What is the meaning of significance level? And what should it be?
This is a very general question with no simple answer.
If you are measuring volumes or weights, to see if they divert significantly from the norm (say you are filling bags of sugur and you want them to be within an acceptable level of toleance from the 1 kg standard set), you usually use a norm that is less than 2 standard deviations from the set weight. This gives a significance level of 5%.
However, if you are measuring something that can be lethal or harmful in even small quantities, you will have to set your tolerance level quite a bit stricter. And maybe adjust your measuring methods or techniques.