Measuring with cross-correlation flowmeters
Cross-correlation flowmeters are a type of
flowmeter which has not yet achieved widespread practical use in
industry. Much development work is still going on, and they therefore
mainly exist only as prototypes in research laboratories. However, they
are included here because their use is likely to become much more
widespread in the future.
Such instruments require some detectable random variable to be present
in the flowing fluid. This can take forms such as velocity turbulence
and temperature fluctuations. When such a stream of variables is
detected by a sensor, the output signal generated consists of noise with
a wide frequency spectrum.
Cross-correlation flowmeters use two such sensors placed a known
distance apart in the fluid-carrying pipe and cross-correlation
techniques are applied to the two output signals from these sensors.
This procedure compares one signal with progressively time-shifted
versions of the other signal until the best match is obtained between
the two waveforms. If the distance between the sensors is divided by
this time shift, a measurement of the flow velocity is obtained. A
digital processor is an essential requirement to calculate the
cross-correlation function, and therefore the instrument must be
properly described as an intelligent one.
In practice, the existence of random disturbances in the flow is
unreliable, and their detection is difficult. To answer this problem,
ultrasonic cross-correlation flowmeters are under development. These use
ultrasonic transducers to inject disturbances into the flow and also to
detect the disturbances further downstream.
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