emonTH calibration

I am new to the Emon and experiencing about a 3 deg F difference in my readings on both an EmonTH and a sensor on the back of the Emon , im doing a function on the reading to convert C to F of *9*.2+32 , and im wondering if i need to calibrate to the sensors output on each. I plan on installing several more and I would like them to be with the accuracy of +/- .05 C they should be.  Any input would be appreciated. 

Robert Wall's picture

Re: emonTH calibration

Which sensors are you using if the accuracy (not to be confused with the resolution) is ± 0.05°C ?

From the DS18B20 data sheet:

"The DS18B20 communicates over a 1-Wire bus that by definition requires only one data line (and ground) for communication with a central microprocessor. It has an operating temperature range of -55°C to +125°C and is accurate to ±0.5°C over the range of -10°C to +85°C."

From the DHT22 data sheet:

"Accuracy: humidity +-2%RH(Max +-5%RH); temperature +-0.2Celsius"

If you are unlucky, using those two, the maximum difference should be 0.7°C (1.26°F). So I think the difference you are seeing is likely to have a large component of local heating in it. There is some evidence that powering the DS18B20 continuously in still air does give rise to some self-heating. I did some measurements a year or two ago and if you search, you might be able to find them here: http://openenergymonitor.org/emon/node/2484. I've not done the same for the DHT22.

(You'll probably find that multiplying by 1.8 and adding 32 is a more efficient way to convert the temperature.)

mcihon's picture

Re: emonTH calibration

Im running both , an emonTH with internal DHT22 and an external DS18B20, on the indoor temperature I am using a DS18B20 into the RJ45 connector on the rear of the emonpi. The EmonTH i have read turns off the sensors in between reading in order to prevent self heating. Im taking readings more often than needed just for set up experimentation , perhaps I will change them to every 10 minute and see if that resolves the issue. 

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