The relative humidity can be explained as the percentage of the steam pressure of the sample air standing the saturated steam pressure.
The concept of the relative humidity is very difficult to be understood. The hermeneutic and elucidation to it is as follows:
It can get the absolute degree of the humidity content to draw out the steam within one cubic meter of the air and measure its weight. It means the water amount contained in one cubic meter of the air and is showed as "g/m3".
The following chart for measuring humidity shows the biggest steam content in one cubic meter of air under the certain temperature. The steam content in the air will increase along with the rising of the temperature. When the temperature is at 10℃(50℉), the air can contain 9 grams of steam and now it has the biggest absolute water content, that means the air is at the saturated appearance. When the temperature is at 20℃(68℉), the air saturation degree is 17g/m3. Therefore, its saturation degree is 9g/m3 if one cubic meter of air implies 9 grams of steam in a sealed container when the temperature is at 20℃. If pour 3 grams of water into the container, the inside absolute humidity will increase to 12g/m3 through the evaporating of the water. If pour 8 grams of water into the container again, there is 5 grams of water evaporating and the other 3 grams of water will stay in the bottom of the container because one cubic meter of air can only imply 17 grams of steam when the temperature is at 20℃. So the relative humidity in the container is 9/17 and equals 53% when there is 9 grams of steam in it.
The relative humidity relies on the air temperature and it will descend along with the rising of the temperature if there is no more steam poured in. So the chart shows that one cubic meter of air can contain 23 grams of steam and the relative humidity descends to 9/23 to equal 39% when the container is heated to 25℃.
The relative humidity will also rise even if there's no more water poured in when the temperature in the container descends to 15℃. When the temperature is at 15℃, one cubic meter of air can only contain 12.5 grams of steam so at this time the relative humidity is 9/12.5 and equals 72%.
The relative humidity will rise to 100% because the steam in the container is at the saturated appearance when the air temperature descends to 9℃ and there will have some drops of water on the wall if the air temperature further descends because the air must condense some steam. The temperature that the steam starts to condense (it's also the temperature that the air attains the saturated appearance) is called dewdrop. In winter when the air indoors circulates to the window where the temperature is enough low, it will be cooled off and there will have some drops of water on the window.
The temperature and relative humidity
The following important points concerning the temperature and relative humidity should be kept in mind firmly.
The important point that should be understand first is that there is no unify ideal temperature and relative humidity for the different library data and just there is only a scope of temperature and relative humidity for possibly reducing the variety of the data. To a certain data the certain temperature and humidity perhaps is the best choice, but to the other data this temperature and humidity is possibly to result in a disaster. For example, the photograph, film, magnetic tape and other digital carriers should be stored in the environment with lower temperature and humidity for prolonging their lives. But in order to protect the parchment and leather paper documents, they should be stored in the environment with the humidity higher than RH50%.
A great deal of science evidence certificates that the paper can keep its chemical stability and physical characteristic such as the external appearance much better if place it in the environment with stable temperature (lower than 10℃/50℉) and relative humidity (30-40%). However, to the cultural heritage binding by leather or cow parchment, the internal paper is best to be kept in the environment with lower relative humidity but the leather and the cow parchment themselves should be stored in the environment where the relative humidity is not lower than 50% to keep its activity. When decide the temperature and relative humidity of the storing environment for the special collected articles, it should consider the different demand of the articles and their artificial covers carefully.
The influence of the temperature.
The point that need to be emphasized again and again is that the chemical reaction speed of the data paper in the traditional library and the file building will doubly fasten if the temperature increases 10℃ each time and it will descend half if the temperature reduces 10℃ each time.
The calories coming out along with the lower relative humidity will lead some articles such as the leather, the cow parchment, the gum, the glued part of some videotapes and audiotapes, etc. to become dry and frailty.
The calories coming out along with the higher relative humidity will contribute to the growth of the fungi and create a good living environment for the insect and the harmful living creatures.
The low temperature (lower than 10℃/ 50℉) with the higher relative humidity and bad air circulations will cause the environment to be damp and the mould is growing.
The influence of the relative humidity
The water content in the air will increase or decrease because of the rising or descending of the relative humidity. And the article will inflate or constringency along with the increasing or decreasing of the water content in the air.
It will reduce the mechanical damage to the data to the least if the relative humidity is stable at 55-65% and it is beneficial for keeping the activity of the data. If keep the relative humidity higher than 65%, the gum of the data in the modern and traditional libraries will become soft and lose its gluing function.
Even if keep lower temperature, the living creature still encroaches upon the articles very probably when the relative humidity is higher than 70%. In the area where the air circulation is very bad, the relative humidity should be not higher than 65% to prevent the collected article getting moldy.
The relative humidity lower than 40% can reduce the chemical damage largest but it will probably cause the article to atrophy, become hard and frailty and spilt open.
The influence leaded by the change of the temperature and relative humidity
The preceding context has already mentioned that if the water content in a space is fixed, the suddenly descending of the temperature will lead the relative humidity to rise quickly and the steam to condense so that the collected articles would get moldy or meet other problems perhaps. The just right variety of the temperature and humidity in a longer period of time will produce the least pressure of inflation or constringency to the article. The change of the temperature and the relative humidity will affect the size and mechanical property of some organic materials and it will damage the articles if this change happens in a short time. The seen damage includes the slice of ink, the winding cover and the sensitive agent to light in the photograph.
Measure and record the temperature and relative humidity
The surrounding condition of each area should be monitored and recorded and the equipment for monitoring and recording should be credible and periodically maintained. To monitor the condition is very important because it records the current environment condition, meets the demand of the controlling equipment to the installing environment and keeps watch on that whether the weather controlling equipment operates normally and provide the ideal condition.
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The concept and problem concerning the humidity
