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