The thermal wind speed sensor's operation
The thermal wind speed sensor uses a hot wire (tungsten wire or platinum wire) or a hot film (platinum or chromium thin film) as a probe, exposed to the measured air, and connects it to the Wheatstone bridge, and detects the flow velocity of the air in the measured section through the resistance or current balance of the Wheatstone bridge. The hot film of the hot film wind speed sensor is coated with a very thin quartz film insulation layer to insulate the fluid and prevent pollution. It can work in the airflow with particles, and its strength is higher than that of the metal heating wire.
When the air temperature is stable, the power consumption on the heating wire is equal to the heat consumed instantaneously by the heating wire in the air. The resistance of the heating wire changes with the temperature, and the resistance of the heating wire and the temperature of the heating wire show a linear relationship within the usual temperature range (0-300 ℃). The heat release coefficient is related to the airflow velocity. The larger the flow velocity, the larger the corresponding heat release coefficient, that is, the faster the heat dissipation; the smaller the flow velocity, the slower the heat dissipation.
The airflow velocity measured by the thermal anemometer is a function of current and resistance. Keeping the current (or resistance) constant, the measured air velocity only corresponds to the resistance (or current) one by one.
The hot wire wind speed sensor has two design circuits of constant current and constant temperature. Constant temperature hot wire wind speed sensor is more commonly used. The principle of the constant temperature method is to keep the temperature of the hot wire constant during the measurement process to balance the bridge. At this time, the resistance of the hot wire remains unchanged, and the airflow velocity is only a single-valued function of the current. According to the known relationship between the airflow velocity and the current, the airflow velocity through the terminal device can be obtained. The constant current hot wire wind 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 heating wire. According to the known relationship between the airflow velocity and the resistance of the hot wire, the airflow velocity passing through the wind speed sensor can be obtained.
Hot wire wind speed sensors measure fluctuating wind speeds. The thermal inertia of the constant flow wind speed sensor is relatively large, while the thermal inertia of the constant temperature wind speed sensor is relatively small and has a high speed response. The measurement accuracy of the hot-wire wind speed sensor is not very high, so attention should be paid to temperature compensation when using it.
