verdicts are based on RS Hydro's own experience and that of its customers.
Applications
Doppler Ultrasonic flowmeters 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.
Typical applications include:
- Slurries
- Primary Sludge
- Dredging
- 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/Channel Size | 50+mm |
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, 12 months or permanent electrical connection |
| Dimensions | Typically 325 x 275 x 150mm |
| Weight (Control Unit) | Typically 3-7kg |
Technical Notes
In order to obtain the optimum accuracy from a portable ultrasonic flowmeter, 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. |
