Hi guys I wanted to recalibrate couple of digital hydrometers , so I google some information I found 1 that says 1/2 cup of salt 1/4 water put in a ziplock bag and wait 12 hours and also found this other one with less salt and less time, what do you think , which one have you try? A guide to calibrating a digital hygrometer using the salt test method Its always a good idea to check the accuracy of your hygrometer. You can use the Salt Test method on both digital and analog hygrometers. Most hygrometers today can either be adjusted or calibrated to an accurate reading. The salt test method wont fail you, and its very easy to do. MATERIALS NEEDED: - 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 youll be on your way to effectively checking the accuracy of your hygrometer. STEP 1: Fill bottle cap with standard table salt; fill about 3/4 of the way up. STEP 2: Add tap water to the bottle cap to saturate the salt. If you see water floating on top of the salt, youve 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. STEP 3: 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. STEP 4: 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, youll just have to make a note and remember how far off it is. Thats all there is to it! Youve just calibrated/tested your hygrometer using the famous Salt Test Method.