Sensor catches both low and high flows

Monitoring volumetric flow in conduits that are less than full ­ open channel flow ­ has always been difficult. Throw municipal or industrial waste into the mix and measurement becomes even more problematic.

The two most popular techniques to measure open channel flow are:

1) Mechanical restrictions.

2) Area/Velocity.

The nature of a flow-measuring restriction ­ usually a flume or weir ­ only allows the flow rate to be measured over a narrow band of flow rates. Storm flows or low-flow conditions usually fall outside the measuring range of a typical flume. The cost of the civil work to build large flumes during plant construction can be high and retrofitting flow-monitoring equipment to such sites is even more expensive.

A revolutionary way of measuring velocity and level.

The most versatile technique for calculating volumetric flow (Q) is to use the 'simple' equation Flow (Q) = Cross-Sectional Area (A) x Mean Velocity (V). To get the values for A and V, however, is not so simple. The cross-sectional area depends on an accurate knowledge of the level of the media, usually determined by a pressure sensor or by a non-contact ultrasonic level.

Within the last 20 years, technology has improved and an active measurement of velocity is now possible using a variety of submerged sensors: electromagnetic, ultrasonic Doppler and ultrasonic transit time, to name the more popular ones. Unfortunately, using submerged sensors in sanitary or industrial sewers creates many problems. Installation is difficult, dirty and dangerous; and even when successfully completed, there is the likelihood of 'ragging' and hence, loss of important data.

Our company has now developed a revolutionary means of measuring velocity in open channels. The new non-contact radar flowmeter, Flo-Dar, measures both the velocity and level of the media from a single sensor mounted above the flow.

Radar is an acronym for the phrase "Radio Detection and Ranging" (Generally credited to Sir Robert Watson-Watt). Originally developed for military use just prior to World War II, radar devices are rapidly becoming part of our everyday lives. Examples include Doppler radar weather forecasting, remote security devices, and radar guns used by the police to measure an automobile's speed. Over the next several years, it is expected that collision-avoidance radar devices will be standard equipment in most automobiles.

In recent years, radar has seen tremendous growth in industrial applications where precise level measurement is required. Today, radar-based flow sensors are entering the open channel flowmeter marketplace to provide accurate, 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 two, such as water, crude oil, 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 two, such as air, vapour, certain gases, or foam.

Radar flowmeters determine the velocity of the flow in a manner similar to the way police radar guns measure the velocity of an automobile. A radar beam is transmitted from the sensor's "horn" to a known location on 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 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 resulting frequency shift is a direct measure of both the velocity and direction of the flow particles from which the signal was reflected.

In all open channels, the flow velocity varies throughout the cross-section. These "velocity contour lines" extend to 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, and applying a rather complex algorithm, the radar flowmeter can determine the average velocity of the flow stream to an accuracy of ± 5% or better. Like all flow measuring devices, the highest accuracy is achieved when the flow is stable and unaffected by bends and obstructions.

The non-contact nature of the radar open channel flowmeter is attractive to new users. The user no longer has to worry about periodic maintenance, sensor fouling or the caustic nature of the flow medium. Also, radar flowmeters can operate from above existing channels without the need for flumes or weirs, and without any limitation on the minimum or maximum flow range. Accurate signals can be detected over a wide range of distances and Flo-Dar is capable of measuring flow velocities with only a minimum amount of surface disturbance.

Flo-Dar's non-contact nature gives it great versatility; it can be used for various measuring applications including industrial effluents, raw sewage or influent to treatment plants, all of which can be too harsh or dangerous for close work in confined spaces or deep manholes. Once installed, the sensor can be removed from the surface without entering the manhole.

Low flow conditions are well suited for Flo-Dar, as it can 'see' and measure the movement of surface water as shallow as 2mm (0.1 inch).

Surcharging will not harm the Flo-Dar sensor; and it will continue to work even when completely flooded, utilizing an on-board pressure sensor that takes over from the ultrasonic level sensor during submergence. A patented method of allowing the radar horn to continue measuring the velocity while submerged is under development.

Industrial radar devices are just as safe in the hands of consumers as such everyday devices as televisions, microwave ovens, and cellular phones. All devices that emit electromagnetic radiation are controlled by government regulatory bodies such as the FCC in the United States and Industry Canada in Canada. Manufacturers of industrial radar devices obtain a license for their manufacture and use, thereby eliminating the need for individual users to obtain licenses.

C&M Environmental is the Canadian distributor of Marsh-McBirney, Inc.