In recent years level devices using radar have seen tremendous growth in uses requiring precision measurement, in part because of their ability to overcome such level measurement problems as foaming, temperature changes, vapors, condensates and surface agitation.
Unlike acoustic devices, where the accuracy of the device is affected by the temperature effects on the speed of sound, radar devices are virtually immune to such errors. Now, flow sensors incorporating radar are entering the open channel flowmeter marketplace providing non-contact flow measurement.
A basic principle of radar is its ability to reflect off the surface of materials based on the material’s dielectric constant. Any material that has a dielectric constant greater than 2, such as water or ammonia, will easily reflect radar signals. The higher the dielectric constant of the material, the more signal that is reflected and available for processing. On the other hand, radar signals tend to pass through materials that have a dielectric constant less than 2, such as air, vapor, certain gases, or foam, and therefore these materials have a minimal effect on level and velocity measurements as compared with other measurement technologies.
Radar flowmeters determine the velocity of the flow similar to how police radar guns measure the velocity of an automobile. The radar beam is transmitted from the sensor’s “horn” at a defined angle to the flow surface.
This transmitted beam interacts with the fluid and reflects back a portion of the transmitted signal. The portion of the signal that is reflected back is at a slightly different frequency than that which was transmitted. For instance, the frequency is slightly higher if the flow is coming toward the beam and is slightly lower if the flow is going away from the beam.
The reflected signals that return to the radar horn are detected and compared with the transmitted frequency.
The frequency shift is a direct measure of both the velocity and direction of the flow particles from which the signal was reflected. Operating at a relatively high frequency, the radar flowmeter can measure velocities with only a minimum amount of surface disturbance.
In all open channels, the flow varies throughout the cross-section. These “velocity profiles” generally terminate along the surface of the flow. In other words, a fingerprint of the flow profile exists on the flow surface itself. By measuring a portion of this fingerprint, the radar flowmeter can determine the average velocity of the flow stream.
Because the position of the beam relative to the flow surface is known, the relationship between the sensed velocity and the average velocity of the flow stream is defined and flow can be determined to an accuracy of ± 5 percent or better. Like all flow metering devices, the flow needs to be reasonably uniform in nature to obtain the highest accuracy.
The non-contact nature of the radar open channel flowmeter reduces the need for periodic maintenance and helps limit sensor fouling. Also, radar flowmeters can operate from above existing channels without the need of flumes or weirs and without any limitation on the minimum or maximum flow range.
