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Anemometer, constant voltage
Flow distribution around a heated cylinder
  HEAT FLOW METHODS
   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
SENSORS OF PHYSICAL PARAMETERS