I just built a bigger bater. I went out and got new thermometers and an acurite hygrometer/thermometer. So I calibrated them. Placed the glass thermometers in a cup of slushy ice water. 36 degrees, on both. Put hygrometer in zip lock bag with cup of 1/2cp salt and 1/4cp water for 10 hours. Should read 75%, but instead reads 64%. The hygrometer is not even close. The thermometers are close if you need to be within 4 degrees. I guess close on a scale of 0 - 100. But out of 10 degrees, it is 40% off. WOW. This is what frustrates me, it is a thermometer. It's entire purpose for existing is to show the current temperature. The little marks are at 2 degree intervals. You would think it would be accurate at least to 2 degrees. My weather rock is more accurate than the hygrometer.