BME280 issues

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iesre
 
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BME280 issues

Post by iesre »

I have been using DHT22 T/RH and BME280 T/RH/P sensors for quite some time. There are serious problems with the RH values from BME280 sensors, especially when they are used in outdoor weather stations. When compared to high-quality RH sensors, data from different BME280 sensors tend to vary wildly over a large range of RH values. When used in outdoor stations, even with what I believe is adequate protection against wind-driven rain and/or condensation, the BME280 RH sensors will fail very quickly -- weeks or less. Typically, the values will be much too high, with continuous values of 100% under conditions where that is simply wrong. Right now I'm looking at output from data from a packet radio weather station where the BME280 RH value has suddenly gone to 0.
Somewhat surprisingly, RH values from a DHT22 sensor seem to be more accurate and reliable, although they, too, sometimes vary considerably from values from higher quality sensors.
So, the bottom line is that the stated RH accuracy of these kinds of inexpensive capacitance-based sensors (typically just a few percent) simply cannot be believed. Despite claims to the contrary, and the temptation to use them with Arduino-based weather stations, BME280 sensors are NOT, I repeat NOT, suitable for use in outdoor weather stations.

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adafruit_support_carter
 
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Re: BME280 issues

Post by adafruit_support_carter »

Can you repeat these readings with a setup directly connected to a computer to rule out any possible issues with transmitting the values via radio.

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kflmiami420
 
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Re: BME280 issues

Post by kflmiami420 »

iesre I am following this thread , As I am in the process of doing a raspberry pi 3 Station with the BME280 , DHT22 , DS18b20 you have me worried I will have to find another Barometer sensor .

BMP280

MPL115A2

Adafruit BME680

I am wondering why the Raspi foundation used the bme280 in their 2016 board if it fails so fast ?

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iesre
 
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Re: BME280 issues

Post by iesre »

I have an indoor setup for testing these sensors -- no packet radio. I doubt that there are transmission problems because the other data in the packet are always OK. I have seen the same problem with a stand-alone solar/battery powered system that writes data directly to an SD card using the Adafruit datalogging shield. Once these sensors go bad, there is no resurrecting them by keeping them inside for a while. Even when they are new, multiple sensors on the same system will give widely varying results.
It is notoriously difficult to measure relative humidity with capacitance sensors. The problems are not always easy to detect unless you can look simultaneously at multiple sensors and compare them with a reliable "professional" source of data.
I stand by my bottom line: The BME280 and similar sensors are NOT reliable sources of relative humidity data in outdoor weather station applications. Sorry...

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kflmiami420
 
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Re: BME280 issues

Post by kflmiami420 »

Can you use the BME280 for Barometric readings the DHT22 for Humidity and the DS18b20 for Temp ?

Grab the best of each and coment out the other readings ?

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iesre
 
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Re: BME280 issues

Post by iesre »

Both the DHT22 and BME280 temperature readings seem OK. They hardly ever differ by more than a degree C. I'm looking at data from my packet radio system now and the DHT22 temperature is 20.6 and the BME280 temperature is 21.6. The physics of the temperature measurement do not have the inherent problems of the humidity measurement. The humidity sensor measures the "absolute" amount of moisture in the air, the "humidity," and then applies a temperature-dependent correction to get "relative humidity." (That's why you find sensors that return temperature and relative humidity rather than just the relative humidity -- you can't calculate relative humidity without knowing the air temperature too.)
The BME280 pressure readings look OK, but I don't have a "professional" instrument against which to compare them. They shouldn't be weather-dependent. Keep in mind that they are "station pressure" readings, that is, actual pressure at your elevation, not "weather report" (sea level) values. As explained in BME280 documentation, the adjustments required to calculate your station altitude from station pressure depends on an INDEPENDENT knowledge of the sea level pressure, so the elevation calculation is just an approximation. If you know your site elevation, easily obtainable from Google Earth, you can find online equations for converting station pressure to sea level pressure if that's what you want.
So, again the bottom line: I believe that the relative humidity values from these inexpensive sensors are inherently and "unfixably" unsuitable for outdoor weather systems. Values from a DHT22 sensor often seem more "reasonable" than from the BME280. At least in my system, they don't display the 100% "saturated" values I often see from the BME280. But they are often simply not right. For example, during hot and humid days in July, my DHT22 often reported relative humidities below 10% during the day -- simply not possible! Right now, at about 09:00 PM EDT, my DHT22 is reporting a relative humidity of 89.6%, but a nearby small airport instrument (about 7 straight-line miles from my location) is reporting 100%. So, who knows...
I have recently ordered a much more expensive relative humidity sensor with analog voltage outputs for relative humidity and temperature. It can be interfaced with an Arduino, but perhaps needing a higher resolution A/D board like Adafruit's ADS1115. We will see...

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