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Hot wire flow meter anemometer

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:

1. 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. In the automobile industry, it is used to detect the amount of air entering the engine and provide accurate fuel injection quantity for automobile engines. 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.

2. 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 eYc 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.