Investigation of Sources of Errors in Abbe Vertical Metroscope for Absolute Length Measurement - A Case Study of the National Institute for Standards, Egypt

Abstract

Metrologists at the National Institute for Standards-NIS (Egypt) develop length measuring devices to provide reliable and less expensive metrological systems for accurate measurement. Length Laboratory Group investigates the sources of errors in the absolute measurement of length using the improved Abbe vertical metroscope. The investigation includes analyzing of errors induced by optical alignment, environment, and instrument. Errors due to environment and vibration are reduced by controlling the area in which the technique is placed. The instability in the readings caused by temperature fluctuations is handled by averaging 1000 points/sec. To ensure ideal contact between the probe of the comparator and the surface of the artifact, holographic technique is used in testing the uniformity of the probe at the area of contact. Other factors influencing the measurements such as material expansion of the artifact and optical source used in the measurement are considered in the uncertainty evaluation. The uncertainty in the absolute measurement of gauge block length of 80mm is found to be ±0.2µm. The results of measuring some pieces using the absolute method are in good agreement with the corresponding reference values given in their calibration certificate.