Lower price humidity gauges are notorious for being inaccurate, many times being off by 10 to 20%. Unless you are willing to spend 150 to 200 dollars for a NIST traceable with a certificate, you can never be sure if your hygrometer is reading accurately. However, you can test your own quite easily to see if it is reading properly, and here is the procedure:
1. Place a teaspoon of salt in a bottle cap or small cup and dampen it with a few drops of water (without dissolving it) to make it the consistency of wet sand.
2. Carefully place the wet salt and the hygrometer inside a see-through container and close tightly. Don't allow any of the salt to directly contact the hygrometer. You can use a zip lock bag provided it seals good and you leave some air inside as well, or a clear plastic food storage container is even better as long as it seals airtight.
3. Let it sit for at least 8 hours at room temperature and note the reading on the hygrometer without opening the container. It should read 75%, and the difference is how much your hygrometer is off. The water and salt mix will reach an equilibrium humidity of 75% within any small airtight container.
4. If it does have an adjustment screw or other adjustment device, adjust to 75%, through a small hole made in the bag if necessary.
I have an expensive certified dial hygrometer I purchased many years ago and have checked it periodically using this method, and this is a very accurate way to check humidity gauges.
Kelly
