An inductive sensor for displacement measurement during uniaxial and triaxial compression test

The paper presents the sensor for measuring strains ranging from below 0.1% to over 50%, which is considered as the one-ply coreless inductor. The inductive sensor is installed on the tested material. Changes in dimensions of the material cause changes in the length of the sensor’s coil. This brings changes in inductance, which together with the capacity, constitutes a resonant circuit of a transistor oscillator. Change of oscillation frequency is a source of information about the deformation of a sample. This sensor in linear form is used for measuring compressibility of rock samples and other materials in triaxial cell and during triaxial test, when strains exceed ten per cent. For measuring circumferential strain of cylindrical rock sample in high pressure cell is used toroidal form of sensor. The sensor operate under hydrostatic pressure up to 1 GPa high or may be applied in normal conditions (outside a cell). Moreover the paper describes influence of temperature and high pressure on the sensor properties and influence of friction on measurement result. Some results of strains measurements of different materials are presented as well.