Heather: I kept hygrothermograph charts for three years simply because I had the room in my files. In addition, I took the information and converted it to a bar chart which ran back as far as my first month at the museum (a page for each month). The bar charts were easier for the board to follow and and, kept in a three-ring binder, helped identify trends and flucuations. The information came in very handy when we needed to persuade the board to update our HVAC system. If you can convert the information on hygrothermograph charts to another format then I can't see any reason for keeping them more than a year. Belinda Nickles