engleski

Use of the laser interferometry in geotechnical laboratory - why and how?

The reasons and solutions for installation of laser interferometer in geotechnical laboratory were investigated. Soil and rock deformability properties are frequently measured at very small level of displacements up to 0, 5 mm or 0, 5 % strains. Consequently, respecting widely accepted and some recently issued standards, maximum permissible errors of measuring equipment (LVDT or other devices with known deviations and uncertainties) take order of +/-0, 002 mm and even smaller. It affects both applicability and calibration of transducers in use. Usually transducers are internally calibrated with mechanical 1D calibration tester, which is externally calibrated by interferometry. In the case of small permissible errors, mentioned traceability may not be satisfactory for decision about (non)conformance with specification if recognized rules for assessment are applied (ISO 14253-1). Moreover, the expanded uncertainty of the instrument used to check compliance should be better that 1/10th of the tolerance zone (U=0, 0004 mm for a 0, 004 mm tolerance). Starting from standpoint that conventional 'in house' calibration can not achieve these demands, reaching one level up in traceability chain is investigated and realized using laser interferometry and its inherent high accuracy and precision based on known wavelength of laser light. After external calibration of wavelength, laser displacement interferometer system is successfully installed, prepared for use and tested. Measuring characteristics, components, set up and uncertainty are resolved and a reliable instrument for the most demanding displacement measuring applications is obtained.

Geotechnical laboratory testing; Displacement transducers; Internal calibration; Laser interferometry

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