Misalignment, instrumental function

In the sine-squared- / method, lattice spacings are measured at five or more f angles. The governing equation is then least-squares fit to find the best value of O. Observation of the behavior of the quantity di - d d as a function of sin f allows instrument misalignment and preferred orientation to be detected. This method is slower than the two-exposure method, but is more statistically significant. 

The intensity scale of the instrument should be calibrated at regular intervals so that the analyst is aware of the possible changes in instrumental response due to misalignments, disassembly, or changes in the detector efficiency (particularly as a function of electron energy) with time. 

Biased data from measurements might result from uncalibrated instruments. For example, a biased Rockwell hardness tester with a misalignment might constantly produce lower hardness values. It is usually required to check the tester with a standard test block to ensure that the hardness test machine is functioning properly before collecting any test data. Figure 6.6 shows the ASTM hardness test standards for Rockwell hardness testers manufactured by Instron Corporation. 

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