I could use a bit of help in this area. I got two digital hygrometers and I'm using the salt test. I've put them in a ziplock bag along with a bottle cap with some salt and water (it looks like a gel or paste right now) with some air and zipped it up. It's been over 12 hours now and they still say 66 to 68%. Are they usually 7 to 9 off or am I doing something wrong?