A paddle wheel flow meter measures liquid flow based on the rotation speed of an impeller (paddle wheel) placed inside the flowing fluid.
1. Basic Concept

• The meter contains a multi-bladed paddle wheel (impeller) mounted on a shaft.
• When liquid flows through the pipe, it hits the blades and causes the wheel to rotate.
• The speed of rotation is directly proportional to the flow velocity of the fluid.
  In simple terms: Higher flow → faster rotation → higher output signal
  2. Step-by-Step Working
  Step 1: Fluid Enters the Pipe
• Liquid flows through the pipeline where the paddle wheel is installed.
• The meter is usually installed perpendicular to the flow direction (insertion type).
  Step 2: Force on Paddle Blades
• Flowing fluid exerts kinetic energy on the paddle blades.
• This force causes the impeller to start rotating.
  Step 3: Rotation of Paddle Wheel
• The paddle wheel rotates freely on a low-friction shaft or bearing.
• The rotational speed depends on:
  o Flow velocity
  o Fluid density
  o Blade design
  Step 4: Signal Generation (Sensing Mechanism)
  There are two common sensing methods:
  (A) Magnetic Pickup (Most Common)
• Small magnets are embedded in the paddle blades.
• As the wheel rotates, each magnet passes near a Hall-effect sensor.
• The sensor generates electrical pulses.
  Each rotation = fixed number of pulses
  (B) Optical Sensor (Less Common)
• Uses a light beam and photodetector
• Rotating blades interrupt the light → generates pulses
  Step 5: Frequency Output
• The generated pulses are converted into frequency (Hz).
  Relationship:
• Frequency ∝ Flow Velocity
  Step 6: Flow Rate Calculation
  The flow meter electronics use a calibration factor called K-Factor:
•  K-Factor = pulses per unit volume (e.g., pulses/liter)
  Flow calculation:
• Flow Rate = (Pulse Frequency) ÷ K-Factor
  This converts rotational speed into volumetric flow rate (LPM, m³/hr, etc.)
  Step 7: Output Signal
  The transmitter converts flow into standard industrial outputs:
• 4–20 mA (analog output)
• Pulse output
• Digital communication (Modbus, etc.)
  3. Important Physical Principle
  The operation is based on:
  Fluid Velocity Principle
• The angular velocity of the paddle is proportional to the linear velocity of the fluid
  So: Q = A × V
  Where:
•  Q = Flow rate
• A = Pipe cross-sectional area
• V = Fluid velocity