All digital hygrometers at this price range are hit and miss...period! If you look at the spec's they will all say +/- 3-5%. Those spec's are not across the full range.... they are usually the tolerance for mid range...approx 50% humidity. As you start moving the range on the upper (or lower) spectrum the +/-% can be way off and still be considered accurate by the manufacture.
This instrument, IMO, is the most important piece of equipment you will own for helping you in your cigar storing. I continue to see people offer advice in this forum to people that have trouble with their humidors not regulating humidity correctly but never once do they ask how the humidity is being measured. You can NOT diagnose a humidity problem without
first knowing if the instrument doing the measuring is accurate.
I'm with David about the reason why a company would not want you to do a salt test. It very well could be because the salt air would corrode their coponents but speaks more to me about the quality of the product. Like I said in the beginning, all of these units in this price range are hit and miss.....you will have some spot on, some a little high/low, and some way off.
I have 6 digitals in my cabinet that all are within 2% of each other. I do a salt test every year or when/if one of the hygro's drifts out of sync with the others by more than a few %. Three of my digitals can be found
HERE! One is 1% low and the other two are spot on. My other three hygro are from Radio Shack. I bought one of these
HERE and was surprised to find it spot on. I went back and bought another and it was 5% high. I took it back and got another one that was 1% low. I recently bought another one from them and had to take it back twice before I found one that was 1% high.
Yes, you can have a fairly accurate $15-$40 dollar hygro but not without testing it. The salt test is the only way I'm aware of of knowing for sure. If it isn't close to being accurate take it back and get another one. Eventually, you will get one that is.