The amount of moisture in the air is compared with the maximum amount that the air could contain at the same temperature and expressed as a percentage. Use your answers to the following questions, to explore the relationship between temperature and relative humidity and describe your conclusion clearly and concisely. Within a meaningful context, students will be able to: Interpret graphs which represent everyday situations.
Local hourly relative humidity and temperature data could be provided for this activity and the graphs plotted by the students before investigating the relationship between the two variables. 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. If the objects are stable within their usual environment then there is little point in altering those conditions. Thus instead of blindly following the strict temperature and relative humidity guidelines specified by many of the larger cultural institutions, anecdotal evidence suggests that it may be better to try to maintain the local conditions to which objects have become acclimatised.
This is, of course on the proviso that an appropriate investigation has revealed that these conditions have not caused damage to sensitive objects. In conjunction with determining the most appropriate relative humidity set point, it is most important to attempt to reduce diurnal and seasonal fluctuations. It is well known that the smaller the fluctuations, the lower the risk of physical damage to susceptible objects.
For most materials, as long as the conditions return to a value close to the original, extreme short-term fluctuations may not be a problem as there is not enough time for objects to respond. Very thin objects however, are much more susceptible to damage from short-term fluctuations than larger more massive objects. These latter objects can take up or lose more moisture without physical impacts becoming evident than can smaller moisture sensitive objects. Note that for objects containing more than one material type, the relative humidity level of the storage environment should reflect the recommended conditions for the most sensitive component.
Figure 6: Temperature and relative humidity data loggers front left , sling psychrometer front right and thermohygrograph rear middle.
One of the simplest instruments for measuring relative humidity is the sling psychrometer. It consists of two matched thermometers mounted side-by-side, one of which has a fabric sleeve covering it.
The end of this sleeve is inserted into a reservoir which is filled with distilled water. The fabric-covered thermometer is known as the wet bulb, the other the dry bulb.
When the thermometers are swung around, the water in the sleeve of the wet bulb evaporates, making it cooler than the dry bulb. The amount of evaporation and subsequent cooling depends on the amount of moisture present in the air - the drier the air the greater the level of cooling and vice versa.
The difference in the temperatures of the thermometers therefore indicates how dry or humid the air is - the greater the difference the lower the ambient relative humidity, the smaller the difference the higher the relative humidity. A standard hygrometric chart, which displays a series of wet and dry bulb temperature differences and corresponding dry bulb temperatures, is then used to give an accurate measure of the relative humidity.
The sling is used to calibrate many other types of hygrometers. Coupled with a thermohygrograph seven-day or one-month an accurate day-to-day or hour-to-hour record of temperature and humidity can be obtained all year round.
An advantage of a thermohygrograph is that the recent temperature and relative humidity history of the space being monitored is immediately visible on its chart. Electronic instruments are also available which record changes in temperature and relative humidity. These devices vary greatly in price and can be obtained from electronic shops or conservation suppliers.
These instruments have certain advantages. For example, they may be placed relatively unobtrusively in display cases or in small storage areas in which a thermohygrograph either would not be appropriate or would not fit. Other relative humidity sensors linked to data loggers can be programmed to record temperature and relative humidity conditions at regular intervals over periods of many months.
These sensors are very small and by operating continuously over the different seasons allow useful long-term profiles to be established of storage and display conditions.
Monitoring relative humidity is important to determine both the actual levels and the fluctuation rates. This information may be used to see how well a building buffers the external ambient conditions and also to see how well a display case further buffers the gallery environment Figure 7.
If the temperature is steady then the relative humidity within a well-sealed display case will remain constant. We strongly recommend the use of passive methods for temperature and relative humidity control as these are often more sustainable and cost-effective. Appropriate design of buildings and storage media, the use of insulating materials and good management practices are critical components of passive environmental control.
These methods are much preferred to more costly and often less reliable air conditioning systems. Fluctuations in temperature and relative humidity are caused by daily and seasonal fluctuations in the local weather. Even without air conditioning, the insulating effect of a building ensures temperature and relative humidity variations inside a building are generally smaller than those outside. The conditions in the innermost rooms will be the most stable, outer rooms and lofts the most variable and basements the most vulnerable to the development of high relative humidity levels.
Thermal insulation of a building will assist in maintaining more stable conditions. The provision of shading north side of a building in the southern hemisphere and the use of reflective building surfaces will also assist in reducing the impacts of exterior conditions on interior environments. Cupboards, boxes and display cases are secondary insulating barriers which provide an additional buffer, helping to stabilise conditions even more Figure 7.
The dew point is the temperature to which the air would have to be cooled to become saturated. Below the dew point, water will condense out of the air onto surfaces. In the early morning, grass surfaces will be coated with water if the nighttime temperature has dropped below the dew point. When humidity is high, the dew point temperature is only a few degrees below, or equal to, air temperature.
In dry places, like deserts of the southwest, air temperature can be 50 or 60 degrees above the dew point. If you're a person that likes to spend time outdoors, then you know that humidity can have a great effect upon human health. As growers work outside during the hottest part of the day, they need to take extra care to stay hydrated when temperature and humidity are high.
People keep cool by perspiring, but it's harder for our bodies to regulate temperature under very humid conditions. In areas that are very dry, such as Arizona, the humidity is so low that when you sweat, the water evaporates so quickly that you may not even feel it.
In this case, you must be careful to stay hydrated because the water loss goes unnoticed. Humidity can also affect plant turgor pressure, which is an indicator of the amount of water in plant cells. When humidity is low, and dew points are in the 50s and low 60s, moisture evaporates from plants very quickly. When this happens, plants can wilt rapidly if too much water is pulled out of plant cells through transpiration. Conversely, when humidity and temperature are both high, plants can get overheated because transpiration is reduced, thus restricting evaporative cooling.
Humidity also influences plant diseases, especially fungi and molds that grow and spread rapidly when humidity is high. Humidity can also affect the fruit set of some plant species. An example is the bean Phaseolus spp. Humidity is also an important consideration for post-harvest storage of crops.
Cold temperature and low humidity are important for the long-term storage of grains corn, wheat etc.
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