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Uncertainty of measurement in calibration of long gauge blocks by mechanical comparison
Ačko, Bojan, 1961- ; Godina, Andrej
Long gauge blocks up to 1000 mm are calibrated in the Laboratory for Production Measurement in Maribor by using the method of mechanical comparison. This method has being applied since 1999. A ...special comparator based on a Zeiss ULM 3000 machine has been design ed and produced in the laboratory for this purpose. Pairs of gauge blocks (reference and calibrated gauge block) are fixed in a special adjustment unit and probed by Mahr probes with a resolution of 10 nm. The comparator is in a climatic chamber assuring constant temperature of (20 :1: 0,1) °C. and is additionally protected against temperature chang es due to airfiow by additional isolation box. This box also prevents the gauge blocks from being heat by the operator. The article will present the comparator design, a short description of the calibration procedure, and detailed uncertainty evaluation. The mathematical model of the measurement comprises seven input quantities: length of the standard gauge block, measured difference between the gauge blocks, difference of the thermal expansion coefficients of gauge blocks 'A' and 'B', temperature difference between gauge blocks 'A' and 'B', average of the thermal expansion coefficients of gauge blocks 'A' and 'B', deviation of the average temperature of the gauge blocks 'A' and 'B' from the reference temperature 20°C, and correction of non-central contacting of the measuring faces of the unknown gauge block. Thermal conditions influencing the uncertainty have been evaluated experimentally, whether the other components were calculated from the intervals given in calibration certificates, handbooks and producer's specifications. The best measurement capability that results from the uncertainty evaluation has been checked by many internationalinter comparisons.
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