i don't see how or why a salt test would hurt the device- these things are highly variable when it comes to rh. this is true in various ways. e.g., it might be read 2% higher today and 5% lower in a few months. i'd suggest doing a salt test, then using the hygrometer to watch for a decrease (or increase) in rh. use your cigars as the true gauge of rh... are they too dry or too wet. the salt test can drive you crazy too. temp is fairly accurate with such devices.
i have a smaller humi(really high quality though, 50cigar) and i put the 2 digitals in it. This humi has that PG solution in it, and is essentially 70rh, the new hygrometer(western digi) still reads 66% and the old one reads 70-71%.
I think the older unit is more precise, or playing games with me.
I still have a Western Digital but also two little havana hygrometers. The little havanas are round and have a hotdog shape. I placed all three in a humi. both the little havanas had the exact same humidity and were seperated by 1/2 degree on temprature. The western digital was 7% lower humidity than my little havanas and a degree higher on temprature. I also had new batteries in the western digital. In addition I did a salt test which produced the same results. I much prefer my little havanas hygrometer.
The biggest difference I noticed was when I smoked the cigars. Because my western digital was reading a lower humidity I was over humidifying my smokes and therefore made them difficult to smoke. I would have to keep relighting them. I also noticed a difference in the feel.