When is a measuring instrument defined as non-critical?

First, when it can be demonstrated that the results do not affect the final conformity of the product. By determining the components of the uncertainty of the measuring process, then by quantifying the contribution of each of these components. It is then possible to quantify the contribution that the measuring instrument makes to the value of the final uncertainty and to decide and justify whether the instrument is critical. All instruments must be considered as being critical if the opposite cannot be demonstrated!