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Термоанемометр, питаемый постоянным напряжением
Flow distribution around a heated cylinder
распределение потоков
Home >> Manometers. Pressure gauge >> Termoanemometer method for determining the flow
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See also:
Temperature sensors, thermocouple             Sensors of mechanical stress
Force, pressure, flow                                    Phototransducers
Ionization transducers                                    Hall sensors
Датчик магнитного поля
   Heat flow determination methods are called methods based on the measurement of heat flow depends on the parameters ( temperature, differential temperature or derivatives of these parameter values) controlled flow or the connecting body.
Termoanemometer method for determining the flow
   The measuring principle . The method involves determining the flow by the temperature of the heated metal wire electricity ( flow transducer ) placed in a controlled flow of gas. Cooling depends on the rate converter flowing stream of gas physical properties ( thermal conductivity , density and temperature ) and the temperature difference between the converter and the gas.
   Fig.1. Anemometer, constant voltage

   There are two known methods:
   Method A. DC . Current ( or voltage) , the heating wire is kept constant and the measured change in resistance caused by cooling : R = f (V). Parameters and the power bridge circuit (Figure 1 ) are chosen so that at zero velocity V gas it is in equilibrium. Deflection galvanometer is a measure of flow velocity .
   The method has a sensitivity sufficient only for low speeds of controlled flow and is not suitable for measuring high velocities (V> 0,5 cm / sec .)
   B. Method of constant temperature . Resistance cooled inverter controlled flow is kept constant by adjusting the heating voltage ( voltage of the bridge circuit ) . This voltage or current supply bridge is a measure of the flow rate controlled . With a sufficient degree of approximation of the velocity dependence of the current is of the form
i ^ 2 = B V + C
where B and C - are constants.

   As in the previous case , the sensitivity decreases with increasing thermal anemometer controlled flow rate , but this reduction is substantially less than when using a constant voltage circuit with the heating anemometer.

   Fig.2. Flow distribution around a heated cylinder

   Rating thermoanemometer ica method. Hot-film method has several advantages : high sensitivity allows the measurement of small and medium flow rates ; miniature size sensor capable of measuring rapidly changing flow rates ; measurement signals of primary converters apply simple circuits and devices ; remote measurement is relatively easy . The disadvantages of the method include : decreased sensitivity with increasing flow rate ; due to the heterogeneity of the wire material need individual calibration of primary converters ; calibration characteristic instability caused by structural changes of wire material caused by heating , dynamic loads , settling dust. Application annealed wire reduces the rate of aging. At high temperatures, the evaporation of the wire is possible . Low operating temperature of the wire increases the influence of the gas temperature . This effect can be eliminated by temperature compensation. Emerging around the hot wire own convective flows of gas ( at zero flow rate controlled ) also distort the evidence , which is especially important when measuring low velocities. As shown in Fig.2 , the direction of flow surrounding the wire is sufficiently ambiguous and varies depending on the ratio of the intensity and direction of the forced flow and the upward flow Archimedean . Due to their low inertia wire thermoanemometers useful in studies of turbulent flows .

Measurements in the industry. Edited P.Profosa , Handbook , Volume 2 , 1990
Датчики, преобразователи. Sensors, transducers