Reading the Hygrothermograph In stack areas, temperature, relative humidity,
and light levels are controlled to create the safest possible environment
for the preservation of records. Air-handling systems are designed to
deliver filtered air free of harmful pollutants. The hygrothermograph
seen here is one of several types of monitoring devices used by the preservation
staff to keep them aware of conditions in the stacks so that problems
can be quickly detected and corrected.