The ratio of the amount of water vapor in the air to the amount which would be present at the same temperature were the atmosphere to be fully saturated. Relative humidity is expressed as a percentage. Saturated water vapor pressure for various temperatures is given below:
Temperature Saturated Vapor Pressure (mm. of mercury) 0 4.58 5 6.54 10 9.20 15 12.78 20 17.5 25 23.69 30 31.71 35 42.02 40 55.13
Thus, at 25° C., for example, if the pressure of water vapor actually present in the atmosphere is 13.77 mm of mercury, the R.H. is:
actual vapor pressure R.H. = --------------------- X 100 saturated vapor pressure or 13.77 ----- X 100 or 58% 23.69
All organic fibers, including paper and leather, are hygroscopic and absorb or emit water vapor so as to adjust their moisture content to that of the air in which they are stored. Although the R.H. factor varies with different kinds of organic fiber, it remains constant for the same kind of fiber. Since the moisture content of paper affects its weight and printing properties, its R.H. must be known. It may be measured by a sword hygrometer inserted in a pile of paper.
Corresponding Water Content of equal Weights of R.H. of the Air a Cellulose Fiber 30% 5% 40% 6% 50% 7% 60% 8.5% 70% 10% 80% 13%
The change in rate of increase, e.g. between 50 and 60%, stems from the fact that the curve is not flat but "S" shaped; and, also, because of hysteresis, equilibrium water content is higher when approached from the wet side than from the dry side.
Any changes in water content of a fiber immediately manifests itself as changes in its dimensions, especially in the cross direction of the paper. The fiber diameter swells considerably at increased moisture content resulting in stretching of the paper. Other properties of the paper are also affected, but the most significant change affecting printing is the change in dimension. The absorption and emission of moisture require time, absorption generally requiring less than emission. The edges of paper in storage come into contact with the atmosphere to a greater extent than other areas, and are therefore exposed initially to changes in R.H. A rise in R.H. will cause the edges of the paper to buckle, while a decrease may cause undulations in the middle of the pile. In order to avoid dimensional changes in paper it is not unusual to hang it before printing. In offset printing, where attention must be given to the moisture transferred to the paper directly from the dampened plates (by way of the rubber cloth), a relative humidity of approximately 60% has been found to be suitable for purposes of register, while in other printing methods and paper handling firms, a lower moisture content may be more suitable.