Single-wire Measurement

When single hot-wire probe is kept in a flow of unknown velocity it produces a voltage that can be converted into a velocity using the calibration curve. Velocity fluctuations manifest as voltage fluctuations that can be recorded as the instantaneous signal through a computer or as RMS value through a true-RMS voltmeter. If is the calibration curve, the instantaneous velocity fluctuations are determined as

where is the mean voltage corresponding to a mean velocity . The RMS value of the velocity fluctuations can be simply calculated as

Here is the derivative of the calibration curve calculated at the operating point of the probe. This formula is based on a truncated Taylor's series expansion of the function and is valid only for small values of . Experiments show that it is valid for turbulence levels upto 10 percent. For larger turbulence levels it is more accurate to construct pointwise, followed by , and and determine using numerical integration of the signal.