Calibration of humidity sensor DHT21

The DHT21 sensors used in the “Beelogger” are capacitive sensors used in modern electronic absorption hygrometers. This type of sensor uses a hygroscopic layer as dielectric between the two electrodes of a capacitor. The absorption of moisture in the dielectric changes its properties and so the electrical capacity of the sensor.

The accuracy of this sensor is specified by the producer with ±3%. In the peripheral areas there may be deviations of 5% points. With a measurement of 95% humidity, 90-100% humidity can be in reality present.

High-quality devices (such as the Trotec T210 thermohygrometer) also work with this type of sensor. Here the manufacturer states a slightly higher accuracy of ±2%, which is probably due to a more complex calibration.

More precise measurements can only be achieved with more complex procedures. With the dew point mirror hygrometer, for example, a mirror is cooled down until the humidity is condensed on it. A light source and a photo sensor determine the moment of condensation. Such devices are much larger, require more energy and are very expensive. An application in bee hives is hardly possible.

The accuracy of the DHT21 sensors can be checked with a simple test setup: They are placed in an airtight sealed box with individual saturated salt solutions. Depending on the salt used, a certain compensating air humidity is set in the container. This process is accelerated by a small fan, which blows the surface of the salt solution and allows the air to circulate in the box. Depending on the salt used, the corresponding compensating air humidity is reached after 60-120min.

In the program code of the Beelogger Solar, the offset values can be simply added to the measured values. In this way, the accuracy in the expected measuring range can be increased. For measurements in bee hives, the range between 40 and 100% is relevant.

The deviations of the sensors presented here amount to a maximum of 2% in the relevant range after calibration.