Bill_J
Posts: 18
Joined: Tue Jun 26, 2018 7:18 pm

Another question about RPi and the DHT11

Fri Feb 15, 2019 11:47 pm

HI. I have a DHT11 working okay to a 16x2 display and the python script includes the math to get °F. Update speed is not an issue. A DHT22 has been purchased and will arrive in a few days.

My temperature was set at 99.6° with a circuit unrelated to the RPI and DHT11. Built around an LM34 I can set the temp fairly accurately but the circulation of the air is slow relatively speaking in system response time.

The DHT11 is mounted above the center of a lazy susan very slowly rolling eggs. The DHT11 was reading 98.6°F constantly at 3.3v. I thought perhaps it wouldn't read higher. I switched the DHT11 to 5v. No change in the temperature reading. I adjusted my Set to 100.0°F. Due to the slow air rotation there's a temperature overshoot even with almost no hysteresis. So, my reading from the LM34 show the circulating temperature to reach 100.4° and lo and behold the DHT11 sees 100.4° briefly then back to 98.6°.

Took a while to figure out but that jump from 98.6° to 100.4° is 1°C. I had thought the accuracy might be +/-1°C but not its output no finer than 1°C. That jump is rather large. I tried adjusting in the .py script to :0.05f from :0.1F but all that got me was 98.60000*F.

Is this 1°C set immutably in the module? It seems a rather gross jump in °F.

Thanks for reading.

Bill_J

danjperron
Posts: 3276
Joined: Thu Dec 27, 2012 4:05 am
Location: Québec, Canada

Re: Another question about RPi and the DHT11

Sat Feb 16, 2019 12:08 am

DHT11 is +/- 2 Celsius.

You should have used another type of sensor for that purpose. At least the DS18B20 will have more resolution.

Bill_J
Posts: 18
Joined: Tue Jun 26, 2018 7:18 pm

Re: Another question about RPi and the DHT11

Sat Feb 16, 2019 12:30 am

Wow. Nice device. Wish I had known. Will possibly use it next time I run the incubator, 5 years from now or so.

Bill_J
Posts: 18
Joined: Tue Jun 26, 2018 7:18 pm

Re: Another question about RPi and the DHT11

Sat Feb 16, 2019 12:57 am

I don't suppose the DS18B20 can be swapped in for the DHT11 and respond to the script if the proper module is loaded?

Oh, that's right, the DS18B20 doesn't have humidity.

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rpdom
Posts: 13887
Joined: Sun May 06, 2012 5:17 am
Location: Chelmsford, Essex, UK

Re: Another question about RPi and the DHT11

Sat Feb 16, 2019 6:51 am

Maybe use something like a BME280 which is an i2c pressure, temperature and humidity sensor?
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