Conversion of Fluid Thermal Parameters into Frequency and Time by Means of Thermistors

A versatile direct converter of fluid thermal parameters into frequency and time is described. The circuit is based on a complementary multivibrator, the frequency of which is controlled by the temperature-sensitive current generator. Temperature-to-time (θ-T) and temperature-to-frequency (θ-f) conversion is obtained by simple rearranging of passive circuit elements. The linearizing conditions and sensitivity for various probe configurations are given. In battery telemetering systems, the circuit is economical in power supply consumption due to the very small mark-to-space ratio of output pulses. The sensitivity as well as the transducer Rmax/Rmin ratio are considerably increased in comparison with other direct thermistor converters. With the positive temperature coefficient (PTC) thermistor probe, the frequency change can be about three decades in the temperature range of 30°C, which represents the highest obtained sensitivity of θ-T/f converters. The circuit is also successfully applied with self-heated miniature bead thermistors as a sensor when it represents the linearized air speed-to-time converter within the range of 0-3.6 m/s.