A method to calibrate steel length measuring tapes by mechanical comparison

Abstract

This paper describes a mechanical method for calibration of steel length measuring tapes with a hook or hook-ring and lengths up to 10 m. A reference and a test tape are laid horizontally by a series of horizontal supporters. In the measuring process the reference standard tape is moved with respective to the test tape by rotating a micrometer head. This movement is measured by a digital dial gauge. The tension weights are used to stretch out the tapes up to appropriate tensions. The tapes are properly clamped at the other end of the system against the pulling force of tension weights. The coincidence of scale marks is done by viewing the image of a CCTV camera which is coupled with a monitor. The temperature of the two tapes is measured to account the temperature effect. The deviation in length from the zero of the test tape is measured by considering temperature correction and sag correction. The performance of the system was checked with a pre-calibrated steel tape and results from the system are in agreement with the calibration certificate of the test tape. A method to estimate the combined uncertainty of the deviation in length calculated by the proposed method is also presented