there are two kinds of hot wire anemometer working mode

( 1) Constant flow

by hotline of current remain unchanged, the temperature change, the hotline resistance to change, and both ends of the voltage changes, the flow velocity measurement.

use of wind speed sensor to measure. Wind speed sensor is a sensitive components. When there is a constant current through the heating coil, probe temperature and in still air reaches a certain value. Measuring element thermocouple at this point, produced the corresponding thermoelectric potential, and was transferred to the measurement indicator system, the thermoelectric potential and benchmark circuit of counter potential offset each other, make the output signal is zero, anemometer pointer can also refers to the corresponding to zero or shows zero value. If the wind speed probe end of heat sensitive components are exposed to the outside air flow, due to heat exchange, the change will cause the thermocouple thermoelectric potential, and after the comparison with the benchmark against potential weak difference signal, this signal amplification by measuring instrument system and promote electric meter pointer changes to indicate the current wind speed or after dealing with the single chip microcomputer through the numerical display shows the current wind speed.

( 2) Thermostat

the temperature of the anemometer hotline remains the same, to current adjustable wind speed sensing elements, under different wind speed in different thermal equilibrium state of the working temperature of wind speed sensor and basic maintenance inconvenience, namely basic constant value, the power consumed by a sensitive element is the function of wind speed.

constant temperature anemometer is the use of feedback circuit so that the temperature of the wind speed sensor and resistance remains constant. When the wind speed change thermal sensitive element temperature changes, the resistance changes, resulting in voltage change on both ends of the thermal sensitive element, the feedback circuit, overheat the flow sensor of the corresponding changes in current, and make the system restore balance. The above process is instantaneous, so the increase in speed is like a bridge output voltage increase, and the speed of the lower bridge output voltage is then reduced.

3, the working principle of the circuit

now with constant temperature hot wire anemometer type as an example to illustrate the working principle of it, As shown in figure 1) 。 Place the probe in the wind instrument road bridge in an arm, the resistance of the probe for Rp, the resistance of the other three arm respectively R1, R2 and Rb. The R1 = R2, Rb to one decimal precision adjustable resistance. At this point, requires hotline probe temperature coefficient of resistance is very high, while the opposite request R1, R2 and Rb temperature coefficient of resistance is very small.

on both ends of the bridge AC voltage E, when bridge balance, there is no potential difference between BD, at this point, there is no signal output. When there is no heating probe, probe the resistance value of the Rf is called cold resistance, each sensor has its different cold resistance. Test, an unknown resistance value of the probe in the access bridge road, adjust the Rb balance of bridge, the decade resistors Rb is cold resistance values, is the Rf. According to the selected overheat ratio regulation Rb, makes it higher Rf, generally recommended value of 1. 5Rf。 That is, the instrument of the circuit can automatically back to zero feedback, make Iw increases, thus make hotline probe temperature, resistance increases, until reach the Rw = Rb, then the temperature on the hotline has been elevated to & theta; w，θ W is higher than the temperature of the fluid medium & theta; f。

due to air flow through the probe away heat, so hot temperature & theta; F is reduced, the greater the flow velocity, probe the greater heat loss. System in order to maintain hotline at constant temperature, namely the resistance value is changeless, flows through the hotline probe current Iw will automatically increase, thereby increasing voltage E, in this way, can build up voltage E and velocity & upsilon; ∞ The relationship between.