As to your second question, yes.. you have a real problem there.
Since you don't batch manage the material you need to explain to us what you want to do with the testing results and what you are doing with the material.
If all the material goes into production, then do you need the results for anything else? As long as it always mixed with good material what does it mater at this point?
If you use the same tank to fill out going tankers or fill other types of containers and you resell the material, well than you have other issues. You would need to take a sample of the tank after mixing and test that. Than what happens to that depends again on what your requirements are. You need to explain more as to what you intend to do with the results.
Craig