Abstract: Self-calibrating a humidity sensor of an integrated humidity and temperature sensor, using the integrated heating element, and the temperature and humidity sensors, of the device, together with a firmware-based control loop running on a controller. The controller actively calculates and monitors one or both sensor output, and the slopes of the sensor outputs, in real time, while the heating element is on, and compares one or both slopes to a pre-programmed threshold, while in a programmable control loop, to then capture the appropriate relative humidity offset to apply to the device during normal operation (with the integrated heating element switched off) for correcting the relative humidity output from the device. Any singularly mounted in-system device can be calibrated, independent of external references, for in-system calibration.