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Paddle Trimming Calculator

Enter your pipe, flow and liquid. The calculator tells you how far the paddle must reach into the pipe for a Kelco flow switch to operate, or which trailing-wire lengths will.

Fill in the pipe and the flow you need to detect, then press Calculate. You get the paddle depth that works for each switch, and a check of the depth you entered. Results assume the switch's sensitivity adjustment is set to maximum.

Installation assumptions: Calculations presume the flow switch is installed vertically (top-entry) in a horizontal pipe run. If an upstream straight pipe length is entered, the calculator interpolates between plug flow (0 diameters, directly after a fitting) and a fully-developed profile (long runs). This correction cannot account for bends, valves, reducers, tee junctions or other fittings upstream. Angled or horizontal mounting will also alter flow impingement on the paddle in ways this tool cannot quantify. If no upstream length is entered, a fully-developed profile is assumed. Results carry increasing uncertainty below about 10 pipe diameters upstream.

Your pipe and flow
 
Pipe inside diameter
mm
The inside diameter (bore) of the pipe, not its nominal size (20–5000 mm)
Switch thread size
BSP or NPT
Flow to detect
L/minm³/hr
The flow at which the switch must operate: use the lowest flow you need it to detect. Enter either unit; the other follows. (max 9,047 L/min — a mean velocity of 30 m/s in this pipe)
Sensor
Paddle switches: F, FM, F27, F29, F60 and IPG20. Suit pipes 25 mm (1") and larger.
Paddle depth in pipe
mm
How far the paddle tip reaches into the pipe, measured from the inside wall where the switch enters (10 mm to 76 mm, keeping 4 mm clear of the far wall). Start with any value: the answer shows what works.
Paddle Width
mm
Paddle width in millimetres (min 5 mm, max 30 mm — 25 mm/1" connection)
Media
Process medium (affects density & viscosity)
Water Temp
°C
Affects density & viscosity (0 to 100 °C)
More detail (optional): pipe material and straight run upstream
Pipe Material
Affects wall roughness & velocity profile
Upstream run
× D
Straight pipe diameters upstream of the switch (0 = directly after a bend, valve or other fitting). Leave blank if not known — a fully-developed velocity profile is then assumed.
Paddle in Pipe — Cross Section
Paddle in Pipe — Cross Section
50 mm28.5 mm⌀ 80 mm5 mm
Disclaimer — This calculator is provided by Kelco Engineering Pty Ltd (“Kelco”) for general estimation purposes only and does not constitute engineering advice. While every reasonable effort has been made to ensure the accuracy of the calculations and data presented, Kelco makes no warranty, representation or guarantee, express or implied, as to the completeness, accuracy, reliability or suitability of the results for any particular purpose. The selection, application and installation of flow sensing equipment based on the output of this tool are solely the responsibility of the user. Kelco accepts no liability whatsoever — whether in contract, tort (including negligence), statute or otherwise — for any loss, damage, injury or expense arising directly or indirectly from the use of, or reliance upon, the information provided by this calculator. Users should independently verify all results and consult qualified engineering professionals before making design or procurement decisions. Use of this tool constitutes acceptance of these terms.