Here again, there is a lot of common sense involved. One should simply see that junk- oops, foreign material – does not get into the penetrant. But it does happen. There is the obvious poor practice of people throwing stuff into the tank. We are told that tanks have been seen with plastic water bottles floating in them, and we know of an instance where a janitor cleaned his mops in a tank of emulsifier. Then there are those instances where the source of the contamination is much less obvious. Water can get into a tank of water washable penetrant as overspray from a poorly directed water wash nozzle. Water can also get onto a tank from condensation, a bit each day until there is enough to cause problems. The penetrant composition can change because parts are immersed in it while they are too hot, causing some of the penetrant ingredients to evaporate. Parts can be immersed that are not properly cleaned and that have contaminants on them that are then left in the penetrant.
Parts can be immersed that have acid or alkali residues on them from the cleaning operation that changes the chemistry of the penetrant, including quenching the response of the dye.
Proper design and operation of the inspection system and an awareness of what is going on can minimize the possibility of these things happening. Keeping tanks covered when the inspection line is not in use is also a very good practice.