Tham khảo tài liệu 'uncertainty and dimensional calibration episode 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Volume 102 Number 6 November-December 1997 Journal of Research of the National Institute of Standards and Technology of the thermistor thermometers to be C. For longer blocks a more accurate system consisting of a platinum SPRT Standard Platinum Resistance Thermometer as a reference and thermocouples is used. Coefficient of Thermal Expansion CTE The CTE of each of our blocks over 25 mm in length has been measured leaving a very small standard uncertainty estimated to be 10-6 C. Since our measurements are always within C of 20 C the uncertainty in length is taken to be X10-6 L. Thermal Gradients The long block temperature is measured every 100 mm reducing the effects of thermal gradients to a negligible level. The gradients between the thermometer and test blocks in the short block interferometer up to 100 mm are small because the entire measurement space is in a metal enclosure. The gradients between the thermometer in the center of the platen and any block are less than C. Assuming a rectangular distribution with a half-width C we obtain a standard uncertainty of C in temperature. For steel gage blocks CTE mm m C the standard uncertainty in length is X10-6 L. For other materials the uncertainty is less. Elastic Deformation We measure blocks oriented vertically as specified in the ANSI ASME Gage Block Standard 19 . For customers who need the length of long blocks in the horizontal orientation a correction factor is used. This correction for self loading is proportional to the square of the length and is very small compared to other effects. For 500 mm blocks the correction is only about 25 nm and the uncertainty depends on the uncertainty in the elastic modulus of the gage block material. Nearly all long blocks are made of steel and the variations in elastic modulus for gage block steels is only a few percent. The standard uncertainty in the correction is estimated to be less than 2 nm a .
