Hot-wire anemometer (Air flow transmitter series) product principle and application

Principle of Hot-wire anemometer

The basic principle of hot-wire anemometer is a thin metal wire line in a fluid, through the current heating wire (hot

 wire), make its temperature is higher than the temperature of the fluid, when the fluid flows through the wire along

 vertical direction, will take away part of the wire  heat are introduced to make wire temperature drop.

The heats dissipating capacity is related to velocity, hot wire temperature change caused by the resistance

 resulting from heat dissipating capacity of the velocity signal into electrical signal, a typical hot-wire

 anemometer principle as shown:

Flow or wind speed is a very important sensing project in many production equipment or systems such as agriculture, 

industry and people's livelihood. The hot-wire anemometer has been widely used in various industries, for example,

 (1) automobile industry, it is used to detect the amount of air entering the engine and provide accurate fuel injection 

quantity for automobile engine. In order to get a more accurate oil-gas ratio, the injection quantity of oil and gas 

should be controlled to give full play to the efficiency of the engine. At the same time, it can reduce the pollution to

 the environment and save fuel.

(2) proportional control of boiler air input to coal in power plant.

(3) optimal control of indoor air quality and ventilation energy saving. (4) check the flow rate of combustible gases such

 as natural gas, liquefied gas and coal gas in people's livelihood and industrial sites. (5) in the clinical diagnosis and

 treatment of respiratory diseases, the respiratory function meter provides the medical basis for the diagnosis and

 treatment of respiratory diseases by measuring the static and dynamic pulmonary function parameters of the subjects, 

such as mid-term and peak respiratory velocity and respiratory capacity.

The Hot-wire flowmeter can also be used to measure gas velocity or anemometer; Hot-wire anemometers are 

classified into CVA (Constant Voltage Anemometer), CCA (Constant current anemometer) and CTA (constant

 temperature anemometer).

Hot-wire anemometers are most suitable for accurate measurements of very low flow rates (for example, below

 2000 Ft/min).

 Some models are designed to measure flow rates up to 15000 ft/min, but for very low speeds they still have

 very accurate measurement capabilities.

For example, FTM09, FTM06, FTM8X and FTM3X, the core products of FTI hot-wire flowmeter, are designed

 based on different  principles. The voltage output of these anemometers is the result of certain circuits in the

 equipment to keep certain variables  (current, power supply or temperature) constant.

In addition, FTM09 can send repeated current pulses, which raise the resistance of the wire to a specific value,

 then cut off the pulse, and then send the pulse again when the resistance falls to the "lower limit" threshold, and finally

 deduce the flow rate through the duration of repeated pulses.

In view of the nonlinear characteristics of the hot-wire anemometer sensor, the signal is converted from the 

characteristic curve of the sensor after temperature zero compensation, and the signal can be output linearity.

 By recording wind speed, sensor voltage, fluid  temperature, and pressure, the sensor voltage varying with wind

 speed and temperature can be corrected. Therefore, it also has additional functions such as temperature

 measurement and data recording.

When the sensor works at a constant temperature, the thermal equilibrium equation of the King's law is:

e²=a^'+b^' vⁿ

In the above equation: e is sensor voltage output (V)； a' is the zero output of thermostatic hot-wire anemometer, 

which can be eliminated  by circuit；b' is the sensitivity of the sensor, which is related to the operating temperature；

v is the velocity of the fluid；n is usually  0.45 ~ 0.5, and the characteristic curve of the thermostatic hot-wire 

anemometer can be drawn by the formula, as shown in the figure.

                       Constant temperature hot-wire anemometer curve characteristicr

FTI hot-wire anemometer (Air flow transmitter) series features as the following low pressure loss, large flow range, 

high accuracy, 

high repeatability, high reliability, good linearity and sensitivity, easy maintenance and not easy to damage.