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Old 27-01-2018, 12:26 AM
Garbz (Chris)
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Garbz is offline
 
Join Date: May 2012
Location: Brisbane
Posts: 644
Hi Graham,

The calculation for Dewpoint follows some well established formulas. They are of varying complexity and efficiency. The detailed formulas produce no error, the simplified formulas are accurate >50%RH which if you're looking to calculate the dewpoint to turn on heaters is good enough since you're going to be interested in the point where the RH is close to 100% anyway.

As for the DHT11, there are some serious variances in quality. The RH it spits out is highly dependent on temperature and I found some variance in the accuracy of the temperature part.

Both Temp and RH are easy enough to test yourself. Stick your DHT11 next to a temperature gauge you know is accurate (surely everyone has those lying around right? ) and compare it as temperatures change.

To test the RH accuracy create a saturated salt solution. Get some table salt and mix it with water. Well over mix it. Add way more salt than will dissolve in the water. Then seal your solution air tight with the sensor. (I did this by mixing it in a small dish and putting that dish and the sensor in a zip lock bag and then sealing the last part of the zip lock where the sensor cable came in with some blutack. Leave it over night and the next morning you should have a calibrated environment with 75.3%RH at 25C or 75.1%RH at 30C, and frankly if you're reading between 70 and 80 on a DHT11 you're already better than expected.

http://www.kandrsmith.org/RJS/Misc/H...t11_sht71.html


Fundamentally there's a different problem though. The DHT11 tops out at 90%RH and is lucky to read above 80%. If you turn on dew heaters at 80% that's fine, but if you turn on your dew heaters based on a calculation of dew point and difference of your actual temperature you won't actually ever get to your dew point. The calculation will use RH which may top out at 80% after which as the actual temperature drops your calculated dew point will drop with it and the two will never converge as you will for ever think your relative humidity is not 100% and by definition then the current temperature is above the dewpoint.

My suggestion is jump on ebay and buy a 10 pack of DHT22 sensors and then put them through the salt test till you find one that performs acceptably. Or spend big and get better sensor, but you may struggle to find an Arduino library for proper sensors as the cheap ones are favoured for obvious reasons.
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