Re: Hygrometer Calibration - Best way to verify Hygrometer 'reference' accuracy
THe first point is that humidity measurement is very quirky. It is a secondary (or perhaps tertiary) measurand, which means at best, it is a combination of all its cumulative inaccuracies.
The wet towel test is a generally accepted functional check. However, I wouldn't depend on its reading to very accurately set a hygrometer. Reason being that many hygrometers are the least linear at the ends of their ranges. So if you use the wet towel method to establish 100% RH on a (for example) +/-3%RH hygrometer, you might be +/-5 or more.
Then, checking with NaCl can be tricky. You have to be sure it is all set up properly to get accurate results. In a previous context (when I used salts), I had a chamber and a pyrex dish to create saturated salt solutions. You need a good airtight seal and fully saturated NaCl in distilled water, and plenty of time to stabilize. If your ambient humidity is significantly less than 75%RH, if you had any leaks in your chamber you likely would not reach 75% with the NaCl.
Regarding the digital Hygrometer, I calibrate lots of them. at 80%, you are in the grey area (unless you have a good quality cal with data that you trust) - I mentioned above that the extremes of range in hygrometers is where they are most infamously non-linear.
Short of paying to get something properly certified by a lab, I will assume that is not a reasonable option. I recommend you discount the wet towel test, as it is inherently troublesome (end of range). If this is a home hobby type application (not spending any money), I recommend fine tuning the 75%RH measurement. Get a bottle of distilled water and some non-iodized salt (not sea salt).
Please pardon if I provide details you already understand. I want to be sure and cover it completely (also for the sake of others who may be interested).
Here is the slightly tricky part...
To make a proper saturated salt solution, pour the salt into a very clean pyrex glass container. Add enough distilled water to saturate/cover the salt, with a little extra. Stir and be sure there is not enough water to dissolve all the salts.
Suspend your hygrometer probe immediately above the salt solution (not touching) seal the container airtight. I would allow a few hours stabilization time. Remember that every detail is important (non-iodized, airtight, enough salt, distilled water, enough time, etc.). Also, you need some idea of the ambient temperature. NaCl is 75.3% @ 15 Deg C, 75.5% @ 20 Deg C, and 75.8% @ 25 Deg C. So if ambient is in that area, there are your numbers.
Monitor the hygrometer for further drift in one direction until it stabilizes completely. Whatever your hygrometer reads is what it is. This isn't quite up to requirements for a lab, but should yield pretty good results.
The other check is with an airtight container filled partially with good quality desiccant and properly sealed will give you a NEAR zero check (again, in the non-linear region so don't expect high accuracy.
If you already covered as I described above (for NaCl check at 75%RH) and your digital hygrometer reads 82%, the hygrometer is inaccurate. One caution on humidity accuracy of hygrometers is they are pretty sensitive to temperature variations. Handling or holding the probe can shift readings (varies by different mfrs).