NCAR Hygrothermometer

The NCAR temperature/humdity sensor is the primary sensor for slow-response (~1 Hz) samples of these variables.  A commercial SHT85 sensor is used.  Each SHT85 is run through the NCAR calibation laboratory to create a calibration.  The RH calibration has been found to have a weak dependence on temperature, which is calculated.  The calibration is loaded into a custom PIC-based processsor board and the PIC outputs both raw and calibrated values.  The SHT85 and PIC are placed inside an NCAR-designed aspirated housing with a double-tube surrounding the sensor for radiation shielding.  The tube has a flaired opening to prevent wind across the tube from opposing the fan-generated flow.  The fan speed is controlled and reported by the PIC to ensure that the sensor aspiration is sufficient.