Suggest you read the following:
Catherine Sease. 1990. A new means of controlling relative humidity
in exhibit cases. Collection Forum 6(1):12-20
The system described in this article regulates humdity in several cases
from one central unit. Catherine Sease is at the Field Museum, which
has a most excellent Egyptian exhibit, so you should contact her directly
there.
John Simmons
Museum of Natural History
University of Kansas