What is the principle of hot wire anemometer?
Anemometer is a conventional testing instrument, and the types of anemometer mainly include:
One is the wheel anemometer. It is self-evident that the probe of the impeller anemometer rotates like a fan blade to measure the wind speed, mainly measuring the wind speed at the middle wind speed and surface wind speed;
The second is the hot wire anemometer. Its probe is a thin metal wire that generates heat, so it is called a hot wire. It mainly measures the wind speed at a clean low wind speed point;
The third is the differential pressure anemometer. This instrument mainly uses the principle of differential pressure to measure wind speed, mainly for high wind speed and ultra-high wind speed measurement;
The octagonal anemometer/matrix anemometer for laminar flow measurement is the surface wind speed measurement, or the pitot tube type measurement in the pipeline is also the point wind speed.
Here we mainly introduce the hot wire anemometer:
The hot-wire anemometer is a speed measuring instrument that converts the flow velocity signal into an electrical signal, and can also measure the temperature or density of the fluid.
The principle is that a thin metal wire heated by electricity (called a hot wire) is placed in the airflow, and the heat dissipation of the hot wire in the airflow is related to the flow velocity;
The heat dissipation causes the temperature change of the hot wire to cause the resistance change, and the flow velocity signal is converted into an electrical signal. It has two working modes:
①Constant flow type.
The current through the hot wire remains constant. When the temperature changes, the resistance of the hot wire changes, so the voltage at both ends changes, and the flow rate is measured;
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 hot wire is input into the computer after amplification, compensation and digitization;
It can improve the measurement accuracy, automatically complete the data post-processing process, and expand the velocity measurement function, such as simultaneously completing the measurement of instantaneous value and time average value, combined velocity and partial velocity, turbulence degree and other turbulence parameters.
Compared with the pitot tube, the hot wire anemometer has a smaller probe volume and less interference to the flow field;
Fast response, can measure unsteady flow rate; can measure very low speed (such as as low as 0.3 m/s) and other advantages.
