Hot-wire anemometer is most well known thermal anemometer, measure fluid velocity by noting the heat convected away by the fluid. Hot-anemometers are popular for unsteady flow of gas and liquid flow measurement. The basic system consists of electrically heated wire suitably is liquid.
Due to the change in velocity of flowing liquid the heat lost by the wire also changes. The loss in temperature causes the change in resistance of a hot wire. And this changes the current through the hot wire for a constant voltage. Flow can be measured by measuring the change in the resistance or current.
There are two basic technique of measurement of flow:
By measuring resistance keeping the current constant
By measuring the current keeping the resistance, that is temperature constant.
Constant current method:
A constant current is passed through the sensing wire. That is, the voltage across the bridge circuit is kept constant, that is, not varied.
Due to the gas flow, heat transfer takes place from the sensing wire to the flowing gas and hence the temperature of the sensing wire reduces causing a change in the resistance of the sensing wire.
Due to this, the galvanometer which was initially at zero position deflects and this deflection of the galvanometer becomes a measure of flow rate of the gas when calibrated
Constant resistance method:
The principle of this method is to maintain the temperature and resistance of the sensing wire at a constant level
Due to the gas flow, heat transfer takes place from the sensing wire to the flowing gas and this tends to change the temperature and hence the resistance of the wire.
For a change in flow rate resistance of hot-wire changes and the output of the unbalanced bridge is amplified and used as supply to the bridge. The bridge current required for balancing gives the change in flow velocity.
The current is measured by measuring the voltage across the hot-wire resistance.