Monitoring Temperature and Relative Humidity
Monitoring is part of a museum's work. There are a number of commercial devices available for monitoring the temperature and relative humidity levels over time. The most common are electronic data loggers and recording hygrothermographs, both calibrated with a swing or electric psychrometer. Some museums prefer recording hygrothermographs because the chart provides an immediate visual clue to the environment. But they require staff commitment to regularly change the charts and calibrate the units. Other museums find changing the charts tedious and prefer the compact, self-powered electronic data logger units that record temperature and RH without the need for charts and pens. Data loggers require a computer for processing; graphs are displayed on the computer screen or printed out. All data is stored in the computer, so there is no need for storing bulky paper charts. The units can be set so that downloading does not need to occur as often as it does with the recording hygrothermographs. Also, data loggers can be downloaded over telephone lines, making them ideal for large museums or museums with many off-site locations. Data loggers are often preferred for unheated structures, since they are less likely to be damaged by cold and heat.
Psychrometers
The minimum equipment required for a museum is a swing or electric psychrometer. It should have two thermometers, one covered in a cotton sock that is wet with distilled water for use. Air flow (using a fan or by swinging) causes moisture from the wet cotton sock to evaporate, lowering the temperature recorded on the wet thermometer. Using a slide rule or a chart will give you the relative humidity based on the reading of both thermometers. The dry thermometer gives you the room's temperature. Psychrometer readings taken three times a day in different parts of a small museum, and recorded on a chart, will provide some monitoring information if the museum cannot afford other logging devices. All other RH and T recorders, even computerized ones, MUST be regularly calibrated with a psychrometer.
Do not substitute an electric psychrometer using sensors instead of thermometers. Do not use tap water, which will destroy the psychrometer and provide inaccurate readings. Finally, keep the cotton sock clean and do not touch it. The dirt or the oils from your hands will cause it to read inaccurately.
Books and products we recommend:
 Silica Gel by Raymond H. Lafontaine provides information on the use of silica gel, how it controls RH, and how it is conditioned for use and maintained. Specific topics dealt with include: the problems of display case leakage, how silica gel fulfills the requirements of a humidity buffering agent, and the maintenance of a silica gel buffered display case.
Controlling Museum Fungal Problems by Thomas J.K. Strang and John E. Dawson provides guidance on identifying a fungal problem, fungal problems in wood outdoors, and controlling fungi.
Vol.2 No.2
Temperature and Relative Humidity
Links to related information on other sites:
Vol 1 No 1 Temperature and Relative Humidity
http://www.collectioncare.org/pubs/pcc.html#v2n2
Environmental Monitoring Stations
http://www.getty.edu/conservation/
resources/newsletter/10_3/feature2_1.html
Data loggers: choice of equipment
http://www.natmus.dk/cons/tp/datalog/datlog2.htm
TINYTAG DATALOGGERS: more information
http://www.meaco.com/tinytags.htm
NOMAD® Humidity, Humidity/Temperature, Dataloggers
http://www.omega.com/toc_asp/
subsection.asp?subsection=Hu09&book=Temperature
Campbell Scientific dataloggers
http://www.campbellsci.com/loggers.html
ACR Dataloggers
http://www.acrsystems.com/
Uses for Data Logging Instruments
http://www.ecd.com/datalogging/uses.asp
The Home of Humidity Monitoring and Control
http://www.meaco.com/
Using
a Psychrometer to Measure Relative Humidity (Conserve-O-Gram 3/1) - (An Adobe PDF file)
http://www.cr.nps.gov/museum/publications/
conserveogram/03-01.pdf
Calibration
of Hygrometers and Hygrothermographs (Conserve-O-Gram 3/2) - (An Adobe PDF file)
http://www.cr.nps.gov/museum/publications/
conserveogram/03-02.pdf
Datalogger
Applications in Monitoring the Museum Environment (Revised) (Conserve-O-Gram 3/3) - (An Adobe PDF file)
http://www.cr.nps.gov/museum/publications/
conserveogram/03-03.pdf
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