Design and advances in OhmPi: an open hardware resistivity meter

The use of time-lapse electrical resistivity tomography (ERT) in environmental studies has increased considerably in recent years. This method serves to produce 2D or 3D images of the subsurface's electrical properties, making it possible to monitor the evolution of hydrologic or biogeochemical processes over time. However, the high cost and limited flexibility of commercial equipment has restricted its accessibility. OhmPi addresses these issues by offering a cost-effective, open hardware resistivity meter designed for laboratory and mid-scale field monitoring experiments. Since its launch in 2020, OhmPi has evolved significantly in response to user requirements. The latest version, OhmPi v2024, features a redesigned architecture with a new multiplexer and an upgraded measurement board. This improves performance and accuracy while maintaining affordability, with a price tag of under €1500 for 16 electrodes. The software has been completely redesigned to offer advanced features such as remote control via a graphical interface, and an integrated web server with MQTT protocol for network integration, ensuring flexibility and ease of use. Backward compatibility has been maintained to ensure a seamless transition for existing users. Extensive testing with ground analogue circuits and small-scale field experiments has confirmed OhmPi 's improved sensitivity, stability and robustness. These upgrades further establish OhmPi 's position as a reliable and adaptable tool for environmental ERT applications, providing an affordable alternative to commercial systems without compromising on quality.