I bought two “Caliber” temperature-humidity sensors a couple of years ago. Both of them read temperature and humidity consistently; they often show exactly the same values. They work great and the batteries last: I have not changed their batteries yet.

Caliber IV and Caliber III Temperature Humidity Sensors

Since both “Caliber” sensors are from the same company I wanted to double check the numbers using 3-d party sensors. It was easy to accomplish with my Arduino UNO board. I picked four DHT11 (blue) and one DHT22 (white) sensors, connected them to Arduino using a solderless board and wrote a simple program using Arduino IDE to read the values from those sensors and to show temperature and humidity on the connected “2004 20×4 LCD” screen.

The DHT11 sensors have +-5% humidity accuracy and +-2°C temperature accuracy. The DHT22 sensor supports wider measurement range and is more accurate; it has +-2% humidity accuracy and +-0.5°C temperature accuracy.

I found that all sensors both DHT11 and DHT22 read the temperature very consistently. The humidity measurement was very consistent between four DHT11 sensors but DHT22 humidity values were different by ~10%. The humidity values from the DHT22 sensor are close to my “Caliber” sensors.

The screen shows pin number, temperature in Celsius and humidity percentage for each sensor. The more accurate DHT22 sensor is connected to the Arduino PIN 6 and uses the whole second row of the screen to show temperature in both Celsius and Fahrenheit. The third and fourth lines of the screen are dedicated for other four sensors that use Aurduino pins: 2, 3, 4, and 5.

So I am puzzled. Why am I getting different humidity values from DHT11 and DHT22? It looks like my little project will take more time, as I need to troubleshoot if it is a hardware problem, a bug in the DHT library or an issue in my code.

Hmm, which value is more correct? Reading different sensors at once reminds me a proverb: Do not take two compasses to the sea, only one or three.