e. Cascade Wash Tank. Cascade washing of radiographs is the most efficient
method because it rinses the fixer solution off the film with a minimum amount of water.
Although an additional operation is required for moving the films from one compartment to
the next, the cascade system is especially useful in installations handling a continuous
volume of films. The tank (figure 2-8) consists of two washing compartments and an
overflow well. The fresh water flows into the bottom of compartment "A," passes upward
over the partition into the compartment "B," under the end wall of the tank proper, and
through the outlet "C" in the overflow wall.
NOTE: Arrows indicated the flow of water.
Figure 2-8. Cascade system of washing.
(1) Washing compartments. Fixer solution is denser than water and normally
drifts towards the bottom of the tank after being washed from the radiographs. Preliminary
washing should be in compartment "B" where the flow is downward. The radiographs are
then placed in compartment "A" for the remainder of the washing period. By placing a film
first in compartment "B," those washing in compartment "A" are not contaminated; this
overcomes the chief disadvantage of single compartment washing.
(2) Overflow well. The chief purpose of the overflow well in a cascade system
is to permit the draining of water from the bottom of the adjacent washing compartment
through a standard standpipe or through an overflow pipe in the back of the tank.
NOTE: Arrows indicate water flow from inlet over both film surfaces and to outlet.
f. Temperature Control. The temperature of the solutions should be controlled
closely. The ideal situation would be to maintain both the air and the tap water at 68F,
which would keep the washing water constant with the solutions. Since this is seldom the
case, some method of temperature control becomes necessary.
(1) Refrigeration unit. A detachable refrigeration unit is normally supplied for
use with the processing tanks to temper the processing solutions.