Bulkhead Flow Sensor Switch. Flow sensor detects the flow of water through your laser cutter's cooling system and acts as a switch closing when a flow is detected. In conjunction with the water protection circuit on either your high voltage PSU or your DSP controller, the sensor is usually placed inline ahead of the water flow into your CO2 glass laser tube. This sensor is intended for use with silicone hose with an internal diameter of 8mm. This switch is intended for replacement of similar failed switches or for installation in new and home built systems.