A Constant-Temperature-Operation Hot-Wire Anemometer

The behavior of a hot-wire anemometer is analyzed using signal flow diagrams, especially for the case of a constant-temperature-operation anemometer, comprising a dc bridge and a dc differential amplifier. The influence of static bridge unbalance and of the amplifier´s gain for in-phase input signals on the over-all dynamic characteristics is analyzed, and requirements for the amplifier are derived. A chopper-stabilized amplifier is employed, which automatically maintains static bridge balance under all operating conditions. The signal-to-noise ratio for comparable instruments having constant-current operation and constant-temperature operation is shown to be equal. Results of measurements are reported; they are in fair agreement with the developed theory. A nonlinear circuit is described, which delivers a voltage directly proportional to the instantaneous air velocity.