Calibrating Hygrometer

Dipsy Doodle Doo

13 Years
Jan 11, 2007
Aiken, South Carolina 29801
My Coop
My Coop
Hi! Can someone point me to a link with specific instructions, please.
I remember it was a bag that can be closed tight, wet salt, and the instrument (if correct) should read 75% humidity --- but that's all I remember.
I looked through 6 pages of search results and can't find 'the instructions'.
I haven't done it in a while and my memory may be failing me.
Many thanks!

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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To calibrate the hygrometer you’ll need the following:
  • Small sandwich ziplock baggy
  • Bottle cap from 2 liter soda bottle (works best)
  • Table salt
  • Hygrometer (whichever one you want to test, digital or analog)
Now that you have all of your materials handy, follow these steps and you’ll be on your way to effectively checking the accuracy of your hygrometer.
Fill bottle cap with standard table salt; fill about 3/4 of the way up.
Add tap water to the bottle cap to saturate the salt. If you see water floating on top of the salt, you’ve added too much water. Easy fix for this is to grab a paper towel, and soak up all of the excess water. You want more of a slurry consistency of water and salt. Again, if you see water actually floating on top of the salt, soak up the excess with a paper towel.
Place both hygrometer and bottle cap (with salt/water mixture) inside of a small ziplock baggy, as pictured above. Wait 4 hours and come back for a reading check.
If your hygrometer is perfectly accurate, it will read 75%. Most hygrometers will be +/- 3 %. If your hygrometer is digital and has a calibration button, follow the directions that it came with to calibrate to 75%. Digital hygrometers have a calibration button you push, while analog hygrometers have a screw which allows you to adjust the needle accordingly.
If your hygrometer is not adjustable, you’ll just have to make a note and remember how far off it is.

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