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.