Principles of Operation 21
© 2018 Nortek AS
The velocimeter's noise level, also called the noise floor, is its most fundamental measure of
measurement uncertainty. If the noise level is known, one can easily compute the expected variance
or standard deviation of the measurements for any sample rate. Velocimeter uncertainty is a random
white noise. This means that each velocity estimate is independent of the next, and it means that
the noise has no preferred frequency.
The noise level varies widely according to the characteristics of the velocimeter, how it is set up, the
characteristics of the flow, and the acoustic scattering environment. Here is how the noise level and
measurement variance are related:
variance = (noise level)*(sample rate)/2
By using the equation in the "Averaging" section, the equation above can be turned into an equation
for standard deviation.
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