Paddle Flow
A paddle flow switch indicates whether flow is above or below a setpoint. It is not inherently a precise flow-rate meter. Threshold, allowable fluid, pipe size and orientation depend on the instrument.
Fluid force moves a paddle against a restoring force and changes the state of a switching mechanism. Magnetic/reed implementations are one option, not a requirement for every model. Follow the instrument's mounting and electrical instructions.
In plain terms
Like a boat's anemometer flag — wind pushes the flag, and you don't need the wind speed to know it's blowing.
Why designers use it
- Indicating flow presence or loss in a suitable pipe installation.
- Providing a threshold signal to a control system.
- Offering one possible technology among paddle, variable-area and electronic flow switches.
Best for
- Pump protection
- Chillers
- Cooling-water lines
Key specifications
- Process compatibility: Fluid, pressure, temperature and wetted materials
- Installation: Pipe size, orientation and paddle geometry
- Switching: Setpoint, hysteresis and contact/output rating
- Maintenance: Inspect for deposits, sticking and mechanical wear
When not to use it
- When flow rate measurement is needed — paddle switches give only a flow/no-flow alarm; pull a turbine, magmeter, or Coriolis.
- On viscous fluids or slurries — the paddle drags or fouls; pull thermal-dispersion or ultrasonic non-contact.
Common mistakes
- Ignoring the specified mounting orientation, insertion depth or straight-pipe requirements.
- Assuming a flow indication proves adequate cooling in every operating condition.
- Choosing materials or pressure/temperature ratings unsuitable for the fluid.
- Assuming moving parts cannot stick or wear, or that a simple switch is automatically a safety-rated protective system.
Where you will find it
- A cooling-water control system can monitor a suitable flow switch as one input to its protection logic. Its response to a failed sensor still needs to be designed.
- A pipe installation may need a different flow technology if the paddle's moving parts or mounting requirements do not suit the process.
A short history
A paddle flow switch senses liquid or gas movement with a hinged paddle set in the flow stream. Flowing fluid pushes the paddle, and greater flow creates more force, moving it further until a preset rate triggers an internal switch, producing an electrical signal. This can confirm adequate flow, helping protect pumps or trigger alarms in fire sprinkler systems.
Good to know
- A switch point is not the same thing as a calibrated flow-rate measurement.
- A no-moving-part technology avoids paddle wear, but still has its own fluid and installation requirements.