In. the. constant-current mode of operation, the current through the hot wire is ‘kept at a suitable value. The hot wire anemometer is connected in a bridge circuit as shown in figure 3.24. The bridge iscalibrated first. The value of .current I through the ianemometer is selected and set at a proper value taking precautions so that the burn out of hot wire does not occur.
The’hotwire iasubjected to different known values of velocities V of the fluid under test-. ·This changes the value of Rw and therefore unbalances the bridge thereby producing an out of balanced voltage eo which is measured by a high
The value of any unknown value of flow velocity can be found from-the calibration curve corresponding to the out, of balance voltage eo. Suppose while measuring the velocity ofa fluid, an-out of balance voltage eOl is obtained; the velocity corresponding to this is VIas found from the calibratior curve. iThe range ‘of velocities .for which constant current type anemometer can-be-used –is necessarily low because of the possibility of .th~ wire burn out when theflow stops. This means that choice of lower value-of I’ for the’ upper .limit of velocity or a lower value of velocity-for an upper limit with a satisfactory value of I. The measuring circuit of the constant current anemometer can be used for the measurement of steady velocities as well as the rapidly fluctuating components such as the turbulent components superimposed on an average velocity.
