The output of the feedback circuit of the anemometer is a measure of the fluid velocity. After proper calibration of the probe, it is possible to measure fluid velocities with an accuracy of 0:05% or better, depending upon the measurement range and the quality of calibration. In view of the high frequency response of the hotwire anemometer it can follow transients in the flow field without practically any time delay. The hot-wire has a limitation that it is insensitive to the flow direction. For an X-probe, the velocity measured by each wire is different from the component of velocity in the laboratory coordinates. The velocity sensed by each wire is known as the effective cooling velocity.The minimum velocity that can be measured by the HWA is determined by the velocities associated with natural convection from the heated wire. If a probe is calibrated and used under the same orientation with respect to the gravity field, it may be used at low velocities. The limit is then reached when natural convection dominates forced convection. In dimensionless form, this limit is expressed in terms of Reynolds number Re and Grashof number Gr as

where The notation used is: U is the fluid velocity, D is the sensor diameter, is the kinematicviscosity of the fluid,  is the coefficient of thermal expansion (equal to 1/T for an ideal gas) and (Tw - To) is the excess sensor temperature over the ambient. For the present experimental conditions, the minimum.Reynolds number for which forced convection dominated natural convection was estimated as Re=23, which corresponds to an air velocity of 0.12 m/s.