I've tried several of the cheap digital humidity gauges ($20 to $40 or so). My experience is that I wasted my money on them. I had 3 of them (different brands and prices) together and they all displayed significantly different readings and they all crapped out in a fairly short time. The best thing I found is the simple but tried and true sling psychrometer. I got one for $100. You can find them for half that, but since I was tired of screwing with cheap inaccurate crap, I paid up front. (Not to say that all inexpensive ones are crap - just the ones I tried

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Same thing with the inexpensive automatic on/off humidifiers (my experience). Get a few of them together and they will all read differently. So, I end up with a $50.00 humidifier and a $100 sling psychrometer and I'm in vapor bliss!
Of course YMMV
Dave Little