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672 LEVENSON December
ment used. It consisted of the large earthenware crock containing 130 liters of developer which were drawn out of the bottom and returned, below the surface so as not to entrap bubbles, at the top.
A system of this sort could, of course, be used with submerged jets without altering its characteristics toward aerial oxidation. The liquid-air interface was 16 square centimeters per liter. It should be noted, however, that if the pump gland were to leak and admit air, or if film were to be run through the tank at high speeds, carrying down air in the perforation holes into the developer, the characteristics of the system would be entirely changed and it would become similar to the air-trapping circulation system described next.
Air-Trapping Circulation System — In most processing machines, a greater or lesser amount of air is trapped and carried, as bubbles, into the bulk of the developer. In many cases, this trapping of air is an inevitable consequence of the design of the system, particularly in those installations where the developer is pumped from one tank to another and then returns by overflowing down an incompletely filled pipe, or a pipe that is so placed that air is sucked in unintentionally. In high-speed machines, the moving film carries a considerable amount of air below the surface of the developer. In any machine a leaky pump gland, or a leak on the induction side of the pump, will inject air into the developer in the form of very minute bubbles which aerate the developer very efficiently.
To simulate such conditions, the arrangement shown in Fig. 4C was used. The developer (130 liters) was pumped from the earthenware crock to a small elevated tank, and returned by passing down a 4-foot long, 1-inch bore, rubber hose, carrying with it a considerable amount of air which was impelled into the bulk of the developer in the crock.
Air-Bubbling System — To represent an air-agitated bath, 130 liters of developer in the earthenware crock were agitated by a current of air injected from nine 4-mm-diameter jets situated 56 centimeters below the surface of the developer. This arrangement was achieved by blowing compressed air down nine tubes as shown in Fig. 4D. The air flow, \vhich was maintained constant according to a differential manometer in a fixed position in the system, was set at a level that, on the basis of general experience, seemed to provide ar sufficient degree of turbulence for good processing. This rate was approximately assessed at 27 liters per minute. The rate of flow of air through each jet was balanced to the same level.