I've been really frustrated by hygrometers. I have three, one in a room clock device (display includes indoor and outdoor humidity levels), a Xikar adjustable calibration device and a none branded none adjustable device.
Living in the UK, humidity levels are generally not a huge problem; levels in my house go from highs of 62% in the summer, to lows of about 39% in the winter; the average is generally about 55%.
Frequently all three of my hygrometers will read the same (within +/- 1%) at other times they give wildly different readings (this when they are all located next to each other). I have tried the salt calibration method, (on the two small devices) with very mixed results. Sometimes they will both read 75%, sometimes only one will, sometimes neither will; what is worse is that there is no pattern between the two; I cannot determine that one is always higher/lower than the other.
I have now fitted the none-branded device into my Taylor case, I keep a humidipak in the case and the hygrometer reads a fairly constant 52% and there is no sign of the guitar being too dry/wet so hopefully everything is ok. Also, my Yamaha L series, which I’ve owned for 3 years, shows no sign of suffering from humidity problems.
I would however like to be able to get an accurate and adjustable hygrometer and some means of accurately calibrating it, so I can be confident that my guitars are ‘safe’.