I haven't seen this point made in other postings, so I'm adding my two cents, recording hygrothermographs are useful because they give you an instant reading of an area. Or if an area develops a problem you'll know immediately, not 2 months later when you download the datalogger. Downloading the datalogger doesn't take a great deal of time for one unit, but it does take some time. Bill Lull was at OHS on an IMS-CPS grant for an environmental survey and he recommended to us the use of both. I have 6 hygrothermographs, 7 dataloggers, and an aspirating psychrometer. I use the dataloggers for monitoring of spaces, downloading about every 2 months; and the hygrothermographs for "spot" checks and if a problem occurs that I want instant data on. I have to admit that I don't recalibrate as often as I should, but I also don't move the hygrothermographs around very much. The other problem with hygrothermographs is going around and winding and changing charts every week, something that I wasn't always successful with. ********************************************************************* Marsha Matthews Director/Museum Collections Oregon Historical Society 1200 SW Park Ave. Portland OR 97205 Internet: [log in to unmask] Telephone: 503/306-5274 FAX: 503/221-2035 **********************************************************************