Observed Behavior of a Thermistor Bead Flow Meter

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Abstract
Rasmussen [Rev. Sci. Instr. 33, 38 (1962)] has analyzed thermistor bead flow meters, treating constant current and constant temperature modes of operation, and summarizing the velocity dependence of the dissipation constant by means of a single curve. This paper treats a third mode of operation in which neither the bead current nor its temperature is kept constant: the bead's resistance is one leg of a dc Wheatstone bridge, and the bridge unbalance voltage is a measure of the flow velocity. Three results are reported. (i) The dissipation constant possesses two regimes of behavior, one being that noted by Rasmussen, but the other characterized by a velocity dependence featuring a nonmonotonic slope when the bead's temperature approaches that of the medium; (ii) the Wheatstone bridge unbalance voltage, initially, is almost linearly dependent on the velocity; and (iii) the ratio of velocity to bridge unbalance voltage achieves a plateau on which this ratio is nearly independent of thermistor power.

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