Hygrometers are extremely unreliable. This is a copy and paste for those that are unaware.
Gather supplies.*To test your hygrometer using salt, you just need a few household supplies:A small food storage bag that zips. A tiny cup or bottle cap from a 20-ounce soda. Some table salt. Water.
Fill the cap with salt and add enough water to make a thick slurry.*Don't add so much water that the salt dissolves; you just want it to be wet. If you add too much, use paper towels to mop up the excess.
Put the cap and the hygrometer inside the baggie.*Zip it up and place it somewhere out of the way, so it won't be disturbed during the test.
Wait 18 hours.*During this time the hygrometer will measure the humidity inside the bag.
Read the hygrometer.*If it's accurate, it should show a humidity of exactly 75 percent.
Adjust the hygrometer if necessary.*If your hygrometer shows a humidity lower or higher than 75 percent, you'll need to calibrate it so that it's accurate when you use it to check the humidity of your humidor.[2]If you have an analog hygrometer, turn the knob to adjust it to 75 percent.If you have a digital hygrometer, use the buttons to adjust it to 75 percent.If you can't adjust your hygrometer, take note of how many percentage points higher or lower than 75 percent it is. Next time you use your hygrometer, add or subtract a few percentage points for an accurate reading.
I can do it too MR1 
