Testing Various Low-Cost Hygrometers

See also the companion page on thermometers.

Described here are my own experiences assessing accuracy and calibrating the following devices. Results only represent my own specific devices which I have bought from various retail sources.

Bosch Sensortec BME280
Measurement Specialties HTU21D
Silicon Labs Si7021
Sensirion SHT71
Aosong DHT11, DHT22, AM2302, AM2320, AM2321

I would like to stress here that my interest in this is rather more about the methodolgy of testing and how accurately I can set up the calibration experiment using low or zero cost equipment than it is about truly calibrating these sensors. It would be presumptuous of me to suggest I know better than the metrology experts at some of the world's premier digital sensor manufacturers. Where I see a deviation between my measurements and multiple sensors I happily assign the error to my experimental procedure and look for improvements. My setup went through many iterations on that basis. Once I can start to see differences between sensors, that might be 'error' in one or other of the devices. Please do bear in mind at all times that a discrepancy from prediction could be fault in the sensor or in my test.

My conclusions are

Update (March 2017): I have a major revision of these conclusions in preparation. There is still a long way to go and data collection is ongoing, but I want to be as open as possible. Newer tests have suggested that in my previous tests I got lucky with an excellent Bosch specimen but possibly had a damaged Sensirion device. In that first one-off test, the BME280 was the clear winner, but I do not want anyone to interpret that as definitive. In my ongoing tests, the Bosch and Sensirion devices are indistiguishable at the limit of what I can measure. Work is still in progress.