Abstract: A static fluid system for detecting a selected change in an ambient condition. The system includes an elongate closed fluid line having a selected absolute pressure and pressure means to maintain the line pressure. Sensing means in the line respond to selected changes in ambient conditions to open the line and change the line to ambient pressure. Signal means responds to the change in line pressure, and flow restricting means in the line maintains the line pressure under normal ambient conditions and precludes re-establishment of the line pressure when the sensing means is activated to open the fluid line.