DHT 11 vs 22 humidity readings mismatch?

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laserdog
 
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DHT 11 vs 22 humidity readings mismatch?

Post by laserdog »

I've got a DHT11 and DHT22 setup next to each on a breadboard connected to a beaglebone black.

I am reading them via the latest Adafruit_Python_DHT library.

The DHT11 gives:
Temp=22.0*C Humidity=42.0%

The DHT22 gives:
Temp=22.6*C Humidity=28.8%

So they agree on Temp, but the chasm between the Humidity values is too wide given the sensitivity of the two.

I swapped *both* sensors out, with identical spares I had, and similarly off values.

I thought maybe there was an acclimation period, so let it run for a week, no change.

I suspect the DHT11's reading is *more* correct, but that is a guess.

So I am left with two possibilities:

1.) I have 2x bad DHT22s
2.) The Python Library has a bug reading Humidity from DHT22s.

When I change the sensor type to AM2302, I get identical values.

I realize this likely isn't enough information to solve what is wrong, but I am stuck as to a next step.

Thanks for reading.

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adafruit2
 
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Re: DHT 11 vs 22 humidity readings mismatch?

Post by adafruit2 »

temperature is easy, humidity is a little tougher. the DHT22 & AM2302 are the 'same thing' but btw, the DHT11 is really not a good humidity sensor - the DHT22/AM2302 is more reliable :)

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laserdog
 
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Re: DHT 11 vs 22 humidity readings mismatch?

Post by laserdog »

Yeah, if the readings were reversed I would chalk it up to "the DHT11 sucks".

But 30% humidity in my basement is not passing the common sense test right now. 40% is just more correct.

Is this something I could introduced by using the wrong resistor or something? I had ruled that out due to the sane temp readings...

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adafruit_support_mike
 
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Re: DHT 11 vs 22 humidity readings mismatch?

Post by adafruit_support_mike »

Nothing you've done would have that effect on the sensor.

IIRC, the DHT11 uses a capacitive humidity sensor. The sensor is made of two metal plates with a thin sheet of plastic between them. The plastic absorbs water from the air proportional to the humidity, and that changes the dielectic strength of the polymer (water is about 80 times as good a dielectric as air).

As with most things that are simple in principle, the details get messy. You have to back-estimate the amount of water absorbed by the plastic from the capacitance, then use that to estimate the humidity. If any of a dozen external factors come into play (the exact absorption of the plastic, component tolerances within the sensor, the absolute temperature where the sensor is located, airflow near the sensor, etc) the readings from two 'identical' sensors will be different.

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laserdog
 
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Re: DHT 11 vs 22 humidity readings mismatch?

Post by laserdog »

> the readings from two 'identical' sensors will be different.

That is the weird thing.

DHT11.A and DHT22.A show a humidity discrepancy.

I swap them *both* out.

DHT11.B and DHT22.B show a humidity discrepancy, which makes me wonder if the library is acting properly...

Which, I hate to do, because clearly people way smarter than me put that together.

It is just frustrating to have 4x temp/humidity sensors and not know what the humidity is. =)

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Re: DHT 11 vs 22 humidity readings mismatch?

Post by adafruit_support_mike »

Try all four combinations and see if the offsets are consistent.

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