Most digital ones I have tested in 6 months are also out by 15-35% Rh the don't work at all.The test is easy and I would highly recommend one that can be calibrated and an analogue style. Look for a metal face card (not paper) and a calibration screw. The bi metal spring type can tolerate dust dirt and some knocking about. Digitals that use a resistive sensor any thing that altered the resistance (dirt cord damage salt uranates) cause them to be come unreliable. The dirtier the worse they get .

Personally I like the western instruments they are certified as they leave the factory I have 26 of them now and the last set I bought (18) all 18 were with in 3% of each other and that was confirmed to be accurate.

The best of the three digital accurites I had was -15% and the worst was 32%+ The only good thing about them was they did not argue when I told them it was reading out side the specs (+/-20%) and replaced it under warrantee, twice, sadly the last one was in specs 15% and 1.7ºF + I gave it away.

I seem to be fussier than most I will not accept worse than 5% and 1ºF in error.

http://www.amazon.com/Analog-Hygrome.../dp/B0007VWEQA


A salt test (google) will tell you accuracy fast it is very hard on digital units so you get one maybe two tests before the test kills it.