The use of electrical resistivity tomography in laboratory or field experiments for environmental purposes has been increasing in recent years. The development of commercial devices has thus far focused on the quality of measurements and their robustness in all field cases. However, both their costs and lack of flexibility to adapt to specific applications have limited their prevalence in the environmental sector. This article presents the development of a low-cost, open hardware resistivity meter to provide the scientific community with a robust and flexible tool for small-scale experiments. Called OhmPi, this basic resistivity meter features current injection and measurement functions associated with a multiplexer that allows performing automatic measurements with up to 32 electrodes (at a cost of less than $500). The device was first tested using a soil-analog electrical circuit to verify there liability and robustness of the measurements. Results show that OhmPi offers a wide range of resistance measurements,from 0.2 to 1000O,for contact resistances between 100 and 5000O. Measurements were then carried out on a small field experiment,in demonstrating good stability ofthe OhmPi measurements,as well as a strong correlation with the output of a commercial reference instrument.