Selection depends on the liquid conductivity. For standard water supply and drainage, a general-purpose relay suffices. However, for high-temperature liquids, you must specify heat-resistant electrodes. If the application involves long-distance sensing (e.g., between a hilltop tank and a basement pump), choose a long-distance type with high sensitivity to overcome wire resistance. For volatile or hazardous liquids, ensure the relay is housed in an explosion-proof enclosure and uses intrinsically safe circuits.