A small factory was screen printing resister inks onto white ceramic boards and firing them in an oven. While conductive inks are fairly straight forward, resistor inks require more care. If the oven is too hot or not hot enough the resistor values would fall too far out of tolerance and the circuits would have to be scrapped.
The factory was generally quite good at maintaining control and getting high yields. Suddenly one day the resistor values were much too high in the afternoon run of circuits. These resistor values were higher than ever before, but the morning run had been fine.
The oven temperature was confirmed and the resister ink was ruled out because it was the same jar that had been used that morning and the day before.
What caused this sudden high resistor levels in the afternoon run of circuits?
Here's the rest of the story.
Could the contamination be airborne? This was a possibility since the oven drew in outside air, and contaminated air could change the resistor values
A process engineer decided that it was best to start checking the neighborhood for any airborne source. He didn't have to look far. Adjacent to the factory was a new tenant in the food processing business. The new company was making French Fries.
A chat with the tenant revealed that they sliced potatoes in the morning while the big fryer was heating up, and then fried and bagged in the afternoon. This looked like a good suspect, contamination from oil vapors, but how to confirm it?
Coincidentally the frying operation was going to be shut down the next day. Sure enough, there were no problems on the afternoon when the friers were shut down.
Apparently, the oil vapors were getting into the resistor inks. The fix was easy, and one that would probably prevent other air contamination problems in the future. Reaction carbon air filters were added to the oven, and that seemed to solve the problem.