Hi Henry,
apologies for the delay, I was finally able to focus on the 21 RuuviTags again. I also implemented a first version of calibration into RuuviCollector saving the uncalibrated/original measurement as well as the calibrated one (up and running for a few days).
I used a Pyrex glass container with various commercial humidity packs and “homemade” salt solutions to get a first feeling of how to proceed further. The goal is to calibrate sensors in a range which the humidity needs to be measured at (ie. using table salt @ 75% RH won’t be of any use if we are measuring around 45-55% RH in the application).
Using three different humidity percentages, each with three consecutive measurement points (1h between) and before all that having the RuuviTags in the closed container for 24h, I collected the following data for the 21 RuuviTags:
For the temperature I just calibrated against the average of all 21 RuuviTags for now, since I don’t have a calibrated “temperature chamber” nor was there any very large deviation in any of the devices.
The first 15 are from orders before/around April 2019.
The last 6 are from Nov 2019.
With the exception of no. 6, it almost seems the complete second batch has bad sensors.
There is no guarantee I did all well and 100% accurate this time, but I will repeat this process frequently to see if the deviations change over time. I will also get new salts to create humidity points closer to our target humidity measurement ranges, so I will see if calibrating the humidity against more than one calibration point makes sense in the future. Restricting it to a single point would cut down the effort immensely, but for now I just wanted to see myself how exactly the deviate and in which ranges.
For now I scheduled the check again in 2-3 weeks, but optimally this could be scheduled once or twice per year, followed by updating the calibration values in RuuviCollector.