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...
You may have had Humidity problems, that's what it sounds like to me anyway. I use the same Thermometer/Hygrometer as you, and I was having issues with chicks fully forming but not hatching, so I calibrated my Hygrometer and found that one was off by 12% and the other by 10%, so I had too much...