Hot Wire Anemometer: Working Principle and Functions

The hot wire anemometer is a type of anemometer that converts the flow velocity signal into an electrical signal, and can also measure fluid temperature or density.

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 microns. If a very thin metal film instead of a metal wire, which is called a hot film anemometer. The function is similar to that of a hot wire but is mostly used to measure the flow rate of liquids. In addition to the ordinary single-wire type, the hot wire can also be a combination of two-wire or three-wire types, to measure the speed component in all directions. The electrical signal output from the hot wire, after amplification, compensation and digitization, is input into the computer, which can improve the measurement accuracy, automatically complete the data post-processing process, and expand the velocity measurement function, such as completing the measurement of instantaneous value and hourly average value, combined velocity and fractional velocity, turbulence degree and other turbulence parameters at the same time.

Compared with the Pitot tube, the hot-wire anemometer has the advantages of small probe size, low interference with the flow field, fast response, and the ability to measure non-constant flow velocity and low velocity (down to 0.3 m/s). Hot wire anemometer with measurement range 0~20m/s, 0~30m/s and 0.1~25m/s are available on ATO.com.

Working principle of hot wire anemometers

It works by placing an electrically heated thin metal wire (called hot wire) in the air stream. The heat dissipation of the hot wire in the air stream is related to the flow rate, and the heat dissipation causes the temperature change of the hot wire to cause a change in resistance, and the flow rate signal is converted into an electrical signal.

Two working modes

• Constant current type, also known as constant current method, that is, the current to heat the wire remains constant, the gas takes away part of the heat after the temperature of the wire will be reduced, the greater the flow rate the more the temperature is reduced. When the temperature changes, the resistance of the hot wire and the voltage at both ends changes, so the temperature of the wire can be measured to know the size of the flow rate.
• Constant temperature type, also known as the constant resistance method (constant temperature method), changes the heating current so that the heat taken away by the gas can be replenished, while the temperature of the wire remains unchanged (also known as the resistance of the wire remains unchanged), the greater the flow rate, the greater the current required to heat, according to the current required to apply the size of the flow rate can be known. The constant temperature type is more widely used than the constant current type.

Main functions of thermal anemometers

• It measures the velocity and direction of the average flow.
• The hot-wire anemometer measures the pulsating velocity of the incoming flow and its spectrum.
• It measures the Reynolds stress in turbulent flow and the velocity dependence and time dependence of the two points.
• The anemometer measures wall shear stress (usually done with a hot film probe placed flush with the wall, working on a similar principle to hot-wire velocimetry).
• It can measure the fluid temperature (the curve of probe resistance with fluid temperature is measured in advance, and then the temperature can be determined from the measured probe resistance).