• Controls Flow with no pressure drop, no sensor fouling and no maintenance
  • Flow control from the outside of metal or plastic pipes
  • Installs in Minutes
  • Pipe Size: Any pipe ID from ½" to 180" (12.5 mm to 4.5 m)
  • 5 amp DPDT Control Relay
  • Ideal for "difficult" liquids
  • Adjustable ON/OFF set-points
  • Hi or Lo flow alarm mode
  • Adjustable time delay
  • Relay status LED
  • Flow rate LED bargraph
  • Watertight, dust tight NEMA4X (IP 67) polycarbonate enclosure with a clear shatter-proof face
  • Easy Calibration

What's in the Box

  • SE4 ultrasonic clamp-on sensor
  • 20 foot sensor shielded coaxial pair cable
  • Silicone coupling compound
  • Stainless steel pipe clamp Kit for 0.5 to 16” pipes
  • Installation and operation manual


The Greyline Instruments by Pulsar DFS 5.1 Doppler flow switch provides accurate flow control from outside of the pipe. Recommended for difficult liquids including wastewater, cooling water, slurries, sludge, chemicals, viscous liquids and abrasives, the DFS 5.1 is ideal for pump protection or to activate flow/no-flow alarms.

The DFS 5.1 ultrasonic sensor can be mounted on the outside of any metal or plastic pipes from 1/2 to 180" I.D. and installs in minutes. The sensor continuously injects high frequency sound through the pipe wall and into the moving fluid where acoustic pulses are reflected back to the sensor from particles or bubbles in the fluid. The DFS 5.1 control relay can be set to turn ON and OFF at any flow rate between 0.25 and 10 ft/sec (0.075 to 3 m/sec). The DFS 5.1 relay time delay is adjustable from 0 to 80 seconds to prevent nuisance alarms and relay "chatter" in turbulent flow. Includes switch selectable "normal" and "failsafe" operating modes.

The DFS 5.1 has no moving parts and no maintenance is required. Installation can be made while the flow system is in full operation. No contact is made with the moving fluid and no holes are drilled in the pipe. There is no fouling or scale build-up on the sensor.

The compact, water tight electronics enclosure includes adjustable controls with separate ON and OFF relay set-points and time delay. A LED bargraph for flow rate and set points plus relay status are included. Switch adjustments and electrical connections are fast and easy.


This product can be used in the following applications:

In Stock

Part Number Power Input Price Available
DFS 5.1A-1-A
Standard, 100-240VAC 50/60Hz
$1,195.00 1 in stock

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