The DHT22 sensor returs a 16 bit temperature with bits 0-14 containing the magnitude and the MSB indicating the sign. The driver as written incorrectly treat the value as a twos-complement signed integer and gives incorrect results below 0 C .
I put the sensor in a freezer.
the dht_simpletest converts to F so this example shows the error as the temperature crosses 32 F
Temp: 32.9 F Humidity: 23.4%
Temp: 32.7 F Humidity: 23.5%
Temp: 32.5 F Humidity: 23.6%
Temp: 32.4 F Humidity: 23.7%
Temp: 32.2 F Humidity: 23.8%
Temp: 32.0 F Humidity: 23.8%
Temp: 5930.4 F Humidity: 24.0%
Temp: 5930.4 F Humidity: 24.1%
Temp: 5930.5 F Humidity: 24.2%
Temp: 5930.7 F Humidity: 24.3%
Temp: 5930.9 F Humidity: 24.4%
Temp: 5931.1 F Humidity: 24.5%
Temp: 5931.3 F Humidity: 24.5%
Temp: 5931.4 F Humidity: 24.7%
Temp: 5931.6 F Humidity: 24.8%
Temp: 5931.6 F Humidity: 25.1%
Temp: 5931.8 F Humidity: 25.7%
Temp: 5931.6 F Humidity: 26.3%
Temp: 5931.6 F Humidity: 26.9%
Temp: 5931.6 F Humidity: 27.6%
Temp: 5931.3 F Humidity: 28.7%
Temp: 5931.1 F Humidity: 29.2%
Temp: 5930.9 F Humidity: 29.7%
Temp: 5930.5 F Humidity: 30.2%
Temp: 5930.4 F Humidity: 30.9%
Temp: 32.0 F Humidity: 31.4%
Temp: 32.5 F Humidity: 32.7%
Temp: 32.9 F Humidity: 33.3%
Note - the DHT 11 appear to only measure down to 1 C - so there is no problem there.
jerryneedell
commented
6 years ago
The DHT22 sensor returs a 16 bit temperature with bits 0-14 containing the magnitude and the MSB indicating the sign. The driver as written incorrectly treat the value as a twos-complement signed integer and gives incorrect results below 0 C . I put the sensor in a freezer. the dht_simpletest converts to F so this example shows the error as the temperature crosses 32 F
Note - the DHT 11 appear to only measure down to 1 C - so there is no problem there.