DHT environmental sensors at freezing temps - Page 1
Forum section: electronics / projects

author: bitwelder
author_rank: Super Contributor
timestamp: December 07, 2012, 11:10:22 am
content: Hello,I see the DHT22/AM2302 sensors are quite popular as temperature+humidity sensors for hobby projects.Together with the similar DHT21 (AM2301), they are rated for temperatures from -40 C to 80 C.If you look at the picture of what's inside a DHT21, you see that the sensor mini-board contains also a crystal, an IC and few other components. My question is, do they work reliably under zero C ?(picture below taken from an Aliexpress vendor)

author: nukie
author_rank: Frequent Contributor
timestamp: December 07, 2012, 01:28:07 pm
content: If you want reliability then Sensirion or Honeywell is your answer. How well these Chinese parts are calibrated is up to how much you are willing to pay)

author: bitwelder
author_rank: Super Contributor
timestamp: December 07, 2012, 03:34:16 pm
content: Thank you but Sensirion or Honeywell are not my answers, but perhaps (better) alternatives.I'm looking for a technical explanation of whether the component above may/may not work in extreme conditions, irrespective of its country of origin.

author: daedalus
author_rank: Regular Contributor
timestamp: December 07, 2012, 11:34:09 pm
content: I agree with nukie, if you want a properly designed and calibrated sensor then Honeywell etc are ideal. That said if you want to use this you have two options, either trust the spec, or do your own calibration / reliability tests.AM2301 is rated at -40degC to +80degChumidity 0-100%

author: Unknown
author_rank: Unknown
timestamp: December 08, 2012, 12:12:14 am
content: Experience tells me to be very suspect of any open board sensor at sub zero temperatures. Condensation is the killer, it is pretty hard to effectively shield a humidity sense element from condensation and still have it read effectively.Most humidity sense elements rated 0-100% usually have issues at the extents of the range. At 100% rh most humidity sensors will saturate and will be use useless until all absorbed moisture is evaporated, some never recover properly. At transition to lower temperatures elements are liable to damage when absorbed moisture freezes and subsequently expands.The usual protection from condensation is some trace heating which isn't going to do any favours for temperature readings.I don't necessary question the sensor range and accuracy quoted but doubt the thing is suitable across all combinations of those ranges. At -10 and 100%, I think you'd have problemsIf you want temp and humidity under extreme conditions you have little option but to use expensive purpose built sensors.If you can get away with temperature only and want low cost options I'd consider fully potted 1wire or RTD sensors, both can give good results and can prove to be hardy in cold environments..

author: bitwelder
author_rank: Super Contributor
timestamp: December 08, 2012, 08:21:58 am
content: Thanks Vernon!That's the kind of reasoning I was looking for.

author: tht
author_rank: Newbie
timestamp: January 06, 2013, 02:18:49 pm
content: Hi bitwelder,I'm using some Sensirion SHT11 sensors and had good results last winter. I got accurate measuring even at the coldest days (-22°C). The battery voltage dropped quite a bit at these low temperatures but my wireless sensor sent even when covered under some snow.Not exactly what you're asking for but maybe it helps.Thomas