Hi Everyone,
I may have mention some of the points here about how I used BME280 spread out through out the thread. I would like to summarized in one post about my specific tune of BME280 since I am releasing my sensor for sale. This is along post. If you are not interested at this point, you can skip it. There is nothing that need to be paid attention to. Perhaps, it will be useful for new owner of these sensors.
In the future, further adjustment may be made. I would still like to have the flexibility of tuning them to be better and better as I get more input. However, the changes if any will be more incremental.
There is a bit misconception that all BME280 is the same. Therefore, the reading will be the same on the final product(or brand) that use BME280. They are physically the same. However, BME280 can be software configured quite differently such that the result of the measurement can be different from application to application.
Here are what we should expect.
Temperature measurement.
BME280 can be configured aggressively to read the sensor data. It can perform measurement at very short interval (milliseconds interval). At this configuration, BME280 can be impacted by self heating. Based on my experience, this impact will cause the reading around 1 to 2 degree Celsius higher. For all of us in US with Fahrenheit as a display, it can look like that there is about 2 to 4 degree higher.
The other spectrum of the configuration where BME280 is requested to perform reading once per second. This configuration does not have any noticeable self-heating.
Through out the development I did adjust the configuration to get the most optimal setting for us. We do not want to be closer to one side of the spectrum. The final shipping configuration today may be different from those who got my sensor in the past.
In regard to temperature, BME280 is very sensitive sensor. It is really good at measuring temperature. The sensitivity and accuracy of BME280 is a double edge sword. Because, there is another source of heat on the board like the Zigbee radio. Keep in mind, the bigger picture that the sensor is not just a Temperature sensor. It still has to function as Zigbee router. It also has other sensors and expansion port that could be in used. They are some of the features that are as important as measuring the temperature.
I have designed the PCB to minimize the impact on the heat. In addition, I recommend below orientation. This is a recommendation. One does not have to follow the recommendation because the device page has offset adjustment. As a reference in below orientation, the sensor should have less than 1 degree Celsius offset.
Relative Humidity measurement.
In the first couple weeks of using the modules, the humidity may be lower than expected. This is due to the re-flow process that the sensor is subjected to. This is documented in datasheet. The sensor can optionally be re-condition according to datasheet. My personal opinion is that I would leave it alone. I have seen enough sensor re-conditioned itself after a couple weeks. After this process, the sensor should be within 3% of the specification of BME280.
Please note that the Relative Humidity is influenced by temperature reading as well. The temperature does impact the humidity reading. Higher temperature will lower the Relative Humidity reading of the same absolute Humidity level. I hope this is not confusing.
Pressure measurement
This is one measurement that BME280 is least affected by its surrounding. It is not impacted by temperature, self heating and etc. This is probably one aspect of the sensor that require least amount of adjustment.
I hope this information is useful. In case if you subject the sensor into different orientation that I indent or encapsulating it into a casing, there is some adjustment on Device setting page is expected to get accurate temperature and humidity reading.
In any case, I would like to offer a sensor that is robust, flexible and capable. Out of the box, with the recommended setup, The sensor is reasonably accurate without any specific effort.
Depending on your requirement, the sensor is design to be tune-able to get the precision that one would need.
Thanks
Iman
