Dynamic characteristics of thermoanemometers with glass-covered transducers

This study deals with the frequency characteristics of a glass-covered thermistor serving as transducer in a thermoanemometer and of a constant-resistance thermoanemometer with such probes.Notation A average-in-time coefficient of heat transfer at the glass-fluid boundary, W/m²· °C - A_tot coefficient of steady-state heat transfer at a bare probe (a fictitious quantity introduced for gauging the heat transfer between a glass-covered probe with the moving fluid), W/m²· °C - a thermal diffusivity of glass which insulated the heat sensitive element from the fluid, m²/sec - C_T total thermal capacity of transducer, W· sec/m²· °C - H₁ ratio of moduli in the expressions for current and resistance fluctuations in the transducer, dB - H₂ ratio of moduli in the expressions for heat transfer and resistance fluctuations in the transducer, dB - I quiescent current through thermistor, A - i transform of fluctuation current through thermistor, A - K_v voltage gain of feedback amplifier - k frequency parameter, 1/m - l thickness of glass layer, m - N intrinsic time constant of thermistor, sec - N time constant of constant-resistance thermoanemometer, sec - M intrinsic time constant of thermistor, sec - M time constant of constant-resistance thermoanemometer, sec - p complex variable in the Laplace transformation - Q average-in-time thermal flux from the transducer, W/m² - q transform of thermal flux fluctuations in the transducer, W/m² - R average-in-time operating resistance of thermistor, - R₁ constant resistance in series with the thermistor in the thermoanemometer circuit, - r transform of resistance fluctuations in the thermistor, - S effective surface area of heat transfer from the transducer, m² - T_D steady-state temperature of hot film, °K - T steady-state temperature of insulating glass layer, °K - T_L temperature of fluid, °K - u velocity of oncoming fluid, m/sec - W_T relation between resistance fluctuations and current in the thermistor, in operator form - y space coordinate in the mathematical model of the transducer, m - fluctuation component of heat transfer coefficient, W/m²· °C - temperature coefficient of resistance, 1/°C - thermal conductivity of insulating material, W/m· °C - _d transform of temperature fluctuations in the hot film, °K - transform of temperature fluctuations in the insulating glass layer, °K - coefficient in the transfer function of a thermistor at high frequenciesTranslated from Inzhenerno-Fizicheskii Zhurnal, Vol. 22, No. 6, pp. 1042–1048, June, 1972.