Design and evaluation of a new linear thermistor velocity probe.

Abstract

Based on the property of a heated thermistor (Th) to dissipate heat as a function of flow velocity ([nu]), instrumentation and probes were developed which combine a 3-db frequency response of 150 Hz, linear output, insensitivity to blood temperature changes, directional sensitivity, and in vitro calibration. In a model, v was correlated with Th dissipation factor [delta]([delta] = P/[DELTA]T, where P = heating power, [DELTA]T = temperature difference between Th and fluid). In 16 experiments [delta] [the power required to maintain the thermistor 1C above the immediate environment] was found to vary linearly with the log of [nu]. Instrumentation measures and solves for velocity. Probes carry 3 Th: a velocity sensor, operated at 42 C; a temperature sensor which maintains [DELTA]T constant; and a direction sensor which effects reversal of signal polarity for negative [nu]. in vitro calibration is provided by a calibrator generating 1-Hz sinusoidal velocities. In the model at constant the velocity probe agreed well with [nu] obtained by timed run off. Qualitative comparison of the velocity probe with an electromagnetic flowmeter in the aorta of 12 anesthetized dogs showed comparable signal contours, magnitudes, and rates of change. It is concluded that the method is suitable for quantitation of instantaneous blood flow velocity in man.