Sensor preheat is an optional feature of the HM70 series humidity probes HMP75B, HMP76B, and HMP77B.
In some measurement applications, unwanted dew formation makes humidity measurement difficult or even impossible. High humidity combined with rapidly changing outdoor temperature may condense the water vapor in the air onto the sensor. Until this dew evaporates or dries, it is impossible to obtain a correct reading. In sensor preheat the sensor is heated to a temperature level of approximately +100 °C (+212 °F) for up to 4 minutes. This is to prevent possible condensation from the sensor affecting the reading.
When setting the probe from outdoor conditions into warm and humid conditions there may be a large temperature difference between the probe and the external environment. In such conditions, turn on sensor preheat immediately before installing the probe to prevent condensation forming on the probe.
Figure 1 shows how the response time can be dramatically shortened and water condensation on the sensor avoided when using sensor preheat.