BME680 versus SHT31...
 
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BME680 versus SHT31 sensor for humidity data  

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tico
 tico
(@tico)
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Joined: 4 months ago
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June 14, 2020 4:51 pm  

Hi all, I've been testing out a BME680 sensor as well as an SHT31 sensor in my mushroom fruiting room project. I had them both in the same breadboard in a regular household environment for a couple of weeks of testing, and aside from bus reset errors with the BME680, I found that they consistently reported the same temperature and humidity. (However, the cheap DHT11 sensor was always off by a couple of degrees Celsius, and by 5% or 10% relative humidity compared to either the BME680 or the SHT31, but that's not too surprising.)

 

A week ago, I moved the SHT31 sensor into my mushroom fruiting room , spaced about 15cm away from the BME680, and I noticed that while they both report almost the exact same temperature (basically overlapping on the graphs), the SHT31 sensor appears to always report a much higher humidity than the BME680. For the record, an additional handheld sensor shows the fruiting room humidity to remain at 91 or 92% but I'm not sure which of any of these sensors to trust the most. Anyone else had experience comparing these sensors in a high humidity environment?

 


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Kyle Gabriel
(@kylegabriel)
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June 14, 2020 7:28 pm  

Can you place both sensors in a very small container together that you can completely seal, such as a small jar that you can cut a slot in the lid and pack it with material to seal it after screwing the lid on? This will provide a more homogeneous environment to test their variance.

Mycodo Developer


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tico
 tico
(@tico)
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Posts: 68
June 14, 2020 9:08 pm  

Sure thing! I'm not near it currently, but I can make that happen tomorrow. I had assumed that having multiple fans in the fruiting room would supply sufficient mixing of the air, but your proposal is certainly a much better test.

Regarding DHT11 or DHT22 sensors, have you also seen them differ by 1-2 degC (from each other, or from either BME680 or SHT31 sensors) when stuck on the same prototyping board? I'm wondering if I should toss out my assumptions about their accuracy when I was playing with them for a couple weeks before trying to use them in the mushroom project ...


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Kyle Gabriel
(@kylegabriel)
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Joined: 6 years ago
Posts: 302
June 14, 2020 9:52 pm  

Sensor accuracies are:

DHT11: ±2.0 °C

DHT22: ±0.5 °C

BME680: ±0.5/1.0 °C

SHT31: ±0.2 °C (up to ±0.5 °C at the extremes)

If you want multiple high accuracy temperature measurements, I'd suggest something like the MCP9808, which is $5 and gives you an accuracy of ±0.25 °C over the whole range of -40 °C to +125 °C.

So, depending on the combination of sensors, you could see up to 3 °C difference between two sensors. You could set up an Equation Math controller to offset a measurement, or an Average (multiple measurements) Math controller to average the two sensor measurements. 

On a side note, I just added in my Mycodo release flow a script to automatically generate information about all Inputs and insert it into the Mycodo manual, which includes links to manufacturer info/datasheets/product pages, so you can easily find info about sensors Mycodo supports. This info is embedded into each Input module and will also appear when you expand the Input options (when v8.5.5 is released).

Mycodo Developer


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not_5
(@not_5)
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Posts: 30
June 14, 2020 10:11 pm  

The bme680 will read a high humidity, others have noted it as well. While the error % are correct, there is an offset required.

https://forums.pimoroni.com/t/bme680-humidity-calibration/9105


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