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I don't know what the jaycar one uses to calculate dew point. Mine uses:
dewPointAmbient = (logEx - 0.66077)*237.3/(0.66077+7.5-logEx);
logEx=0.66077 + 7.5*temperatureAmbient/(237.3+temperatureAmbient) + (log10(humidityAmbient) - 2);
This is reasonably accurate (unlike simpler formulae that have huge errors at humidity below 50%).
As Gabrz says the DHT11 can have trouble above 80% humidity. I figure this could lead to an error of about 2C in the dew point calculation. I'm not going to lose sleep over that. There's probably more errors due to placement of the scope lens temperature probes which provide the feedback control to the heater (difference between ambient dew point and temperature of scope glass which has to be heated).
Also the Jaycar uses as resistor to sense scope temp? I don't think think will work as well as the DS18B20s (which theoretically doesn't require any calibration)
Edit: actually error might be a bit more, maybe 3-4C error between humidity at 80% v 100%. But for the purposes of a controller I don't think it matters too much, the other errors probably have a bigger impact.
Last edited by ChrisV; 27-01-2018 at 01:15 PM.
Reason: mistake in calculating dew point error
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