Temperature monitoring hygrometers
Hair Hygrometers were invented during the 1700s and were used to
measure the amount of water in the air. They were used as an aid
to predict weather changes and are still used today for that
purpose. They are also used to monitor the environment where
cigars and other moisture sensitive materials are stored. They
are not very accurate and if moved around must be allowed to
'adapt' to the new environment of a period of time before they
can be used at all.
In an effort to improve the accuracy of these instruments
digital hygrometers were created in the late years of the 21st
century. They are indeed more accurate but the are also much
more expensive. But in museums where high humidity promotes mold
growth and biodeteriation the cost is not as important as
reliability and stability.
Levels at 60% RH are considered the threshold for damage - over
that level and the museum will eventually have trouble. Because
many collections are hygroscopic, the humidity levels affect
dimensional stability. For example, some types of wood board can
vary up to one inch in length over a foot between 10% and 90%
RH. Variation in relative humidity can loosen furniture joints,
cause paint to chip from canvas, and cockle paper. In addition,
fluctuating relative humidity can lead to chemical reactions
which cause metals to corrode, dyes to fade, and damage to glass
and mineral collections. In short it is critical that the RH
levels be accurately monitored and maintained at the proper
levels.
The ThermaViewer, a recent addition to the ranks of hygrometers.
It uses thermistor and electronic RH sensors to sample
temperature and humidity every 10 minutes and save the results.
A chart is drawn on the large LCD display that employees,
maintenance personnel, and docents can read. Ten months of
information is stored and displayed by the ThermaViewer.
Periodically the information is downloaded to a computer that
maintains an archived copy of the environmental conditions. The
ThermaViewer is a giant step forward from the hair hygrometer as
far as accuracy and reliability goes.