Calibrating Hygrometer

Type in the search box, calibrating a hygrometer and it will tell you what you need to know.

I did a google search and it popped right up.
If it helps though, I believe you mix 1/2 cup of salt with 1/4 cup water.  That and the hygrometers go in the ziplok.  After 12 hours it should read 75%.  If it doesn't mark the hygrometer with whatever number you need to add or subtract.

Good luck!

Here's what I got Lisa, the same thing really. Its just a little more drawn out:

Summary of Steps to Calibrate a Hygrometer

You will need:
  - 1/2 cup table salt
  - approximately 1/4 cup water
  - coffee cup
  - hygrometer
  - large re-sealable freezer bag

1. Place 1/2 cup of salt in the coffee cup, and add the water. Stir for a bit to totally saturate the salt (the salt won't dissolve, it will be more like really wet sand).
 
2. Place the salt/water mix in a re-sealable plastic bag, along with the hygrometer, and seal the bag. Note: make sure none of the salt/water mix comes in direct contact with the hygrometer.

3. Set this bag aside at room temperature for 8-12 hours, in a location where the temperature is fairly constant.

4. After 8-12 hours, check the reading of the hygrometer. It is best to read it while still in the bag.
The relative humidity in the sealed bag with the salt/water mix should be 75 percent.

5. For adjustable hygrometers, adjust to read 75 percent. You will have to do this very quickly, or remember how much you need to adjust the setting.

If yours is not adjustable, simply make a note of how "off" your
hygrometer reads. If it reads below 75 percent, you will need to add the difference to your actual readings. If your hygrometer read above 75 percent on the calibration, you will need to subtract the difference from your actual reading.

In my example: after sitting in the bag, my hygrometer read 72 percent, when it should have read 75 percent -- a difference of 3 percent. I now add 3 percent to the readings I take on the hygrometer to get the actual relative humidity.
Remember: always give a hygrometer about 2 hours to stabilize before taking a reading, as changes in the relative humidity may take a while to register accurately on a hygrometer.

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