VARIATION OF RELATIVE HUMIDITY IN A SEALED SHOWCASE
(download here the showcase calculator)
Almost any closed ambient contains humidity, generally expressed as g (water)/m3. When the water content is so high that a small increase condenses in liquid form, the vapor is said to be saturated.
The amount of saturated vapor increases with temperature, as shown in this table:
|
T °C |
0 |
5 |
10 |
15 |
20 |
25 |
30 |
35 |
40 |
|
g H2O/m3 |
4.85 |
6.80 |
9.40 |
12.77 |
17.31 |
23.08 |
30.36 |
39.61 |
51.05 |
The ratio between the actual water content in a given ambient and its saturation value is called Relative Humidity (R.H.).
Let us suppose that in an empty sealed showcase there are 12 g/m3 of water at 25°C. By means of the above table, we can evaluate the relative humidity: R.H. = 12/23.08 = 0.52, e.g. 52%.
If the temperature goes down to 20°C, what will be the R.H. inside the showcase?
The new value can be evaluated by taking into account that the saturation value at 20°C is 17.31 g/m3, so that R.H. = 12/17.31 = 69%.
But what happens if temperature decreases to 10°C? In this case, the water content of our showcase (12 g/m3) is higher than the maximum allowed value at 10°C (9.40 g/m3). The R.H. jumps then to 100%, and the difference (12-9.40 = 2.6 g/m3) condenses in liquid form.
The above calculations hold for a sealed and empty showcase: the presence of a water-absorbing material (such as paper, leather or parchment) changes everything.
When R.H. decreases, our material releases its absorbed moisture. If the surrounding environment is big, it releases a lot of moisture. When R.H. increases, our material adsorbs a lot of water .... and so on.
We can download here a spreadsheet that calculates the variations of R.H. in a sealed showcase containing paper.
Let us suppose we have a showcase sealed at 25°C and 50%R.H. What happens when temperature jumps to 30°C?
Simply, paper acts as a buffer toward humidity.
When the surrounding air is wide (for instance, 10 m3 per 100 g of paper), a R.H. value of 38% is reached and paper loses some water (~22% of loss).
In a volume of 0.05 m3, the variation of R.H. is null (50% to 50%), but the water content of our paper decreases of about 3%.