Hello Tony
Thanks for contacting us and showing your interest in dew point measurements. The capacitance signal is measured by converting it first to frequency, where the sensor is a variable capacitance component in the oscillator circuit. This frequency is then counted and converted to relative humidity based on the sensor calibration and linearization (this part is based on the sensor model and other proprietary algorithms that are not public, unfortunately). Please note we always measure relative humidity, and therefore, we need also precise temperature reading that is measured on the humidity sensor surface. This is a normal resistive (pt100) measurement.

With a help of this document, you can derive the dew point calculation based on the measured RH and T: https://www.vaisala.com/system/files/documents/Humidity-conversion-formulas-Technical-eBook-B210973EN-L.pdf
I hope you will also find answers to other humidity-related calculations like absolute humidity, pressure-dependency, and other humidity formulas under the same document.

Best regards
Juhani