The Cine Technician (1938-1939)

170 T H E (IX E TECHNICIAN Jan. Feb., 1989 32. Elon monosulphonate, 13. Slight traces of elon and bydroquinone disulphon'ates, 14. Free acid (HBr), L5. Temporary (up to a few hours after use) con-; centrations of unreduced, dissolved silver complexes. The alkaline water solution will produce: 17. Dissolved gelatin, 18. Probable degradation products of gelatin. In addition there will he a gradual accumulation oi substances present in the emulsion of the tilin which dissolve out into the develop* r. Such substances are sensitising dyes (in negative materials), more soluble bromide, etc. Dirt, calcium carbonate, and extraneous matter will also enter the tanks cither on the film or in the water, and there arc probably small amounts of other substances produced by chemical reactions of which thenis at present no know ledge. The problem of replenishing such a solution is twotold. Starting with Irish solution the bath must be brought, to a state of dynamic equilibrium with film, air. and replenisher without permitting the photographic properties to change appreciably. This equilibrium must then be maintained in the lace of changing conditions and, in general, with only the replenisher as an independent variable, since film and air quantities cannot be varied at will. In a Large industrial laboratory the amount of solution in the machines may be approximately 10,000 gallons and the amount oi film to be processed may be from five to ten million feet of motion picture positive per week. Correspondingly lower figures hold for negative handling. It is customary to connect batteries of developing machines by a system of piping in such a way that all the developer may be made to circulate past a single point. The volume of the solution is, of course, held constant. Dry film passes into the developer at a constant rate during the operation of a machine and carried with it a small amount of air, both on its surfaces and in the perforations. The latent image on this film enables the developer to reduce to metallic silver a quantity of silver halide which varies widely, depending on the nature of the subject matter. Motion picture positive film contains per thousand feet, roughly fifty grams of metallic silver in the form of halide salts. Of this, amounts varying from practically none up to nearly the full amount may be developed, depending on the subject of the reel. Thus sound track, or black titles on a clear ground, may represent only a few grams of silver per thousand feet, while a reel consisting largely of night scenes and the like may represent forty grams or more of reduced silver. On the average, approximately one quarter of the silver is ordinarily utilised, or from 10 to 15 grams. The remainder may be recovered by an efficient hypo recovery system. With respect to a given developing machine, however, the total average amount of silver reduced per clay is not constant unless care is taken to vary the type of work being handled. With an efficient circulating system, good mixing, and several developing machines operating simultaneously, satisfactory averaging of the work on all machines is possible. The wet film after development passes out of the developing solution into the rinse water, earning with it a considerable quantity of the solution. This quantity varies with the speed of the film, the design of the machine, and the efficiency of such devices as may be present to prevent "carry over." If the surface of the film carries no surplus layer of liquid there is in the gelatin of motion picture positive approximately one quart of solution per thousand feet. High speed and absence of devices to remove the surface layer may triple this figure. This solution loss, then, represents a definite minimum of liquid which must be added to the system as a whole to maintain its volume constant. This quantity frequently is insufficient and more must bo bled away so that the desired amount of replenisher may be introduced without overflowing the tanks. Since there is seldom occasion to refill such a system completely with entirely fresh solutions the dynamic equilibrium which must be maintained after ageing will be considered first. Since fresh replenisher is constantly entering the system, and developer which has nearly the photographic properties of the bath as a whole is constantly leaving it. considerable economy can be effected by choosing the proper position for the point on the system at which the two occur. They should be so situated that the "'bleed" by which solution is removed occurs in the system just before the point at which the replenisher enters the system. Theoretically, some economy could also be effected by having the fresher developer at the end of the machine into which dry film is being fed and the more exhausted developer removed from the other end. This sets up an unstable balance, however, which breaks down when the machine is stopped and so leads to variations over which there is little control. It the system is so designed that perfect mixing may be assumed at all times, an equation may be written for the grow th or decrease of any constituent of the solution. For convenience in computation, the figures will be given in the metric system for 10,000 gallons of developer replenished at a rate of 1\ gallons per minute. If h — replenisher rate in litres per ininute = bleed rate v— total volume of the system in litres a' = initial total amount of a given substance r' — amount of the given substance at time t V= amount of the substance added per minute then bdt— x'di — dx ox dt /, V k'—-X V this equation has as a solution v v b b v 1 = log or x'— V— [k a) e b e h v v (k -x) V A rather obvious axiom which greatly simplifies the calculations may be stated as follows. A subStance which is being formed in the solution at a constant rate may be considered as being introduced in the replenisher. Since material is also actually added in the replenisher it is convenient to convert the above equation to Concentrations rather than amounts. k' set k — — — concentration of material in replenisher b