The influence of gauge block preparation on their optical reflection properties is investigated. Results of length calibration experiments by optical interferometry performed on 9 steel gauge blocks having a wide range of surface roughness characteristics and pre-treatment are described. These data demonstrate that different conventional cleaning procedures and the use of appropriate wringing fluids have negligible effect on the reflection properties of the measurement surfaces of gauge blocks and platens.
In interferometric length measurements the position of the sample with respect to the interference phase map is very important. Even the pixel resolution of the camera array may limit the measurement uncertainty, depending on the amount of the sample’s non-parallelism. In this case a correction can be applied taking into account a sub-pixel position at the sample’s front measuring face. In this paper three different methods are introduced which can be used for the definition of a sub-pixel central coordinate. Measurement examples illustrate that a value of 0.04 pixels for the standard uncertainty associated with the sub-pixel position seems realistic.
Modification of an interferometer instrument for highest accuracy calibration of long gauge blocks is presented. The interferometer design employs a Kosters prism, and a built-in 1 metre long vacuum cell for evaluation of refractive index of air in the direct neighborhood of the gauge block. The measurement set-up also includes platinum resistance thermometers and thermocouples for accurate measurements of gauge block temperature. Principles of the measurement method, including the application of phase stepping interferometry to both the length measurement and the correction for refractive index of air, are described in detail.
Mechanical comparison calibration of gauge blocks using reference standards that are a different material from the client gauge block can be problematic. This two-part study investigates some of the dominant influences identified in the mechanical comparison calibration of ceramic gauge blocks using steel reference standards. The goal is to develop techniques for lowest uncertainty mechanical comparison calibration in the application of two dissimilar gauge block materials. Of primary interest are: correction for differences in mechanical stylus deformation, and length equivalent thermal corrections in the different materials. In our model, mechanical stylus deformation is evaluated using gauge blocks of known length, calibrated by optical interferometry. The optical phase correction applied in this initial interferometric determination of gauge block length is an important first step. In the first part of this study, optical phase correction for ceramic gauge blocks has been determined using similar techniques by three labs, all of them applying the method of stack experiments using the same gauge blocks, and similar platens. One of the platens is of the same material and made by the same manufacturer as the ceramic gauge blocks. Temperature effects dominate the mechanical comparison calibration of dissimilar materials. In the second part of this study, the importance of the approach to length corrections as a result of temperature variation is demonstrated.
The definition of gauge black length,and the lowest- uncertainty gauge block calibrations, derive their link to the definition of the meter through the technique of optical interferometry using calibrated optical wavelengths. Following interferometric calibration of gauge blocks, the technique of mechanical comparison of gauge blocks is largely used for the dissemination of the meter. A measurement scenario for the case of low-uncertainty gauge block calibration in which the working standard and client gauge blocks are made of dissimilar materials is described. The correction for stylus deformation is determined experimentally. A detailed tutorial on evaluating the uncertainties for this gauge block calibration in accordance with the ISO Guide to the expression of uncertainty in measurement is reported, including discussion of degrees of freedom and correlations.
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