Laser Doppler Flowmeter
Measuring with laser Doppler flowmeter
This instrument gives direct measurements
of flow velocity for liquids containing suspended particles flowing in a
transparent pipe. Light from a laser is focused by an optical system to
a point in the flow, with fiber optic cables being commonly used to
transmit the light. The movement of particles causes a Doppler shift of
the scattered light and produces a signal in a photodetector which is
related to the fluid velocity. A very wide range of flow velocities
between 10 µm/s and 105 m/s can be measured by this
technique.
Sufficient particles for satisfactory operation are normally present
naturally in most liquid and gaseous fluids, and the introduction of
artificial particles is rarely needed. The technique is advantageous in
measuring flow velocity directly rather than inferring it from a
pressure difference. It also causes no interruption in the flow and, as
the instrument can be made very small, it can measure velocity in
confined areas. One limitation is that it measures local flow velocity
in the vicinity of the focal point of the light beam, which can lead to
large errors in the estimation of mean volume flow rate if the flow
profile is not uniform. However, this limitation is often used
constructively in applications of the instrument where the flow profile
across the cross-section of a pipe is determined by measuring the
velocity at a succession of points.
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