The characteristics of the material to be tested must be taken into consideration when choosing appropriate measurement locations. Consider two hypothetical Rockwell hardness machines that agree perfectly in their measurement performance. Will the two machines then measure the same hardness value for a sample of material. The initial response would be yes; however, if the hardness of the material sample varies significantly from location to location, it would be possible to obtain significant measurement differences if the measurements were made at two different locations.
Therefore, when a high level of accuracy in measurement comparisons is important, the same measurement locations should be tested. One solution is to make all measurements of both machines in one test area; however, the hardness result may not be representative of the entire sample of material. A better solution is to choose several test locations over the entire surface of the material to be tested by both machines. Each machine should make measurements at each of these locations adjacent to the measurements of the other machine. The measurement average of each machine could then be reasonably compared and would also provide a more valid estimate of the overall average hardness of the sample material.