Calsense FM 1B 1" Brass T Flow Sensor Meter. Calsense Flow Meters, model FM, are used with all Calsense irrigation controllers to monitor real-time flow of potable or non-potable water, making them important tools in detecting mainline and lateral breaks on an irrigation system. Real-time system flows are learned automatically at night when irrigation comes on, so that constant flow rates are measured and can provide the most accurate and reliable monitoring of breaks and problems as they occur.