Cheap DH22 sensors return incorrect readings of negative temperature

[fairfield87]

Some DHT22 temperatures are being sold (Ebay) that are not genuine. Genuine sensors will read temperatures down to -50degC. The defective types measure temperature OK above zero but return large and incorrect values when the temperature drops below zero. This has been a topic of discussion before:

DHT22 negative readings; is there a solution?

Hello, everyone! Is there a solution for negative Celsius readings of DHT22 sensor? I read many posts on this isue, I try everything I found about that, but no succes. I use an Arduino Uno. My sensor is wired as everybody says: + to Arduino's 5V; minus to Arduino's GND and data pin to pin no.2 of Arduino. And I drop it in my freezer. The soft is an example from DHT_sensor_library_1.4.4: // DHT Temperature & Humidity Sensor // Unified Sensor Library Example // Written by Tony DiCola for Adaf…

I have investigated this and discovered that the sensor will produce a reading of -3276.8 at the point that the temperature drops below zero. The readings do change with temperature: e.g at -0.4 degC the reading is -3276.4; at -0.8 degC the reading is - -3276.0. So it is possible to correct for this error using the following code:

float t = dht.readTemperature();
// Read temperature as degC
Add this code to make the correction if required.
if (t < -50) { // The correction will not be applied if a genuine DHT22 is used
t = -3276.8 - t ; // correct value if t< 0
}
This will return negative temperatures correctly

[MarkT]

Or put another way they use sign and magnitude coding rather than 2's complement like the genuine ones, in units of 0.1 degree, hence 3276.8 which is 2^15 / 10