QuoteJackal - 2/9/2016 3:04 PM I have always had difficulty making a proper supersaturated salt solution for calibration (and I mix chemicals for a living), so I just don't trust the method.The other issue that I have with the salt solution is that you are probably nearing the upper end of the capability of the hygrometer. It may not read well at that extreme, so that could have a negative effect on your calibration curve. For any measurement instrumentation (without going into a full gauge R&R), it is good practice to calibrate on the upper and lower limits of your expected measurement range plus a known point in the middle to check for any accuracy offset.
We would like to believe that the range of the hygrometer that we are working in is small enough that we can reasonably expect linearity of measurement, but that is wishful thinking.
I have always had an issue with most hygrometers using a single point calibration. Your multi-point check is a good one. Unfortunately, you have no way to know if your standards (the Boveda packs) are precise and or accurate for your conditions (temperature and pressure).
I see the salt test being touted as the best method all over the place, interesting to see a contradictory perspective.
My temps are pretty stable, and when stable, I can see the humidity fluctuations are in sync with my temp fluctuations. Here are my temps the last 30 days as recorded by the tags...
That funny spike was when I moved the tags to the calibration bag and tried to blow into it to give it more air.
I also found it interesting that in the RH chart, tag 0 (upper shelves), was placed upside down, the sensor holes in direct contact with the Boveda. I actually find it rather amazing how much these readings fluctuate at a tenth of a %.
I have 62%, 65%, 69%, and 75% 8g Boveda packs. Maybe when I'm done I'll calibrate at 65% and then check the differential for the other 3. After that, I'll just keep tabs on how well things are smoking and adjust the humidifier to taste, regardless of RH readings.