In the Navy, we use two methods of mixing chemicals: hand mixing and machine mixing. Hand mixing is used when only small quantities of solutions are needed or when machines are not available. Machine mixing is necessary to handle the large production requirements of most Navy imaging facilities.

Agitation Mixers

Proper agitation of the solution during mixing increases the rate at which the chemicals are dissolved and prevents undesirable side effects. For proper agitation, an agitator type of mixer does not cause excessive amounts of air to enter into the solution (fig. 9-4). Developers are quickly ruined by oxidation; a few minutes of improper and violent agitation can

weaken a developer and cause it to underdevelop and sometimes stain film. Too little agitation during mixing may cause the powdered chemicals to settle to the bottom of the mixer and form hard lumps. When these lumps of chemicals are undissolved and undetected, they can clog pumps and plumbing during transfer from the mixer to the storage tank. These lumps can also cause the solution to be less active.

Agitation mixers circulate solutions through a pump that causes a stirring action. There are several types of agitation mixers available. These include large capacity models for preparing large volumes of solutions and small models for making small amounts of solution.

Impeller Mixers

Impeller mixers provide thorough, rapid mixing, but they must be used with care to prevent frothing or foaming and introducing air into the solution. The solution must be mixed so a minimum amount of air is drawn into it. When the shaft is placed in the center of the container, the impeller causes a whirlpool effect that introduces excessive amounts of air into the solution. Furthermore, when the shaft is in the center of a container, there is very little agitation in the bottom-center area of the container and undissolved chemicals pile up directly beneath the end of the shaft (fig. 9-5).

Avoid bumping the shaft or impeller on the sides or bottom of the mixing vessel. This procedure may bend the mixer shaft, and a bent shaft produces excessive vibrations that can ruin the motor bearings.