Introduction
A speed measuring instrument that converts a flow rate signal into an electrical signal that also measures fluid temperature or density. The principle is that a thin wire (heating wire) that is heated by electricity is placed in the airflow, and the heat dissipation amount of the hot wire in the airflow is related to the flow velocity, and the heat dissipation causes the temperature of the hot wire to change, causing the resistance change, and the flow rate signal is converted into electric signal. It has two working modes: 1 constant current. The current through the hot wire remains unchanged. When the temperature changes, the heat line resistance changes, so the voltage at both ends changes, thereby measuring the flow rate; 2 thermostatic type. The temperature of the hot wire remains the same, such as maintaining 150 ° C, the flow rate can be measured according to the current applied. The thermostatic type is more widely used than the constant current type.
Hotline features
The length of the hot wire is generally in the range of 0.5 to 2 mm, and the diameter is in the range of 1 to 10 μm. The material is platinum, tungsten or platinum-iridium alloy. If a thin metal film (thickness less than 0.1 micron) is used instead of the metal wire, it is a hot film anemometer, which functions similarly to the hot wire, but is mostly used to measure the liquid flow rate. In addition to the ordinary single line type, the hot line can also be a combined two-wire or three-wire type to measure the velocity component in all directions. The electric signal output from the hot line is amplified, compensated and digitized and input into the computer, 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 the time average value, the combined speed and the sub-speed, and the turbulence. And measurements of other turbulence parameters. Compared with pitot tube, hot wire anemometer has small probe volume and small interference to flow field; fast response, can measure unsteady flow rate; can measure very low speed (such as as low as 0.3 m / s)
principle
A type of flow meter that works by placing a sensing element, a thin wire that is heated by a current, in a channel that carries a certain amount of heat as it flows through it. It is related to the speed of the fluid. There are two methods commonly used to determine the flow rate: one is the constant current method, that is, the current of the heating wire is constant, the temperature of the wire is reduced after the gas takes away a part of the heat, and the higher the flow rate, the more the temperature is lowered; The temperature of the wire is measured to know the flow rate. The other is the fixed resistance method (that is, the fixed temperature method), changing the heating current to replenish the heat taken away by the gas, and keeping the temperature of the wire unchanged (also called the resistance value of the wire); The larger the flow rate, the larger the current required to be heated, and the value of the flow rate can be known by measuring the value of the heating current.
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