We do not know to what extent this subject is a problem for anyone in the USA, but it seems to have come up in Europe. There is a lot of common sense involved in this subject, and probably most, if not all, of our readers will agree with that. But let us take a look at the subject.
The easiest situation to look at is where one has a tank full of penetrant, and the level falls because it is being used to inspect items. The level then needs to be brought up by adding penetrant. When the replacement penetrant is ordered, it is rarely possible to get the same batch that is in the tank unless a special arrangement has been made with Met-L-Chek® to put a certain quantity of a specific batch into inventory, tagged for a specific customer. Otherwise, the product ordered will be of a different batch than what is in the tank. Assuming that the penetrant ordered is the same product that is in the tank, the replacement material will have been manufactured, tested, and certified to be the same as the product that is in the tank. So, mixing of different batchs is not a problem but rather common practice.
There are two situations where one might add a different product to the tank than what is in it. Both of these situations are to be avoided, but there is evidence that they have happened in the past. The first of these is when he tank contains a penetrant made by manufacturer “A”. When the tank needs to be replenished, the purchasing agent does not realize that the same product must be added. This agent orders a penetrant of the same type, method, and sensitivity, but made by manufacturer “B”. His or her logic could be that a Type I, Method A, sensitivity level 2 penetrant is listed on the QPL as being equivalent among all qualified manufacturers, and therefore they can be mixed. While this might be an appealing bit of logic to the uninformed, those who have the responsibility for the inspection process know that this cannot be done. Different manufacturers use different formulations that result in equal performance, but they cannot be mixed, because it would result in a formulation that was not approved, and which might not work.
The second situation is where a penetrant of the correct manufacturer, correct type and method, but of a different sensitivity is added to the tank. Now one has a tank filled with a penetrant of unknown sensitivity. It is possible that the resulting penetrant could have a sensitivity somewhere between the two levels, but even if it does, the tank no longer has an approved penetrant in it. These situations can be avoided by being careful to order the correct product, and being sure that the product is added to the correct tank. Of course, one of the reasons for performing the periodic ASTM tests on the penetrant inspection line is to detect problems of this type.