Two operating modes of the anemometer
An anemometer is a speed measuring instrument that converts flow velocity signals into electrical signals and can also measure fluid temperature or density. The principle is to place a thin metal wire (called a hot wire) that is heated by electricity in the airflow. The heat dissipation of the hot wire in the airflow is related to the flow rate, and the heat dissipation causes a change in the temperature of the hot wire and a change in resistance. The flow rate signal is then converted into an electrical signal. It has two working modes:
① Constant current type.
The current through the hot wire remains constant, and when the temperature changes, the resistance of the hot wire changes, resulting in a change in the voltage at both ends, thus measuring the flow rate;
② Constant temperature type.
The temperature of the hotline remains constant, such as at 150 ℃, and the flow rate can be measured based on the required applied current. Constant temperature type is more widely used than constant current type. The length of the hot wire is generally in the range of 0.5-2 millimeters, and the diameter is in the range of 1-10 micrometers. The material used is platinum, tungsten, or platinum rhodium alloy. If a very thin (less than 0.1 microns thick) metal film is used instead of a metal wire, it is called a hot film anemometer, which functions similarly to a hot wire but is mostly used to measure liquid flow velocity. In addition to the ordinary single line type, the hotline can also be a combination of double line or triple line type, used to measure velocity components in various directions. The electrical signal output from the hotline, after amplification, compensation, and digitization, can be input into the computer to improve measurement accuracy, automatically complete the data post-processing process, expand the speed measurement function, and simultaneously measure instantaneous and mean values, combined and partial velocities, turbulence intensity, and other turbulence parameters. Compared with pitot tubes, the hot wire anemometer has a smaller probe volume and less interference with the flow field; Fast response, capable of measuring unsteady flow velocity; It has the advantage of being able to measure very low speeds (such as as as low as 0.3 meters per second).
