What is a thermal anemometer
A speed measuring instrument that converts a flow rate signal into an electrical signal, and can also measure fluid temperature or density. The principle is that a thin metal wire heated by electricity (called a hot wire) is placed in the airflow, the heat dissipation of the hot wire in the airflow is related to the flow velocity, and the heat dissipation causes the temperature change of the hot wire to cause a change in resistance, and the flow velocity signal is transformed into electric signal. It has two working modes: ① constant flow. The current through the hot wire remains constant. When the temperature changes, the resistance of the hot wire changes, so the voltage across the two ends changes, thereby measuring the flow rate. ②Constant temperature type. The temperature of the hot wire is kept constant, such as 150°C, and the flow rate can be measured according to the required applied current. The constant temperature type is more widely used than the constant flow type.
The length of the hot wire is generally in the range of 0.5-2 mm, the diameter is in the range of 1-10 microns, and the material is platinum, tungsten or platinum-rhodium alloy. If a very thin (thickness less than 0.1 micron) metal film is used instead of a metal wire, it is a hot film anemometer, which has a similar function to a hot wire, but it is mostly used to measure the flow rate of a liquid. In addition to the ordinary single-wire type, the hot wire can also be a combined double-wire or three-wire type to measure the velocity components in all directions. The electrical signal output from the hotline is input into the computer after amplification, compensation and digitization, which can improve the measurement accuracy, automatically complete the data post-processing process, and expand the speed measurement function, such as simultaneously completing the instantaneous value and time average value, combined speed and partial speed, and turbulence. and other measurements of turbulence parameters. Compared with the pitot tube, the hot wire anemometer [1] has the advantages of small probe volume, small interference to the flow field, fast response, and can measure unsteady flow velocity; it can measure very low velocity (such as as low as 0.3 m/s).
When using thermal probes in turbulent flow, airflow from all directions impinges on the thermal element simultaneously, which can affect the accuracy of the measurement results. When measuring in turbulent flow, the indication value of the thermal anemometer flow sensor is often higher than that of the rotary probe. The above phenomenon can be observed in the pipeline measurement process. Depending on the design of the managed pipe turbulence, it can occur even at low speeds. Therefore, the anemometer measurement process should be carried out on the straight part of the pipeline. The starting point of the straight line should be at least 10×D (D=pipe diameter in CM) before the measurement point; the end point should be at least 4×D behind the measurement point. The fluid section must not have any occlusions (edges, overhangs, objects, etc.).
