- Dry powder developer,
- Water soluble developer,
- Water suspendible developer.
- Nonaqueous developer.
Each of these has distinctly different characteristics and different susceptibilities to deterioration and contamination.
Dry Powder Developer
Dry powder developer is not prone to deteriorate. It is composed of chemically stable absorbent powders that are unlikely to decompose or otherwise change their composition in a way that would affect their performance. However, dry powder developers are prone to contamination of various types. The most obvious of these is contamination by fluorescent liquid penetrant. When parts have been dipped into the dry developer, the developer adheres to the wet opening of discontinuities because there is liquid penetrant there.
The liquid penetrant makes the dry powder wet and fluorescent powder fall free from the part, they mix with the rest of the dry powder, creating spots of fluorescence that can later adhere to another part and cause a false indication.
Dry powder developer can also become damp or wet, causing it to clump. When this happens, it does not from a smooth even coat on the parts being inspected and it also may not stick to the wet openings of the discontinuities, thereby missing indications. Wetness can result from parts that are not completly dry before being put into the developer and it can occur because of excessively high humidity.
Water Soluble Developer
Water soluble developer may deteriorate in use as a result of infections of bacteria, algeae or fungus, commonly termed bugs. In order for the soluble developer to from a smooth coat on the parts, a surfactant is included in the formulation. Surfactants are favorite food for bacteria, algae and fungi and airborne bugs can land in the solution and infect it. Soluble developer formulations also biocides to counteract this undesirable occurrence but they do not last indefinitely. The best practice to avoid infections is to keep the tank covered when not in use. If an infection occurs, the cure is to drain the tank, sterilize it and then refill it with fresh developer solutions. The developer manufacturer has complete instructions for this procedure.
Soluble developers can also become contaminated in various ways, The concentration can go up or down depending on evaporation of water from the tank or the addition of water from wet parts being inmersed. Fluorescent liquid penetrant can get into the developer bath from the parts being inspected. Also, it is possible to contaminate the developer solution through electrolytic action. The developer is a solution of chemicals that ionizate in solution, creating an electrolyte. If a basket of dissimilar metals is placed into the soutions, electrolysis can occur in which metal is disolved from one of the dissimilar metals. The result can be cloudy or murky solution that reduces the developing ability of the solution.
Water Suspendible Developer
Water suspendible developers are prone to some of the same contaminants that soluble developers are, because they also contain surfactants and because they also have ionized components. This means that it is possible to have an infected developer bath and that it is possible to have electrolytic effects. It is also possible to have contaminate the suspension with liquid penetrant from the parts being inspected and it is possible that the concentration of the suspension will change as a result of evaporation of water from the tank or the addition of water from the parts.
Nonaqueous Wet Developer
Nonaqueous wet developers are usually used in aerosol from. Properly used, the developers is not prone to either deterioration or contamination. If the cans are not agitated before use, the developer powder will not be properly disperser and this improper dispersion will cause the developer coating to be either too thick or too thin. The only other failure that can occur is the loss of pressure in the can, in which case the developer may be good but can not be expelled from the can.
Nonaqueous developers used in bulk from are prone to concentration changes because of solvent evaporation and should be kept in tightly closed containers. if applied with a spray gun, agitation must be used to keep the developer particles in suspension.