EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: JacquesBBB on July 28, 2016, 03:38:38 pm
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Hi All,
I have recently ordered 10 DHT12 sensors on aliexpress.
which were presented as improved form of DHT11.
After some preliminary testings, I have set up a more complete experiement with 9 sensors at the same time.
They give answers that are extremely different for the humidity. Not at all within the advertised specs of the maker (AOSONG) which says 5% for the DHT11 (did not find the DHT12 datasheet)
The temp seems OK, but
I got 48%, 52%, 56%, 59%, 61%, 68%, 69%, 72%, 72% for the Humidity output
so a max difference of 24% which is huge.
I am presently claiming for a partial (50%) refund.
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I have no experience either. :) but hope this helps: Check sampling period of DHT22. Datasheet says :
RH response time min. 6sec. max.15sec. typical -10sec. Also check your code. Are you using shielded wires as recommended? there is a way to rehabilitate sensor if contaminated, see datasheet.
DHT11 (Aosong) sensor specs: https://akizukidenshi.com/download/ds/aosong/DHT11.pdf (https://akizukidenshi.com/download/ds/aosong/DHT11.pdf)
DHT project:
http://www.micropik.com/PDF/dht11.pdf (http://www.micropik.com/PDF/dht11.pdf)
http://www.best-microcontroller-projects.com/dht22.html (http://www.best-microcontroller-projects.com/dht22.html)
http://playground.arduino.cc/Main/DHTLib (http://playground.arduino.cc/Main/DHTLib)
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for me dht11 have always been not precise.
so better than dht11 can be ... less not precise ? ;)
go and try the dht22 and you will have decent results.
then try sensirion sht11 and taste a real precision sensor (but for some 15 euros each !)
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Thanks to all,
The sensors are new, and just on the breadboard, so no problems of soldering, heating, deteriorating of any kind.
For the wiring, I believe that the present wiring is perfectly fine for these kind of low speed communication.
As an example, I have done the same experiment with 5 brand new DHT22
Hum: 63.00 % 60.60 % 63.60 % 65.50 % 66.30 % Temp: 24.80 C 23.70 C 23.40 C 22.90 C 23.10 C
Hum: 63.00 % 60.60 % 63.50 % 65.50 % 66.60 % Temp: 24.80 C 23.70 C 23.40 C 22.90 C 23.20 C
Hum: 63.00 % 60.50 % 63.50 % 65.40 % 66.30 % Temp: 24.80 C 23.70 C 23.40 C 22.90 C 23.10 C
Hum: 63.00 % 60.50 % 63.50 % 65.50 % 66.30 % Temp: 24.80 C 23.70 C 23.40 C 22.90 C 23.10 C
Hum: 63.00 % 60.50 % 63.50 % 65.50 % 66.40 % Temp: 24.80 C 23.70 C 23.40 C 22.90 C 23.10 C
Hum: 63.00 % 60.50 % 63.60 % 65.50 % 66.50 % Temp: 24.80 C 23.70 C 23.40 C 22.90 C 23.10 C
Hum: 63.10 % 60.60 % 63.60 % 65.50 % 66.50 % Temp: 24.80 C 23.70 C 23.40 C 22.90 C 23.10 C
Hum: 63.10 % 60.60 % 63.60 % 65.50 % 66.50 % Temp: 24.80 C 23.70 C 23.40 C 22.90 C 23.20 C
and the output is more reasonable : less than 6 % (± 3%) variation among the sensors (close to the ± 2% of the datasheet )/
I had tested once a DHT11 and found that the output was not very good, but I had some expectations on the DHT12.
I am afraid it looks even worse.
I should try the sensirion, but up to now, I have not because of the price. For a single one, its OK, but if you intend to have dozens of
them, you like to have a cheaper one. I am afraid I will stick to the DHT22 (3$ vs 1$ for the DHT12).
I would be obliged if any one has a link to the real datasheet of the DHT12.
Some tests on the real precision of other humidity sensors are also welcome !
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so why not use these dht22 ? they seems fine.
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Do you get the same results when you use the I2C interface?
Pin 1 - VCC
Pin 2 - SDA
Pin 3 - GND
Pin 4 - SCL
Don't forget to add pull ups on both SDA+SCL.
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The DHT11 and DHT22 are very inaccurate for the humidity. I have a DHT11 that says 31% when it should be 80%. For more accuracy a capacitive humidity sensor is needed. Adafruit has selected this one : https://www.adafruit.com/products/2857 (https://www.adafruit.com/products/2857).
I don't know why the DHT11 and DHT22 are so inaccurate. Perhaps they are not calibrated, or the humidity-resistive part got a few chemical fumes. I think the DHT22 has a DS18B20 with a microcontroller inside. Therefor the temperature of the DHT22 is the only good thing.
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from adafruit web site : https://learn.adafruit.com/dht/overview
DHT11
Ultra low cost
3 to 5V power and I/O
2.5mA max current use during conversion (while requesting data)
Good for 20-80% humidity readings with 5% accuracy
Good for 0-50°C temperature readings ±2°C accuracy
No more than 1 Hz sampling rate (once every second)
Body size 15.5mm x 12mm x 5.5mm
4 pins with 0.1" spacing
DHT22
Low cost
3 to 5V power and I/O
2.5mA max current use during conversion (while requesting data)
Good for 0-100% humidity readings with 2-5% accuracy
Good for -40 to 125°C temperature readings ±0.5°C accuracy
No more than 0.5 Hz sampling rate (once every 2 seconds)
Body size 15.1mm x 25mm x 7.7mm
4 pins with 0.1" spacing