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.

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 could start to see systematic repeatable differences between sensors, that might be 'error' in one or other of the devices. When reading these pages, please do remember at all times that a discrepancy from prediction could be fault in the sensor or in my test.

These pages have grown over time as I try repetitions and variations on certain experiments. Though I have gone back an updated some of the older write-ups in the light of new results, some results described in the older pages are likely to be outdated. I have therefore split the conclusions into two sections. I have a single page I can keep up-to-date with my latest overview and summary of each sensor. Then I have all the detailed results which present the actual data and detailed discussion presented as they were at the time that web page were written. Comments on those older detailed write-ups could contradict statements I make in my latest overview summary.

Conclusions:

• Overview summary of each sensor with my own personal opinion on its accuracy, strengths and weaknesses. [In preparation, Spring 2018]
• They all work. Almost all do actually measure relative humidity to an accuracy sufficient for most realistic tasks.
• Individual devices will not necessarily match the advertised 'typical' performance.
• The actual accuracy achieved depends at least as much on your instrumental setup as much as the devices themselves, in some ways more so.
• Just taking the one I say was 'most accurate' may not be the best for your application. Think for a moment, is it really useful information to you whether the humidity is 90% or 85%? Both are basically 'a bit damp'.
• Ease of use, reliability and long term stability may be more important to you than absolute precision.
• Long term, these types of sensors are all susceptible to calibration drift and those changes likely exceed many of the differences between new devices.

• All the details; Direct tests of specific devices in reverse order from newest to oldest
• Adding Sensirion SHT31 in comparison to previously tested AM2302/DHT22, SHT71, Si7021 & BME280.
• Baking a possibly contaminated SHT71 to recondition. Long term reliability.
• Initial detailed comparison of AM2302/DHT22, AM2320, AM2321, SHT71, HTU21D, Si7021 & BME280.
• Compare DHT22(AM2302), DHT11 and SHT71
• Six DHT22 (AM2302) sensors. Includes the most complete description of my test methodology.
• Other observations on the sensors and the test method
• DHT22 (AM2302) reliability and failure of one device
• How fast can DHT22, SHT71, BME280 devices be read?
• Comparing the response speed for the HTU21D and Si7021.
• Effects of solution supersaturation.
• Absolute temperature accuracy for the BME280.
• Noteworthy idiosyncrasies: a few miscellaneous things worth knowing when using these sensors.
• First generation experiment with DHT22 only. Retained because some mistakes I discovered in my original setup may be of interest to anyone repeating the experiment.

2018-02-25 1:07 PM