American cinematographer (Jan-Dec 1924)

January, 1924 AMERICAN CINEMATOGRAPHER Thirteen Problems in Motion ¥^. T f . By M. Briefer Picture Laboratories Second Installment. From Transactions, Society of Motion Picture Engineers. Not all have yet learned to keep the developing, fixing and other solutions at something like relatively uniform temperatures and that additions should be made with previously prepared stock solutions and not with dry salts which may have strong positive or negative heat reactions. However, since we cannot hope to effect this reform let us insist at least upon the liberal use of good thermometers. There is a lot of fiction indulged in as to the value of dopes, special mixtures, secret formulae and the like for controlling grain, contrast and gradation. How may we bud an effective, a convincing way of demonstrating the futility, the uselessness, the wastage of such procedures? Changes in processing formulae should be made with some regard to proportionality. The formulae recommended by the manufacturers of photographic material should be studied and the relative proportions maintained. This preachment has been delivered regularly since the dawn of practical photography and according to the rate of its acceptance is still good to adorn the pages of our periodicals for some ages to come. Humidity Humidity and temperature have important functions in processing laboratories. The subject seems rather vaguely understood by the average man, if indeed any attention is paid to it at all. Altogether it appears as if shrouded in a veil of mystery. There is really nothing mysterious about it. Relative humidity means nothing more than the per cent water vapor contained in air. The term is defined as the ratio of moisture present in air, to the amount it will hold when saturated at the same temperature. Thus if a cubic meter of air at a temperature of SOF. is saturated with moisture it will contain approximately 25.5 grams water vapor. The relative humidity will therefore be 100%. Should the same volume of air at the same temperature contain only 12.75 grams of water vapor the relative humidity will be 50%. Absolute humidity is defined as being the measure of the actual weight of water vapor present in air and is usually expressed in grains per cubic foot. There are tables which serve to show that the number of degrees difference between wet and dry bulb is the factor for calculating the relative humidity at the prevailing temperature. Reference to these tables gives the relative humidity direct for any combination of wet and dry bulb readings. Assuming a requirement of 05% relative humidity for good working conditions we find from the tables that this per cent moisture is realized in many combinations of wet and dry bulb readings, in fact, at all temperatures. Most workers are possessed witli the idea that as long as the required per cent moisture is present in the atmosphere nothing else matters. Yet it matters very much indeed. A dry bulb at 72 (let us understand the figures in degrees F.) and a wet bulb at 04 indicate a relative humidity of 05%. The same relative humility is indicated when the dry bulb registers 82 and the wet bulb 73. But there is considerable difference in performance as between the two conditions. The difference is in the position of the dew point or the points at which moisture begins to condense and deposit as dew on all objects contained in the room. The difference in performance is of course not limited to the two examples. There is a proportionate difference between any two sets of conditions. The question arises, how shall we determine the most favorable working conditions. We wish to make this explanation as clear as possible. If it appears elementary to those present, please remember that it is still one of the perplexing and troublesome factors in some film and paper sensitizing plants where such matters are supposed to be understood. The Hygrometer The best known type of wet and dry bulb hygrometer consists of a wood block upon which is mounted side by side two ordinary indicating thermometer tubes. The bulb of one thermometer is covered with a tubular wick thoroughly wetted with water. The other end of the wick is immersed in a reservoir which keeps the wicking saturated by capillary. Constant evaporation necessitates frequent refilling of the reservoir. It is well known that evaporation is accompanied by the loss of heat and the more rapid the evaporation the greater the heat loss. The dry bulb of this instrument indicates normal temperatures without regard to the humidity present. When the air is saturated no more water can be taken up by it, that is, no evaporation can take place at the wet bulb, therefore there is no heat loss registered. If both thermometers indicated the same temperatures before the wet wick was attached to one of them, they will read alike under the conditions named. If the air is now heated its capacity for taking up moisture is increased, it is then no longer saturated and evaporation at the wet bulb takes place at once. The reading of the wet bulb will fall below that of the dry bulb thus demonstrating the heat loss due to evaporation. The dryer the air, the greater will be the difference between wet and dry bulk readings. Referring to the example first given, when the dry bulb is at 80 degrees and the air has an absolute humidity of 25.5 grams water vapor per cubic meter (saturation for that temperature) then the wet bulb will also indicate 80 degrees. On the other hand, if at the same temperature the absolute humidity is only 12.75 grams per cubic meter the wet bulb will indicate 07 degrees. This difference degrees between wet and dry bulbs is the measure of the rate of evaporation for these readings and the factor for computing the relative humidity which, (Continued on Page 22)