If your humidity calibration workload requires a high level of accuracy, consider using an external reference to get the best measurement by comparing the reading on the DUT with the reference instead of with the calibrator’s display (we recommend 0.5 °C dew point uncertainty).
It’s common to use a chilled mirror hygrometer as the humidity reference and a reference grade digital thermometer as the temperature reference. A chilled mirror hygrometer is a humidity reference standard that measures dew point on a polished metal surface that is chilled until water begins to condense on it. At this point the dew point temperature is measured; an optical detection system measures the reflected light from the mirror’s surface.
The chilled mirror hygrometer also has an air temperature sensor that can be inserted in the humidity chamber next to the DUT. With dew point and air temperature measurements, relative humidity is calculated by the chilled mirror and measured to a high level of accuracy.
During the DUT calibration, the DUT’s reading is compared with the highly accurate relative humidity measured with the chilled mirror system. The DUT temperature sensor is compared either with the chilled mirror air temperature thermometer or with a reference grade digital thermometer system that is inserted in the chamber with the DUT.
Here are three ways to configure a calibration with a chilled-mirror hygrometer.
Calibrating a hygrometer or other humidity sensor using a chilled mirror sensor head inserted into the humidity generator's chamber
Remote head. In this method the chilled mirror sensor head is inserted into the humidity generator’s chamber. This method reduces the risk of condensation and an external pump is normally not required.
Calibrating a hygrometer or other humidity sensor using a humidity generator with remote head with pumped air
Remote head with pumped air. Draw a gas sample from the humidity chamber to the chilled mirror remote head sensor by inserting a tube inside the generator. Note: when drawing gas out of the chamber, the dew point of the gas must be lower than ambient temperature; otherwise a heated tube must be used to prevent condensation from forming in the tube.
Humidity generator performing a calibration using a chilled-mirror sensor with pump style setup
Pump style. Most chilled mirror hygrometer systems have a built-in chilled mirror sensor so air is pumped from the humidity generator (or calibrator) into the chilled mirror measurement system. In most cases the air is returned to the generator using a return line. Again, watch for condensation in the lines by keeping the dew point of the air lower than ambient temperature.
