To most accurately calibrate a hygrometer or other humidity sensor, you need to generate and measure humidity at a variety of temperatures and humidity levels. To do this efficiently, you would use a humidity generator / calibrator. These measurements are compared with the measurements of the device under test (DUT). The difference between the two measurements lets you calculate the error of the DUT measurements.
The calibration process described here uses a mixed-flow humidity calibrator to generate and measure the humidity and temperature. This type of instrument has a chamber in which air is circulated through both a drier system and a humidifier system. A controller determines how much dry air and moist air to mix in the chamber. A calibrated chamber sensor determines the chamber humidity and temperature. This type of generator performs the calibration quickly and accurately.
Here are the steps for how to calibrate a hygrometer or other humidity sensor.
Fill the humidity generator humidifier reservoir with distilled water according to the manufacturer’s directions.
Remove any existing condensation from the chamber.
Insert the device(s)-under-test (DUTs) into the chamber of the humidity generator. Depending on the type of sensor you are calibrating, you might be able to place it entirely inside the chamber or insert just the sensor portion through a port hole that keeps it in place.
Set the chamber to the desired temperature and humidity set point.
Set the lower relative humidity points first and ramp up to the higher levels on each subsequent calibration step.
Set the lower temperature calibration points first to minimize the possibility of water vapor condensing inside the chamber.
In general, best practice is to start at the temperature and humidity points that produce the lowest dew point and then move up. If necessary, points can be taken going down in temperature and/or humidity if the DUT requires this.
Wait for the chamber and DUT to stabilize.
Record the actual temperature and humidity displayed on the calibrator.
Record the reading on the DUT(s).
The difference between the calibrator’s readings and the DUT(s) readings tell you how much your DUT is “off.” If the difference it too great your sensor may need to be adjusted, repaired, or replaced.
