The amount of water vapor in the air is called absolute humidity. The amount of water vapor in the air as compared with the amount of water that the air could hold is called relative humidity. This amount of space in air that can hold water changes depending on the temperature and pressure.
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Privacy Policy. All rights reserved. The table below indicates maximum moisture content in a cubic metre of air at various temperatures:. This dramatic change is important to explain why heated air is so much more effective than cold air in drying processes. Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro.
We don't collect information from our users. Only emails and answers are saved in our archive. In essence, cold air cannot hold as much water vapour as warm air. In a closed environment such as a display case, there will be a fixed amount of water vapour, referred to as the absolute humidity. If the temperature inside the case falls then the relative humidity will rise.
If the temperature rises the relative humidity will fall. Such changes in relative humidity could be caused by many factors including direct sunlight, spotlights and air-conditioning failures. Figure 3: Hygrometric chart adapted from Thomson Condensation will then occur.
Damage caused by either changes in temperature and relative humidity or by exposure to inappropriate levels of these agents may be either chemical, physical or biological in nature Figure 4. The main impacts of temperature, unless they are extreme enough to freeze or melt objects, are on relative humidity and the rate of chemical deterioration of objects. Figure 4: Tar creep induced by warm gallery temperatures.
After light, relative humidity is the most significant factor to be considered in the environmental control of collections. It is important not only to control relative humidity levels but also to minimise fluctuations. Large and rapid changes in relative humidity caused by sudden temperature variations can have significant effects on materials.
A sudden drop in temperature in a display case for example, may result in the dew point being reached. The subsequent condensation will accelerate metal corrosion and encourage biological attack on susceptible organic materials. Organic materials such as paper, wood, textiles, bone and ivory expand and contract as they absorb and release water in response to changes in relative humidity levels. Rapid fluctuations can lead to cracking and warping of these materials and also cause bonded materials to separate.
Paint for instance, may craze or peel from wooden surfaces Figure 5 and paper glued to a backing board may buckle. Figure 5: Paint damage due to differential expansion of the underlying wood and the paint layers.
While temperatures in museum display spaces are often designed for visitor comfort rather than for object preservation, conditions in storage areas are usually more carefully defined and controlled see recommendations for particular material types in other chapters.
These conditions, that were experientially based rather than scientifically determined, are difficult to maintain unless expensive air conditioning systems are used and may not be possible or even desirable in certain areas. This not only saves on energy costs but also means that material which is conditioned to the ambient relative humidity will not be damaged by change.
The following temperature and relative humidity ranges were recommended, on a daily basis for particular climatic zones Heritage Collections Council :. The key is to determine just what the most appropriate environments are for the objects under consideration. Most of the uncertainty with the specification of relative humidity conditions is associated with organic and mixed media materials whereas the conditions for inorganic materials like metals and ceramics are generally more clearly defined.
Since the early s, many scientific studies have been devoted to determining the most appropriate environmental conditions for the storage and display of objects. More stable conditions must be maintained however for certain degraded objects veneers and inlays etc and where possible, lower relative humidity conditions should be maintained for most metal objects. These more relaxed guidelines were determined after considering the possible impact these changes may have on the preservation of general collections, the need to make collection care more sustainable especially in light of climate change and to reduce the carbon footprint and high costs associated with the maintenance of tighter collection conditions.
This latter point is significant as previous guidelines were difficult to maintain in most situations without the use of expensive, high energy air-conditioning systems. Further, the IIC and ICOM-CC groups also recommended that collection care should be achievable for local climates and that consideration should be given to more use of passive methods for environmental control, to the use of simpler technologies, air circulation and lower energy systems.
Conservation management of environmental parameters has also changed somewhat, tending to move away from the specification of strict guidelines apart from those for particular material types such as acetate films, weeping glass etc to the adoption of a risk analysis approach. Risk analysis requires an examination of the relationship between the environment and the objects in that particular environment.
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