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236 J. M. CALHOUN Vol 43, No 4
in water starting from 50 per cent RH. The change in dimension, in general, parallels the change in moisture content. The extremely rapid swelling of the gelatin compared to the film base is apparent. In 5 min the gelatin swells more than ten-fold in thickness, whereas the film base swells only a small fraction of 1 per cent in length. The safety base swells to a greater extent than the nitrate base and does so more rapidly, as would be expected from the difference in their rates of moisture absorption. It is conceivable that 2 different films might show the same degree of expansion when fully swollen in water, and yet have different swelling rates. This would be important in processing where the immersion time is relatively short, and the faster swelling film would reach a greater extension.
The Rate of Drying of Film. — The rate of drying of wet film after development is very important to the motion picture laboratory. When the film surface is wet, the rate of evaporation of moisture from the film is essentially the same as the rate of evaporation from a free water surface. In the absence of radiant heat, the rate of drying of film by air flowing parallel to the surface is constant as long as the film surface is wet. Under these conditions, the rate of drying may be expressed by the following equation, which is merely a modification of the standard evaporation formula11
W T = k (Ps Pa}V** (I)
where T = drying time, min
W = weight of water in film, gms per ft2
Ps = vapor pressure of water at the surface temperature (wet bulb),
mm Hg
Pa = vapor pressure of water in the air (dew point) , mm Hg V = air velocity parallel to the surface, ft per sec k = a constant
This equation shows that the time to sensible dryness is reduced by:
(1) Decreasing the water content of the wet film by hardening or by the use of a squeegee.
(2) Increasing the difference between the wet bulb temperature and the dew point of the air.
(3) Increasing the air velocity.
This formula has been found to apply very well to the drying of film within the limitations specified. However, there are a number of practical considerations in the drying of developed film which limit the conditions which may be selected. For example, the dry