Vaisala WXT520 - Manual (Page 31)

Chapter 3 ______________________________________________________ Functional Description
VAISALA_______________________________________________________________________ 29
PTU Measurement Principle
The PTU module contains separate sensors for pressure, temperature,
and humidity measurement.
The measurement principle of the pressure, temperature, and humidity
sensors is based on an advanced RC oscillator and two reference
capacitors against which the capacitance of the sensors is continuously
measured. The microprocessor of the transmitter performs
compensation for the temperature dependency of the pressure and
humidity sensors.
The PTU module includes
- capacitive silicon BAROCAP
®
sensor for pressure measurement,
- capacitive ceramic THERMOCAP
®
sensor for air temperature
measurement, and
- capacitive thin film polymer HUMICAP
®
180 sensor for humidity
measurement.
Heating (Optional)
Heating elements located below the precipitation sensor and inside the
wind transducers keep the precipitation and wind sensors clean from
snow and ice. A heating temperature sensor (Th) underneath the
precipitation sensor controls the heating. Note that Th is measured
inside the equipment, where temperature is much higher than the
ambient temperature (Ta).
Three fixed temperature limits, namely +4 °C, 0 °C, and -4 °C (+39 °F,
+32 °F, +25 °F) control the heating as follows: