Pressure Measurement

The pressure sensor is an encapsulated steel aneroid sensor. It utilizes a capacitive transducer with a vacuum inside the capsule. The entire unit is precision welded requiring no mechanical adjustment. The unit is friction free and continuously variable.

Temperature Measurement

The temperature sensor is a capacitive bead in GAUS encapsulation. The temperature sensors are calibrated at the factory.

The manufacturer's specification for the time constant is less than 1 second at 100 mb. The time constant of the thermistor, combined with the ascent rate of the sonde produce a slight lag in temperature measurement through the sounding. However, with typical atmospheric lapse rates the resultant smoothing of the temperature profile is less than the accuracy of the thermistor. The smoothing resulting from the lag time becomes more significant when the sonde crosses frontal boundaries or goes through strong inversions.

Experience has shown that if the sonde sensor arm is not protected or properly ventilated prior to launch, it can be adversely affected by solar heating. This results in a temperature reading that is too high, producing a false near-surface super-adiabatic lapse rate. Due to the small thermal mass of the temperature sensor and its supporting structure this effect is not long-lived. The thermal time constant of the sensor arm is 13 seconds and thus the problem goes away soon after launch, once the sensor is adequately ventilated.

Relative Humidity Measurement

The humidity sensor is a thin-film capacitive type sensor. The Vaisala type H radiosonde utilizes the type H sensor, which uses a humidity algorithm incorporating temperature compensation. This new sensor has improved humidity measurements over previous sensors, particularly in the high end of the humidity range (95% to 100%).

Solar heating of the sonde temperature/humidity sensor arm prior to launch can produce an error in the low level humidity measurement (and hence dew point).The humidity sensor gives a reading of the humidity relative to the temperature of the sensor surface itself. In a situation where the sensor surface is warmer than the surroundings, the humidity reading will be lower than ambient (vapor pressure remains unchanged, "sensed" saturation vapor pressure value goes up). Due to the thermal time constant of the sensor arm (13 seconds), the initial heating of the sensor arm affects the humidity data for roughly the first 40 seconds of the flight. (In a shaded, well ventilated situation, in which the sensor surface is in thermal equilibrium with its surroundings, an accurate ambient humidity measurement at the surface can be obtained.)

The effect of the heated sensor arm persists for a longer time in the humidity measurement than it does in the temperature measurement. The portion of the sensor arm where the temperature sensor is mounted is an isolated small cylinder which quickly comes to a thermal equilibrium with its surroundings whereas that portion of the sensor arm on which the humicap is mounted is much larger and thus takes more time to come to a thermal equilibrium with its environment.

Wind and Position Measurement

Winds derived from GPS have a constant accuracy once the minimum number of satellites (typically four) are received.