What Is Relative Humidity?
Methods of checking conditions in large spaces include the use of thermohygrometers and recording hygrothermographs. The “thermo-” portion of either word refers to temperature measurement. The “hygro-” portion denotes humidity. The suffix “meter” means a measurement indicator, and the ending “graph” signifies a record is kept. A hygrothermograph produces a paper record showing temperature and humidity levels over a fixed period. On the other hand, the thermohygrometer is read but does not produce a record. There are many thermohygrometers on the market. It is wise to choose one that can be calibrated. Some devices are small enough to place within an exhibit case or vitrine and are unobtrusive. Recording hygrothermographs are larger due to the drive and recording mechanisms. Often these are placed on their own pedestal or mounted on the wall in an unobtrusive spot. Use of recording hygrothermographs to document conditions should be continuous. This allows the curator to stay familiar with the conditions to which the collection objects are subjected. Placement of temperature and humidity monitoring equipment is critical. Locate them so they are in the same conditions as the objects. Thermohygrometers may be put inside the exhibit case or vitrine. Position larger instruments at the same height and under the same general conditions as the objects or cases. Placing an instrument close to the ceiling out of reach will produce unreliable results since temperature and humidity levels vary widely throughout a vertical gradient. The same is true of placing the monitoring equipment on the floor. Psychrometers come in two basic forms. The sling psychrometer must be swung around to take a reading. Motor-driven or electronic types are also available. The psychrometer is needed for spot checks of conditions and to calibrate the hygrothermographs and thermohygrometers. When thermohygrometers and the other sophisticated equipment are not available, then monitoring can still be done using thermometers and hygrometers. Both of these are relatively inexpensive and are usually available.