ghostrider513
Posts: 35
Joined: Tue Mar 28, 2017 6:03 am

Please build a better Relative Humidity Sensor

Sun May 20, 2018 4:34 pm

If you have used a DHT11 or DHT22, you have experienced the sensors are so out of whack that you can't get a real reading. Now for kids in a classroom that's not good. Teaching my students weather related phenomena like static electricity and other cools tuff requires the students to accurately follow the Scientific method. This is impossible due to the company making the mini sensors for RPi or Arduino et al.

I have connected more than ten of each sensor DHT11 and DHT22 and none of them provide any similar readings. They are off by up to +- 30%

The kids are stressed because their projects don't work. We can't calibrate the sensor thanks to Ansong (manufacturer) lack of quality control, none of these are even close to accurate.

All that being said, can someone please create an accurate RH reader by hacking the current DHT22 or create a completely new sensor that is easy to calibrate and accurate. I'd do it but I have no idea how to make the thing.
For all the scientists, students and teachers out here, thank you so much.
Mike

hippy
Posts: 3325
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: Please build a better Relative Humidity Sensor

Sun May 20, 2018 5:28 pm

I have never compared DHT11 or DHT22 to a calibrated reference but am surprised you are getting ±30% discrepancies. The datasheets claim ±2% on temperature and ±5% for relative humidity and I don't see a lot of posts from people using them saying they are 'worse than useless'. When I used them they gave similar readings to all the other non-calibrated devices I had.

There is an independent assessment of the DHT22 by someone who appears to know what they are doing here ...

http://www.kandrsmith.org/RJS/Misc/Hygr ... dht22.html

"My devices initially performed impressively. Whilst I never achieved ±2% absolute accuracy over the full range 0–100% RH, they did when first tested deliver a higher accuracy than I had expected for such a cheap device. (My expectations were pretty low!) The maximum errors I measured in my initial calibration was +6% RH and that only on one of my six devices. Typical errors were 2–3% RH at a constant room temperature (20°C)".

Could it be that something else is wrong other than the sensors themselves ?

What code and libraries are you using to read the sensors ?

alphanumeric
Posts: 1515
Joined: Tue Jan 19, 2016 2:17 pm
Location: Sydney, Nova Scotia, Canada

Re: Please build a better Relative Humidity Sensor

Fri May 25, 2018 11:27 am

I had an issue with the humidity sensor on the Sense Hat HTS221 being light sensitive. Bright light shining on the sensor would lower the readings. I actually got negative readings. I put a piece of non conductive foam over the sensor to bloke any light from shining on it.
viewtopic.php?f=104&t=211079&p=1314090#p1314090

Bruce_Pi
Posts: 17
Joined: Tue Feb 21, 2017 5:09 pm

Re: Please build a better Relative Humidity Sensor

Mon Jun 04, 2018 2:18 pm

I will recommend looking into honeywell temp humidity sensor. something like HIH6130
https://store.ncd.io/product/hih6130-hu ... ni-module/
the temp/humidity reading are extremely accurate and they are temp compensated. These sensors are i2c based, so easy to use and provide real world temp humidity values.

alphanumeric
Posts: 1515
Joined: Tue Jan 19, 2016 2:17 pm
Location: Sydney, Nova Scotia, Canada

Re: Please build a better Relative Humidity Sensor

Mon Jun 04, 2018 2:59 pm

I switched to a BME680 on my portable weather clock. The only thing I'm using on the Sense Hat now is the LED matrix.

ghostrider513
Posts: 35
Joined: Tue Mar 28, 2017 6:03 am

Re: Please build a better Relative Humidity Sensor

Sun Jun 24, 2018 4:36 pm

alphanumeric wrote:
Mon Jun 04, 2018 2:59 pm
I switched to a BME680 on my portable weather clock. The only thing I'm using on the Sense Hat now is the LED matrix.
I bought the SHT31D and it's working great. It has the I2C and works better than the DHT's. Thanks

ghostrider513
Posts: 35
Joined: Tue Mar 28, 2017 6:03 am

Re: Please build a better Relative Humidity Sensor

Sun Jun 24, 2018 4:36 pm

Bruce_Pi wrote:
Mon Jun 04, 2018 2:18 pm
I will recommend looking into honeywell temp humidity sensor. something like HIH6130
https://store.ncd.io/product/hih6130-hu ... ni-module/
the temp/humidity reading are extremely accurate and they are temp compensated. These sensors are i2c based, so easy to use and provide real world temp humidity values.
I bought the SHT31D and it's working great. It has the I2C and works better than the DHT's. Thanks

ghostrider513
Posts: 35
Joined: Tue Mar 28, 2017 6:03 am

Re: Please build a better Relative Humidity Sensor

Sun Jun 24, 2018 5:01 pm

hippy wrote:
Sun May 20, 2018 5:28 pm
I have never compared DHT11 or DHT22 to a calibrated reference but am surprised you are getting ±30% discrepancies. The datasheets claim ±2% on temperature and ±5% for relative humidity and I don't see a lot of posts from people using them saying they are 'worse than useless'. When I used them they gave similar readings to all the other non-calibrated devices I had.

There is an independent assessment of the DHT22 by someone who appears to know what they are doing here ...

http://www.kandrsmith.org/RJS/Misc/Hygr ... dht22.html

"My devices initially performed impressively. Whilst I never achieved ±2% absolute accuracy over the full range 0–100% RH, they did when first tested deliver a higher accuracy than I had expected for such a cheap device. (My expectations were pretty low!) The maximum errors I measured in my initial calibration was +6% RH and that only on one of my six devices. Typical errors were 2–3% RH at a constant room temperature (20°C)".

Could it be that something else is wrong other than the sensors themselves ?

What code and libraries are you using to read the sensors ?
Thanks for the input. I ditched the DHT's and bought an SHT31 D. It's performing flawlessly. Now I have to figure out how to loop it every 10 seconds, then download the data into a csv file to upload to my MySQL db on my website. I can't seem to find any help with those two items.
The SHT31-D seems really accurate so far. Temp is right on with a thermometer.

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