I recently purchased a 125 count humidor off of eBay that came with a humidifier and an analog hygrometer. I did the wet towel test and it only read 80% instead of 96% like it should. However, I can't properly calibrate the hygrometer because it has no screw on the back to adjust the dial. Is there another way to calibrate it or do I need to buy a new (digital) hygrometer?