9/6/2023 0 Comments Relative humidity dew point![]() ![]() Since the early 1990s, many scientific studies have been devoted to determining the most appropriate environmental conditions for the storage and display of objects. 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. ![]() The key is to determine just what the most appropriate environments are for the objects under consideration. Storage and/or display of objects in appropriate environments contribute significantly to their longevity. The following temperature and relative humidity ranges were recommended, on a daily basis for particular climatic zones (Heritage Collections Council 2002): 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. In the tropics for instance, where the average yearly relative humidity is about 65 %, it may be better to have this as the optimum level (combined with air circulation) whereas in an arid region it may be better to aim for a relative humidity range of 40 - 50 %. 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. 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).įor many years the recommended ideal temperature and relative humidity conditions for museum collections were specified as 20 ☌ and 50 % respectively. Temperature and Relative Humidity Guidelines Paint for instance, may craze or peel from wooden surfaces (Figure 5) and paper glued to a backing board may buckle.įigure 5: Paint damage due to differential expansion of the underlying wood and the paint layers. Rapid fluctuations can lead to cracking and warping of these materials and also cause bonded materials to separate. 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. The subsequent condensation will accelerate metal corrosion and encourage biological attack on susceptible organic materials. ![]() A sudden drop in temperature in a display case for example, may result in the dew point being reached. Large and rapid changes in relative humidity caused by sudden temperature variations can have significant effects on materials. It is important not only to control relative humidity levels but also to minimise fluctuations. Constant relative humidity above 70 % can cause mould growth and increase corrosion whereas relative humidity levels below 40 % may cause sensitive materials such as paper, parchment and textiles to become brittle. Such changes in relative humidity could be caused by many factors including direct sunlight, spotlights and air-conditioning failures.įigure 4: Tar creep induced by warm gallery temperatures.Īfter light, relative humidity is the most significant factor to be considered in the environmental control of collections. ![]() If the temperature rises the relative humidity will fall. If the temperature inside the case falls then the relative humidity will rise. In a closed environment such as a display case, there will be a fixed amount of water vapour, referred to as the absolute humidity. In essence, cold air cannot hold as much water vapour as warm air. Therefore air at 50% relative humidity, regardless of temperature, is holding half of its total possible water capacity. Put simply, the relative humidity is a measure of the percentage saturation of the air. Saturated air at 10 ☌ holds about 10 grams per cubic metre (g/m 3) of moisture at 20 ☌ about 17 g/m3 and at 30 ☌ more than 30 g/m 3. amount of water the air can hold at that temperatureĪir at 100 % relative humidity holds the maximum amount of water possible at that particular temperature and is said to be saturated. It is expressed as a percentage and can be defined as follows: RH = The relative humidity (RH) of the air is an indication of how much water vapour is in the air at a particular temperature compared with how much water vapour the air could actually hold at that temperature. These deterioration factors are considered together because of their close inter-relationship. ![]()
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