Development of the 3D extensometer in Prague

Konstrukce extenzometru z roku 1967

Development of the original Czech 3D extensometer, which measures microdosplacements across rock failures/faults with high accuracy, has taken place since the 1960s. The first patent PV 4602-67 was registered in 1967, and the final version TM71 (or TM-71) has been produced since 1971. It has been manufactured with small modifications by Gestra CZ s.r.o. until now. To measure microdisplacement, this gauge uses optical moiré effect and the principle of the gauge construction are de facto holders of two glass sheets sets. Pictures show construction in 1967, the following TM71 version and the new magnetic TM72 version. Recently, the newest version uses neodymium magnets to keep two frames with glass sheets together. The previous version TM71 used clips. Moreover, the newer version reduced 32 screws too, which helps cut down mistakes during installation. This version has proved its higher sensitivity to microdisplacement since 2019. Development was made in data-taking too and recently 101 of them are equipped by a reliable electronic system, which is able to suffer in harsh conditions in underground spaces with high humidity as well. 

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