A thermal anemometer is a device used to measure the velocity of a fluid, such as air or gas, based on the principle of heat transfer. The operation of a thermal anemometer relies on the cooling effect of a fluid flow on a heated sensor element. As the fluid flows past the heated sensor, it carries away some of the heat, and the change in temperature is used to determine the velocity of the fluid.
Here's a step-by-step explanation of how a typical thermal anemometer works:
Heated Sensor Element: The thermal anemometer consists of a small sensor element that is electrically heated. This sensor is usually made of a fine wire or a thin-film material with a known resistance. The sensor is heated to a constant temperature using a current supply.
Zero Flow Calibration: Before measuring the fluid velocity, the thermal anemometer is calibrated in still air (zero flow condition) to establish a baseline temperature. This allows the anemometer to account for any ambient temperature changes or other factors that may influence the sensor's readings.
Fluid Flow Measurement: When fluid flows past the heated sensor, it carries away some of the heat from the sensor element. The rate at which heat is carried away depends on the velocity of the fluid. Faster fluid flow results in more significant cooling of the sensor.
Temperature Difference Sensing: The thermal anemometer has a temperature-sensing element (thermistor or RTD) that measures the temperature of the heated sensor element. As the fluid flow rate changes, the temperature of the heated sensor will also change, resulting in a temperature difference between the heated sensor and the reference temperature (ambient temperature).
Feedback Control: The thermal anemometer employs a feedback control system to maintain the sensor at a constant temperature despite the cooling effect caused by the fluid flow. To keep the sensor temperature constant, the current passing through the heated sensor element is adjusted by the feedback control system based on the temperature difference sensed.
Velocity Calculation: The magnitude of the electrical current required to maintain the sensor's constant temperature is proportional to the velocity of the fluid. The thermal anemometer converts this current value into a flow velocity measurement using calibration curves or equations obtained during the calibration process.
Output and Display: The calculated flow velocity is usually displayed on a digital screen or transmitted to a data acquisition system or computer for further analysis or recording.
It's important to note that thermal anemometers have limitations, such as sensitivity to ambient temperature fluctuations and sensitivity to the thermal properties of the fluid being measured. However, they are commonly used in various applications, including meteorology, environmental monitoring, HVAC (heating, ventilation, and air conditioning) systems, and research settings where non-intrusive fluid flow measurement is required.