## OhmPi: an open source data logger for dedicated applications of electrical resistivity imaging at the small and laboratory scale. ## The use of electrical resistivity tomography for laboratory or field experiments has been growing in recent years for environmental purposes. Although, the equipment was developed quickly, there are still a number of limitations link to the equipment. So far, the development of commercial devices has focused on the quality of measurements and their robustness in all field cases but (i) their costs and (ii) their lack of flexibility to adapt to specific applications limit their usage for environmental application. The article presents the developments of a low-cost and open hardware resistivity-meter providing the scientific community with a robust and flexible tool for small experiments. In the article, we present a basic resistivity-meter with the current injection and measurement functions associated to a multiplexer that allows to perform automatic measurements with up to 32 electrodes (for less than 500$) . The first validation results were obtained on an equivalent circuit. The results show that OhmPi offers a wide range of resistance measurements from 0.2 to 1000 ohm, for contact resistances from 100 to 5000 ohm. The second validation results obtained from a small field experiment demonstrated the good stability of the OhmPi measurements and the good correlation between the data measured with OhmPi and a commercial reference instrument.