THEORETICAL TIME CONSTANT AND RADIATION ERROR OF A ROCKETSONDE THERMISTOR

Abstract

The time constant for a ten-mil-diameter, spherical-bead thermistor is determined as a function of height in a standard atmosphere from sea level to 100 km. Evaluations are made for continuous, slip, and free-molecular flow conditions with the transition from one flow regime to another being a function of sensor size and the mean free path length of the air molecules. The ventilation speed of the sensor is dictated by the fall velocity of the parachute from which the element is suspended. Computed values for the time constant are 0.32 sec at sea level, 0.62 set at 2.5 km, 2.0 set at 50 km, 70 sec at 80 km and 720 sec at 100 km. A discussion of the errors which might be contained in these determinations is also given. These results are then used in computations of the radiation error of the thermistor from sea level to 100 km. Variations in the radiation error which result from changes in various physical parameters of the sensor and its environment are also shown in the 20 to 60 km altitude interval. It is concluded that this sensor should generally provide temperature information which is accurate to ±5C up to about 55 km with no correction necessary. Above this level, corrections to the measured temperature are necessary because of possible variations in the physical parameters of the sensor and its environment. The magnitude of these corrections becomes quite uncertain above 65 or 70 km.