As we have seen, an environment with carefully controlled and stable levels of temperature and relative humidity, as well as controlled levels of pollutants and light, is generally agreed to be the single most important factor in extending the life expectancy of paper-based and related materials.
Since our subjective impressions of climate levels tend to be inaccurate, temperature and relative humidity should be systematically documented wherever collections of permanent value are stored. Monitoring involves consistent measurement of temperature and RH, either constantly or at scheduled intervals, in collection storage areas.
Recorded data will serve to establish existing environmental conditions; support the need for environmental controls (should the need exist); indicate whether climate control equipment is operating optimally, if such equipment is already in place; and/or indicate if there is a season during the year when the use of additional climate control equipment (such as portable dehumidifiers) should be considered.
It is generally recommended that independent monitoring be undertaken even if there is a monitoring system within the HVAC system. In-system monitoring may not be sophisticated or accurate enough to provide a good picture of conditions.
Monitor the climate in each space for a full year to get a sense of seasonal changes, before making any changes to your climate control systems.
Monitoring devices vary greatly in cost, complexity, effectiveness, and staff time required to maintain them and to interpret the data. Some monitoring devices can be used to report immediate conditions ("snapshot" monitors), while others are designed to create an ongoing log of environmental conditions.
Examples of "snapshot" monitors include thermometers, dial hygrometers, indicator strips (useful in sealed exhibit cases), and sling or motor-driven psychrometers. "Min/max" digital thermohygrometers provide a middle ground; they can keep track of the highest and lowest temperature and humidity since the device was last reset. Thus, they can give a very general sense of fluctuations in the climate at night or on the weekend.
Recording hygrothermographs were the standard automatic recording device for many years, but in recent years dataloggers (battery operated devices that take readings at intervals specified by the user) have become the monitoring instrument of choice. This is largely due to the ease of downloading data gathered by a datalogger to a computer for graphing and analysis. There are a number of issues that must be considered in choosing a datalogger, including memory capacity, battery life, sensor accuracy across a range of temperature and humidity, and type of display. See the National Park Service's Datalogger Applications in Monitoring the Museum Environment, Part I: Comparison of Temperature and Relative Humidity Dataloggers (PDF) for more information on dataloggers.
It is important to choose the monitoring instrument most appropriate to your institution and situation. See Monitoring Temperature and Relative Humidity for more information on setting up a monitoring program.