Response of the thermistor used in the Australian radiosonde

The specification for the Australian radiosonde states that the response of the thermistor shall not exceed 5 seconds. This limit, however, only applies at sea level and at high altitudes the response increases considerably to a value of about 18 seconds at 35 km. The response has been measured during a balloon flight using a modified radiosonde. Theoretical values of response based on heat transfer theory are in reasonable agreement with the experimental values. The theoretical determination requires a knowledge of the thermal capacity of the thermistor and this has been measured for temperatures over the range +40° to -60°C. Examples are included showing the errors due to response for typical situations in a radiosonde flight.