Get a small bowl and put about a teaspoon of salt in it. Put enough water in with the salt to make a slurry. Place the bowl and the hygrometer sensor in a ziploc bag and seal it as much as possible. Leave for at least 6 hours. The hygrometer should read 75% if it is correct. If not, just make note of how far off 75% the hygrometer is reading and use that as your guide. So if the hygro reads 70% then you know you will need to up your humidity by 5 points.
