I have two humidors, one I've had for 4 years and the other for about 1.5 years. They both have analog humidity gages and the foam humidifier disc that you add water to. Should I ever change these? I don't know how to check if they are working properly, but my older gage is saying 80% humidity and I know it is definatly not that high. How do I calibrate if I need to?