A laser velocimeter, as the name suggests, uses a laser beam to measure velocity remotely. Mounted above the water, the sensor transmits a laser beam that penetrates the water and focuses at single or multiple points below the surface of the flow stream. The light is scattered back to the sensor. The returned light is frequency shifted due to the Doppler effect and the motion of the flow. The frequencies of the transmitted light and received light are compared to determine a Doppler shift, which is then used to calculate flow stream velocity.

At sites where surcharge conditions may occur, uninterrupted flow measurement​ is possible with a Continuous Wave AV sensor mounted either in the flow stream or on the bottom of the laser sensor.

This non-contact measurement device is ideal for monitoring streams where submerged sensors can be hampered by fouling or obstructions in the flow, such as streams containing harsh chemicals, grease, or suspended solids. Mounted above the flow stream, this type of sensor generally requires less frequent cleaning and maintenance than submerged sensors.

The device is able to move the laser beam transverse to the flow in order to obtain readings at multiple points, and multiple depths, within the flow (rather than a mere single-point surface reading), with automatic compensation to maintain precise focus at all times. Other advantages of Laser Doppler monitoring include the ability to measure velocity at very shallow depths, and in bi-directional flows.

Laser Doppler flow technology is very versatile in that it can be used in conjunction with a variety of level measurement technologies for AV flow calculation. For example, with an ultrasonic transducer taking the level readings, all measurements take place without touching the flow stream. ​

Electromagnetic probes measure the local velocity of the flow at the location of the electrodes at the bottom of the channel. Average flow velocity is then estimated based on this local velocity, the measured liquid depth, and the size and shape of the channel. Because this estimate is based on ideal flow conditions, electromagnetic probes require on-site profiling and calibration to obtain the most accurate measurement of flow. 

Because the electrodes on the probe are exposed to the flow, they are subject to fouling and often require frequent cleaning. The accuracy of an electromagnetic probe is questionable during the time that its electrodes are fouled. ​​​

T​he transit time velocity method is based on the principle that a sound pulse traveling diagonally across a flow stream will be accelerated by the velocity of the liquid when the sound pulse is traveling in a downstream direction, and decelerated when traveling in an upstream direction.

Transit time flow meters are typically used to measure flow in large pipes and channels. Transit time systems may incorporate one or more pairs of transducers. Multiple acoustic paths are used to increase accuracy when the velocity distribution is not well known, and to allow for changes in liquid level. From two to four pairs of transducers may be used in such instances. Path angles of 30° to 65° are commonly used. A transit time system cannot measure velocity when the liquid level drops below the transducers. Under this condition, flow rate is usually calculated based on the level measurement.

Because the transducers must be precisely located and aligned, transit time systems are more difficult to install than Doppler and electromagnetic systems. In addition, the ability of a transit time flow meter to detect sound pulses will be affected if large quantities of entrained air and/or suspended solids are present in the flow. ​

Traditionally, surface velocity measurement involved observing an object floating on the surface and measuring the amount of time it took that object to travel a specific distance. Radar Doppler systems remotely measure a surface velocity using the Doppler principle with electromagnetic radiation as the measurement medium.

A radar Doppler device transmits an electromagnetic signal to the liquid surface at a known frequency and defined angle. Turbulence on the water surface reflects the signal back to the transmitter. This returned signal is analyzed for a Doppler shift in frequency that gives the velocity of that portion of the surface illuminated by the electromagnetic radiation. This surface velocity measurement is transformed into the mean velocity using a fixed, predetermined velocity profile.

The main advantage offered by such a remote-sensing device is that it is installed outside of the flow stream.

Similar to other point velocity measurement instruments, a surface velocity measurement device relies on a specific and consistent model of the flow’s velocity profile to maintain a degree of accuracy in its mean velocity conversion from the surface measurement. Installation is a critical factor with such devices. The surface velocity method requires a relatively high surface turbulence to obtain a reading. Slow or reverse velocities may result in unreliable readings.

Although most makers of surface radar devices provide some method for surcharge flow measurement, in order to obtain complete flow data, profiling of the entire channel cross-section is still needed.​​

​Frequently Asked Questions

What is a non contact open channel flow meter?

Non-contact type flow meters measure full pipe clean to dirty liquid applications. Typical examples include water, well water, chilled water, hot water, city water, cooling tower water, glycol, deionized water, sea water, sewage, acids and more.

How can I measure flow without a f​low meter?​

Flow is typically measured using a flow meter. However, flow can be measured by other means such as those used in flow laboratories that incorporate weight or volumetric techniques.

What is a non-invasive flow meter?

A non-invasive flow meter sits above, not in, the flow stream. It measure the time it takes for an ultrasonic signal transmitted from one sensor, to cross a pipe and be received by a second sensor.