The cross-correlation flowmeter has not yet achieved widespread practical use in
industry. Much development work is still going on, and it therefore mainly only exists
in prototype form in research laboratories. However, it is included here because use is
likely to become much more widespread in the future. The instrument requires 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.