McDonnell & Miller FS7-4SJ Snap Switch

The McDonnell & Miller FS7-4SJ (Part Number 120171) is a rugged, industrial-grade liquid flow switch designed for high-pressure applications where standard brass units fall short. This SPDT snap switch features a durable stainless steel body and wetted parts, allowing it to withstand a maximum pressure of 1000 PSI. Built with a 1 1/4" BSPT connection, it is engineered for precise flow sensing in large pipe systems across hydronic heating, industrial process cooling, and water treatment operations.

For maximum versatility on the job site, the unit comes with trimmable stainless steel paddles, enabling contractors to customize the response based on specific pipe diameters and flow rates. The internal magnetic switching mechanism is a key reliability feature, as it eliminates the need for bellows, reducing the risk of mechanical fatigue or leakage. Technicians can easily fine-tune system sensitivity via the external sensitivity adjusting screw to prevent nuisance tripping while ensuring the SPDT contacts reliably signal alarms, pumps, or control circuits during critical flow or no-flow conditions.

• High-Pressure Tolerance: Reinforced stainless steel construction supports demanding industrial environments up to 1000 PSI.
• Customizable Actuation: Trimmable paddles allow for precise field adjustment to match unique flow rate requirements.
• Bellows-Free Design: The magnetic switching mechanism provides a superior seal and longer service life in high-cycle applications.
• Universal Flow Control: SPDT snap action switch facilitates easy integration into both "normally open" and "normally closed" control logic.
• Extreme Temperature Range: Rated for fluid temperatures from 32°F to 300°F, making it suitable for both chilled water and steam condensate lines.

• How do I handle nuisance tripping from turbulence? Use the sensitivity adjusting screw to increase the required flow setpoint and ensure the switch is installed at least 5 pipe diameters away from elbows or valves.
• Can this be mounted in vertical piping? This specific Series FS7-4 model is designed for horizontal pipe runs; mounting it vertically may affect the internal balance and switch accuracy.
• What electrical loads can the switch handle? The snap switch is rated for 7.4 Amps at 120VAC and 3.7 Amps at 240VAC, providing a robust interface for motor starters and pilot duty signals.

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Disclaimer: While we strive for 100% accuracy, product data and specifications are subject to change. Please contact our team to confirm compatibility or technical details before purchasing.