How to Check and Calibrate a Humidity Sensor

How accurate is your humidity sensor? Find out with this project.
Humidity sensors are commonplace, relatively inexpensive, and come in many different varieties. Too often, we check the datasheet, use them with an interface, and (as long as the values “look reasonable”) we accept the results.

In this project, we demonstrate how to go a step further and verify the accuracy of a humidity sensor. We also illustrate a general method for sensor calibration and apply the method to calibrate the results to improve the accuracy of the humidity measurements.

Project Fundamentals
To check the accuracy of a sensor, obtained values are compared to a reference standard. To check the accuracy of a humidity sensor, we use the “saturated salt” method to produce the standards. Put simply, certain salts (i.e., ionic compounds such as table salt or potassium chloride), when dissolved in an aqueous solution, produce an atmosphere of a known humidity (see reference PDF).

These chemical properties are used to create micro-environments of known relative humidity (RH) percentages (i.e., reference standards), and the sensors are read inside the micro-environment. Specifically, we will make a solution in a sealed jar to preserve the atmosphere and then place the connected sensor in the sealed jar. Subsequently, the sensor is repeatedly read and the values recorded.

By repeating the procedure using several different salts, each producing a different relative humidity, we can develop a profile for the sensor under test. Since we know what the relative humidity is for each micro-environment, we can assess the deviations of our sensor readings from those known values, and thus, evaluate the accuracy of the sensor.

If the deviations are substantial, but not insurmountable, we can apply mathematical calibration procedures in software to increase the accuracy of the measurements.”