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Open channel flow measurement techniques


Arguably, the most important measurement taken in and around water/wastewater systems is flow. In open channel applications, the most common flow measurement technique is through Primary/Secondary devices.

Primary devices include flumes and weirs that create a reliable relationship between water level and flow rate; in the form of a flow equation.

Secondary devices are flow meters that continuously measure water level and apply the equation to determine flow rate, for totalization, recording or control output(s).

But what do you use when there is no flume or weir available? What if frequent surcharges, even submergence, occurs; or, other locations where the primary / secondary technique is not appropriate?

Area Velocity Flow Meter with Continuous Wave Doppler

In these cases, an Area Velocity flow meter can be used and the most commonly used technique is Continuous Wave Doppler (CWD).  The CW doppler AV sensor measures depth of water and velocity  to solve flow’s Continuity Equation Q = A * V.

A pressure transducer continuously measures depth and the meter calculates the area (A), then an upward looking doppler ultrasonic sensor sends sound into the water to bounce off of particulates and entrained air. As the sound signal is reflected back at the sensor, it returns a different frequency. That frequency shift is proportional to velocity.  A weighted average of the return signal is used to determine the average velocity (V). Most commonly, the CWD Area Velocity sensor is positioned at 6 o’clock in the invert; looking upstream.

CW Doppler is widely used in round pipes, concrete and earthen channels found in sewer collection, drinking water intake and backwash; stormwater; and industrial wastewater.

There are, however, conditions where the CW Doppler technology is not the best technique available:

  1. Clean Water

Very clean water may lack enough particulate or air bubbles for the Doppler sound to bounce off; sending but not receiving the return signal.

  1. Debris

In sewer and surface water, solids tend to collect on the wetted sensor; and with significant enough accumulation, may interfere and ultimately blind the Doppler sensing when ‘ragged up’.

  1. Low Water Level

Another common issue is very low water level.  The CW Doppler AV sensor needs at least 1- 2 inches of depth in order to effectively measure velocity.

  1. Low Velocities

CW Doppler sensors struggle to work in velocity velocities below 0.5 fps. In addition, at low velocities sediment deposition is more of an issue, requiring more frequent cleaning of the sensor.

  1. Large pipes/channels

In larger pipes and channels, the CW Doppler sensor may not ‘see’ the velocity profile completely to determine the true average velocity.  The acoustical footprint of a CWD sensor varies with depth and the amount of solids, with the maximum acoustic footprint roughly the size of a basketball. For deeper flows and/or wide channels a correction factor may be needed.

LaserFlow Area Velocity Sensor

In applications described above, the LaserFlow Area Velocity Sensor can be more effective.  The LaserFlow Meter uses the continuity equation also, but utilizes a non-contacting ultrasonic to measure water level and a non-contacting laser sensor to measure velocity.

The LaserFlow is able to measure at much lower depths and slower velocities than the CW Doppler sensor, and since it is non-contacting, there is no issue with debris disturbing the signal, or covering a wetted device. In addition, because the sensor is mount above the flow stream, it is easier to install in large pipes and channels than wetted sensors.


Supporting resources:

LaserFlow AV Sensor Brochure

Watch a video demonstration of Continuous Wave Doppler Area Velocity 

LaserFlow Non-Contacting Flow Meter from Teledyne Isco

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