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I bought a batch of ESP8266-01 and DHT11 on ebay.

The sensors give data, but the datasheet say +/- 2 degrees. Who can live with that? Well, they claim the repeatability is 0.2 degrees, so I guess they could be calibrated.

But thet are not stable. They start a few degrees over room temperature, but gradually the reading is up to 33 degrees, more than 10 degrees too high.

Could it be the proximity of the ESP8266 giving off heat? Or are the sensors themselves heating up when reading at one read every two seconds?

  • I would suspect heat from the 8266. Can you add a second sensor at a little bit of a distance and compare the two values as it runs for a bit – Chad G Feb 23 '18 at 19:19
  • the sensor only uses microamps, it's not self-heating. you might try the DHT22; twice as good. try reading every 10 seconds and check the linearity. – dandavis Feb 23 '18 at 23:15
  • I have ordered a handful of DS18B20 onewire sensors. I have had more success with these before, they are waterproof, and I can put more sensors on one ESP. – Lenne Feb 24 '18 at 0:48
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    more than 10 degrees too high ... how do you know? ... you did not say that you used a thermometer to verify the reading – jsotola Feb 24 '18 at 5:01
  • i'd suspect the physical placement of the sensor. perhaps its near an incandescent lamp or tube ? – Shreyas Murali Feb 24 '18 at 5:01
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It could be the ESP8266, try physically separating the DHT11, but I'm guessing that that will only help slightly.

Do you have some way of reliably calibrating the sensors? If not, then even if you are getting consistent readings, you'll have a hard time knowing how accurate your readings are.

While they're typically more expensive, it might be prudent to buy an infrared temperature sensor. If you have the budget, replacing the DHT11s with IR temperature sensors might just be a better solution, I saw that Sparkfun's MLX90614 was accurate within +/- 0.5 degrees Celsius. If you don't have the budget to replace your DHT11s, or don't want to be wasteful, perhaps you just buy one IR temperature sensor to use as a sanity check. These should be more reliable than DHT11s and you could even write a sketch to calibrate them based on the IR temperature sensor's readings.

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