Some circuits have non-metallic parts, such as non-metallic liquids, gases, vacuums, or plasma. An electrode is a type of conductor that can make contact with those parts, and is widely used for water testing, such as a PH tester. It can also test for ORP, conductivity, dissolved oxygen, ammonia and other elements or compounds. A probe can sense and relay data on temperature or conductivity. For more on water testing, see our water testing meters, water testing meter accessories, and water quality testing kits.