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emonTH v1 DHT22 self heating?

Tags: #<Tag:0x00007f6e05df6f68> #<Tag:0x00007f6e05df6e50>

Hi all!

Its been almost 2 years since I was last playing around with a bunch of emonTH’s, TX’s and the Pi as a base station… I was working on a project to use the emonTH idea as a sort of ‘learning’ tool in order to develop my own more large scale low cost temperature solution for datacenter moinitor applications…

However here I am again with the horn for getting my head into such things again :slight_smile:

Anyway… I have set two of my emonTH’s back up, these were the older versions with the DHT22 onboard. Now alarmingly I have noticed pretty major innacuracies with the temperature measurement of the DHT22 from both units, I have tried a few different DHT22’s I had lying around as well and the same thing happens…

Outside, when the temp was in the low 0’s, the DHT22 was reporting around 2-3c above actual temperature… So I wired in an external DS18B20 and that gives more accurate readings.

On my emonTH inside, I have just sat 3 different K type thermocouples next to it, each going into different meters and it turns out the DHT22 is reporting around 1.5c above what the K types are reading (which were all within 0.2c of each other).

So… I got my el cheapo IR thermometer out and compared the surface of the emonTH case to the surface of the DHT22 itself and sure enough… Its at least 1c - 1.5c above ambient…

Some research online tells me that self heating is common with this sensor… But is it really this bad? I fully understand you can offset this in code as a thermal ‘correction’, but I wasn’t expecting it to be CONSISTENTLY hot running across 3 different DHT22’s.

Anyone else noticed this?

Tom

there a good comparison on this website:
http://www.kandrsmith.org/RJS/Misc/thermometers.html
http://www.kandrsmith.org/RJS/Misc/Hygrometers/calib_many.html
 

The bottom line is the DHT22 wasn’t as accurate as hoped and was updated:

Interesting, I am certainly getting much better readings with the DS18B20 Temperature only sensor. Spot on against my two K Type thermocouples.