With external leakage monitoring, the sensor is positioned in a measuring device connected to the rotary union by a tube. If the predefined flow limit is exceeded inside the tube, a signal is emitted which appears directly on the measuring device or which can be sent to the machine control system. The external leakage monitoring can be added at a later date without replacing the rotary union and is also designed for use with rotary unions from third-party manufacturers. The sensor is designed to detect leaking coolant, hydraulic oil and water.