DFRobot Gravity series is a set of professional open-source hardware modules. It consists of 300 kinds of standardized modules, including expansion boards, sensors, actuators, functional modules, and kits as well as each module comes with clear documentation and demo. The Gravity series has been used by more than 1 million developers worldwide and used in a broad range of applications, like AI, environmental monitoring, IoT, smart homes, etc. With simple gravity interfaces and practical sample code, you can build your oxygen concentration monitor easily and conveniently.
Source: https://www.helpnetsecurity.com/2021/03/22/dfrobot-gravity/