The Role of Temperature and RH

Temperature and RH are directly related:

• when a volume of warm air is cooled, its RH goes up
• when a volume of cool air is warmed, its RH goes down

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:

• Biological
• Chemical
• Mechanical

General Set Points

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:

• To refer to the setting of the thermostat or humidistat over a short period of time (hours, days)
• To refer to the average annual setting of the thermostat and humidistat (because the set points may be adjusted over the year for various reasons, such as energy saving)

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.