The HMP45C probe uses a HUMICAP capacitive polymer H chip to measure relative humidity and a PRT to measure temperature. Yearly HMP45C recalibrations are recommended.

To reduce the current drain, power can be supplied to the HMP45C only during measurement when the sensor is connected to the data logger’s switched 12 V terminal. Data loggers that do not have a switched 12 V terminal, such as the CR510 or CR7, can use the SW12V switched 12 V device to switch power to the sensor only during measurement.