Accurite Hygro ...Amazon $8....I have a dozen of these and they are accurate from 1 to 2%....they have never deviated not 1 % in the last half dozen years I've owned them. All you need is one current good hygro to compare them to.....put em all in one tupperador.....check them readings and you're done. The ones that are more than 2% off take em back and get replacements which is what I did....no salt test, no nonsense or incantations or knob twisting...etc.