What is a warmed probe?

A warmed probe refers to a special type of humidity probe in which the sensor and probe tip are heated to reduce the likelihood of condensation forming on the humidity sensor, even in saturated environments. Probe warming is a patented technology available on the Vaisala HMT337 Humidity and Temperature Transmitter. The warmed probe relies on a composite sensor where the humidity and temperature sensor are bonded together, making it is possible to know the relative humidity and the precise temperature of the relative humidity sensor simultaneously. In the warmed probe, a separate electrical resistance heater is installed within the probe. The heater is controlled by the measured relative humidity and is designed to keep the probe a few degrees above ambient temperature. Heating of the humidity sensor corrupts the humidity measurement (relative humidity is temperature dependent), but the composite sensor allows for the accurate calculation of dewpoint temperature. By default, warmed probe instruments provide dewpoint temperature as an output. With the addition of a separate air temperature probe, relative humidity and temperature can also be provided as outputs.