A hot-wire anemometer compensated for ambient temperature variations

A simple system for compensating a hot-wire anemometer for ambient temperature variations is proposed. In addition to the hot-wire probe, this system requires a temperature probe for sensing the working flow temperature. When the overheat ratio of the hot wire is kept constant in the usual manner and the temperature coefficient of the temperature probe in the hot-wire bridge is set equal to that of the hot wire, complete compensation can be achieved. The unequal coefficients may be equalised in the electrical circuit. It is found that the empirical heat-loss equation for plated wire probes is less susceptible to variations in both flow temperature and hot-wire operating temperature than that for welded wire probes. For an ambient temperature increase of 25 degrees C, the output drift of the system using the plated probe is within 1% of the indicated velocity. In the overcompensation mode of the system, compensation with 1% accuracy is achieved for a temperature rise of up to 35 degrees C and for a limited velocity range.