Flow Switches

Flow Switches

McDonnell & Miller Flow Switches are equipped with either one or two SPDT switches except for Model FS7-4A (Pneumatic).


They can make or break an electrical circuit when flow starts or when flow stops, and can be used to:

  • Actuate a signal when flow stops
    • Start a motor with flow, Shut off an alarm when flow is adequate Stop a motor with no flow
  • Size of pipe
    • McDonnell & Miller Flow Switches may be used on pipe sizes 1/2" - 36" NPT.
  • How much flow is present?
    • The flow rate at which the flow switch is to respond should be determined next. McDonnell & Miller Flow Switches are actuated (make or break) with an increase in flow. The term “Flow” represents the actual movement (velocity) of liquid within a pipe sufficient to actuate the switch. The term “No-Flow” represents a decrease in velocity, or total flow stoppage, which will permit the switch to return to its original position. IMPORTANT: In operation, the switch must be actuated by “Flow” before it can be reversed again by “No-Flow”. All McDonnell & Miller Flow Switches can easily be adjusted in the field to require a higher actuating “Flow” or “No-Flow”.
  • Maximum liquid pressure in pipe
    • The maximum pipeline pressure should be considered when selecting a particular model. Different flow switch models can accommodate a range of pipeline pressures up to 1000 psi (70kg/cm2).
  • Maximum temperature
    • Determine the liquid and ambient atmospheric temperature when selecting the flow switch model. Various McDonnell & Miller Flow Switches can be used at ambient temperatures from 32°F (0°C) and liquid temperatures up to 300°F (149°C). If ambient temperatures are lower than 32°F (0°C) use the FS7-4W.
  • Type of liquid
    • McDonnell & Miller Flow Switch models have wetted parts of brass, monel or stainless steel. Depending on the particular model they may be used with water, certain light viscous fluids, some oils, various caustic solutions and other fluids.
  • Atmosphere surrounding flow switch
    • It should be determined if the location will be subject to high humidity, weather conditions or explosive atmospheres. Standard, water tight and hazardous duty flow switch models are available. 

If you have any questions please give us a call at 1-800-781-7405 or email us your request. Sales@NationalPumpSupply.com