Rebecca - Have a read at two recent Commission on Preservation & Access publications, Donald K. Sebera, _Isoperms: An Environmental Management Tool_, June 1994, followed by James M. Reilly, Douglas W. Nishamura, and Edward Zinn, _New Tools for Preservation: Assessing Long-Term Environmental Effects on Library and Archives Collections_, November 1995. I believe that the texts of both are available at the CoOL Web site. The address of the Commission is 1400 i6th St., NW, Suite 740, Washington DC 20036-2217. As I have remarked elsewhere, the "data-logger vs. hygrothermograph" debate sometimes resembles the "latke vs. hamantash" debate, so these two publications may not definitively resolve your quandary. But they might help. "Hop Schweiz"