A sensor unit detects an open vacuum interface valve coupled to a gravity sump that is pneumatically coupled to an air relief tube of a vacuum sewage system. The sensor unit includes an open valve sensor, a communication device, a processor and a power source. The open valve sensor is affixed to the air relief tube. The processor is in electronic communication with the open valve sensor and the communication device. The processor is configured to read a sensed value from the open valve sensor and cause the communication device to transmit an alarm signal when the sensed value meets predefined criteria. The power source provides power to the open valve sensor and to the communication device.