No matter when and where, all measurements should be made under standard conditions. If the measurements are not made under standard conditions, results must be referenced to standard conditions by correcting for any deviations. The reason is that all materials have different coefficients of expansion, it is not sufficient to simply asy that a shaft is so many inches (millimeters ) in diameter. It is extremely important for us to know at what temperature it has that value.
It is internationally considered that all dimensional measurements should be referenced to 68°F or 20°C. Furthermore,the temperature in most metrology laboratories is controlled to a nominal value of 68°F(20°C). It is important that a temperature-controlled environment can eliminates the worry of temperature drifts and ensures that the test piexes and the standards are at the same temperature. For this, it is essential to allow sufficient time for this equilibrium state to be achieved.
The wavelength of light varies with barometric pressure because of changes in the index of refraction of air. Therefore, when making measurements in terms of the wavelength of light , it is necessary to measure the barometric pressure and correct for any deviations from 760 mm of mercury, which is the international standard for barometric pressure. Before measuring , part cleanliness must be ensured. Additionally, we cann’t overlooked the compatible geometry. It is most meaningful to report the diamerter of a hole to 0.0001 ″(0.0025mm) if the hole is out of round by 0.0003″(0.008mm). Neither is it meaningful to measure the length of a gage block to 5μin (130μm) if the two faces are out of parallel hy 10μ in.(250μm) or if either face is not flat within 5μin.(130μm).
All materials deform when subjected to a load. The initail deformation is elastic, and its extent is determined by the Young’s modulus of the material, the geometry of the contact, and the magnitude of the load.