Caring for Cultural Material 1

Caring for Cultural Material 2

Damage and Decay

Managing Collections

Managing People

Handling, Transportation, Storage and Display

Glossary

Index

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Damage and Decay
In this volume:
– Light and Ultraviolet Radiation
arrow Humidity and Temperature
– Biological Pests
– Dust and Pollutants
– Common Deterioration Processes
– Acknowledgments
Humidity and Temperature
In this chapter:
– Objectives
– Introduction
arrow What is relative humidity?
– How does relative humidity change?
– Why worry about relative humidity and temperature?
– Can the damage be prevented?
– Identifying the source of the problem
– What can be done to minimise damage?
– Modifying the conditions in buildings
– A word of caution about travelling exhibitions and loans
– In an ideal world …
– Australia’s climatic zones
  MORE ABOUT RELATIVE HUMIDITY AND TEMPERATURE
– Measuring relative humidity
– Dehumidifiers
– Humidifiers and steam generators
– The use of silica gel
– For further reading
– Self-evaluation quiz
– Answers to self-evaluation quiz

 

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Humidity and Temperature

What is relative humidity?

Heat really needs no explanation. You may have difficulty defining it, but you're certainly familiar with the experience of it. Relative humidity, on the other hand, does need explanation.

Water is an extremely pervasive substance and can be found everywhere on the planet—including in the air, where it's held as vapour.

The capacity of air to hold water-vapour varies according to the temperature of the air.

The warmer the air, the more water-vapour it can hold. As the air cools down, its capacity to hold water will decrease.

Relative humidity is a measure of the amount of the amount of water-vapour contained in air at a particular temperature. It is basically a comparison between:

  • the amount of water-vapour held in the air at any one time and at a particular temperature; and

  • the total amount of water-vapour which the air can hold at the same temperature, that is, the amount of water which will saturate the air at that temperature.

Relative humidity is expressed as a percentage.

This can be written as an equation:

RH = water-vapour present in the air x 100%
  water-vapour required to saturate air at that temperature

As the temperature of air increases, its capacity to contain water-vapour increases. For example:

  • At 0C the air can hold about 6 grams of water for each cubic metre of air, that is, 6g/m3.

  • At 10C this increases to 10g/m3

  • At 20C it increases to 17g/m3

  • And at 30C it increases to 30g/m3

So, if air at 20C contains 8.5g/m3 of water-vapour:

RH = 8.5     x 100%
  17
=  50%

    Thus the relative humidity would be 50%.

 

 

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