Thermal wind speed sensor principle of operation

Thermal wind speed sensor principle of operation

The thermal wind speed sensor uses a hot wire (tungsten or platinum wire) or a hot film (platinum or chromium film) as a probe, exposed to the air being measured, and connects it to a Wheatstone bridge to detect the flow rate of the air in the section being measured by the balance of resistance or current of the Wheatstone bridge. The hot film wind speed sensor is coated with a very thin quartz film insulation to insulate it from the fluid and prevent contamination, and it can operate in air streams with particles and is stronger than a metal hot wire.

When the air temperature is constant, the power dissipated on the wire is equal to the instantaneous heat dissipated by the wire in the air. The resistance of the hot wire varies with temperature, and the resistance of the hot wire and the temperature of the hot wire in the usual temperature range (0 to 300 °C) show a linear relationship. The coefficient of heat dissipation is related to the velocity of the airflow.

The velocity of airflow measured by a thermal air speed sensor is a function of current and resistance. By keeping the current (or resistance) constant, the measured airflow speed corresponds only to the resistance (or current).

Thermal wind speed sensors are available in both constant current and constant temperature design circuits. Thermostatic hot-wire air velocity sensors are more commonly used. The principle of the constant temperature method is to keep the temperature of the hot wire constant during the measurement process, so that the bridge balance, the resistance of the hot wire remains constant, the airflow speed is only a single-value function of the current, according to the known relationship between airflow speed and current can be derived from the airflow speed through the end device.

The constant current hot wire air speed sensor keeps the current value flowing through the hot wire constant during the measurement process. When the current value is constant, the airflow velocity is only related to the resistance of the hot wire. The air velocity through the sensor can be found from the known air velocity in relation to the resistance of the hot wire.

The hot-wire wind speed sensor measures the pulsating wind speed. Constant flow wind speed sensors have a high thermal inertia, thermostatic wind speed sensors have a relatively low thermal inertia and have a high velocity response. The accuracy of the measurement of the thermal speed sensor is not very high, so attention should be paid to temperature compensation when using the sensor.