Introduction |

Consider a wire that's immersed in a fluid flow. Assume that the wire, heated by an electrical current input, is in thermal equilibrium with its environment. The electrical power input is equal to the power lost to convective heat transfer, where T and _{w}T are the temperatures of the wire and fluid respectively, _{f}A is the projected wire surface area, and _{w}h is the heat transfer coefficient of the wire.
The wire resistance where a is the thermal coefficient of resistance and T.
_{Ref}The heat transfer coefficient where Combining the above three equations allows us to eliminate the heat transfer coefficient Continuing, we can solve for the fluid velocity, Two types of thermal (hot-wire) anemometers are commonly used: constant-temperature and constant-current. The constant-temperature anemometers are more widely used than constant-current anemometers due to their reduced sensitivity to flow variations. Noting that the wire must be heated up high enough (above the fluid temperature) to be effective, if the flow were to suddenly slow down, the wire might burn out in a constant-current anemometer. Conversely, if the flow were to suddenly speed up, the wire may be cooled completely resulting in a constant-current unit being unable to register quality data. |

Constant-Temperature Hot-Wire Anemometers |

For a hot-wire anemometer powered by an adjustable current to maintain a constant temperature, R are constants. The fluid velocity is a function of input current and flow temperature,
_{w}Furthermore, the temperature of the flow |

Constant-Current Hot-Wire Anemometers |

For a hot-wire anemometer powered by a constant current If the flow temperature is measured independently, the fluid velocity can be reduced to a function of wire temperature R. Therefore, the fluid velocity can be related to the wire resistance.
_{w} |