Category: Flow Meter

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.

Source: http://www.waterworld.com/articles/print/volume-14/issue-8/automation-technology/radar-takes-on-open-channel-flow-measurement.html

Doppler and Transit Time are two very popular types of flow meter for non-invasive measurement of flow in full pipes. We tend to confuse these technologies because they are both ultrasonic and both measure flow by using sensors clamped onto the outside of a pipe. In the real world they actually work best in opposite applications. Success in your installation depends on understanding the differences and making the right choice. Ultrasound is sound generated above the human hearing range – above 20 kHz. Both Doppler and Transit Time flowmeter technologies are called “ultrasonic” because they operate far above the frequencies or sound range that we can hear. At the heart of each ultrasonic transducer is a piezo-electric crystal. They are glass disks about the size of a coin. These crystals are polarized and expand or pulse a minute amount when electrical energy is applied to the surface electrodes. As it pulses the transducer emits an ultrasonic beam approximately 5° wide at an angle designed to efficiently pass through a pipe wall. The returning echo (pressure pulse) impacts a second passive crystal and creates electrical energy. This is the received signal in a Doppler or Transit Time transducer. So far, both these piezo-electric ultrasonic technologies seem much the same. No wonder the choice can be confusing. But now let’s look the differences. Transit Time transducers typically operate in the 1-2 MHz frequencies. Higher frequency designs are normally used in smaller pipes and lower frequencies for large pipes up to several meters in diameter. So operators must select transducer pairs/frequencies according to the application. Doppler transducers usually operate at 640 kHz to 1 MHz frequencies and work on a wide range of pipe diameters.

Transit Time flowmeters must have a pair of transducers, each containing a piezo-electric crystal. One transducer transmits sound while the other acts as a receiver. As the name suggests, Transit Time flowmeters measure the time it takes for an ultrasonic signal transmitted from one sensor, to cross a pipe and be received by a second sensor. Upstream and downstream time measurements are compared. With no flow, the transit time would be equal in both directions. With flow, sound will travel faster in the direction of flow and slower against the flow. Because the ultrasonic signal must cross the pipe to a receiving transducer, the fluid must not contain a significant concentration of bubbles or solids. Otherwise the high frequency sound will be attenuated and too weak to traverse the pipe.

Doppler flowmeters use a single-head sensor design allowing fast, simple mounting on the outside of pipes. The single-head transducer includes both transmit and receive piezo-electric crystals in the same housing. The Doppler effect was first documented in 1842 by Christian Doppler, an Austrian physicist. We all hear daily examples of the Doppler effect. It is the distinct tone change from a passing train whistle or the exhaust from a race car. We hear this tone change, or Doppler effect, only because we are stationary and the sound transmitter – the train or the race car – is in motion. Doppler flow meters use the principal that sound waves will be returned to a transmitter at an altered frequency if reflectors in the liquid are in motion. This frequency shift is in direct proportion to the velocity of the liquid. It is precisely measured by the instrument to calculate the flow rate. So the liquid must contain gas bubbles or solids for the Doppler measurement to work. Two technologies, one decision: Doppler flowmeters work best in dirty or aerated liquids like wastewater and slurries. Transit Time flowmeters work with clean liquids like water, oils and chemicals. Contact Tardigraz technologies for specific advice and information on selecting and applying these technologies successfully in your application.