This general introduction to the current approach to controlling ambient RH and temperature in museums is intended for all museum professionals. It is based on the "Museums, Galleries, Archives and Libraries" chapter in the American Society of Heating, Refrigeration, and Air Conditioning Engineers Inc. (ASHRAE) Handbook, a rather technical document intended primarily for engineers designing, maintaining, or operating HVAC systems in buildings that house heritage collections. The approach taken in the chapter represents a departure from earlier more traditional thinking about museum environments, which called for stringent control of RH and temperature. In the current approach, RH fluctuation is linked to measurable damage in artifacts. Certain types of artifacts are much more sensitive to RH fluctuation than others, and it is neither economical nor environmentally acceptable to have very tightly controlled conditions if they are not necessary.
Temperature and RH are directly related:
Because of this interdependence, temperature and RH are usually considered together. The damage caused by incorrect temperature and incorrect RH falls into three broad categories:
The target value of temperature or RH that a mechanical system is designed to maintain over time is known as the "set point". However, even the best mechanical systems will produce values that fluctuate above and below the given set point.
The term "set point" can be used in two ways:
Note that the "set point" is often defined by museums as 50% RH with the temperature between 15 and 25°C, although it can also be based on the historic averages. In practice, it may be defined by factors such as the needs of the collection, the performance of the building plus the HVAC system, and the climatic variation in temperature. On the other hand, class of control is defined by the degree of fluctuation in temperature and RH. And it is fluctuation rather than set point that we now strive to control because fluctuation provides the main threat to most artifacts and Class of Control defines the allowable degree of fluctuation.