Paddle Trimming Calculator

Enter process and sensor parameters below. Product suitability is determined by the calculated equivalent drag force (gf) and indicated by colour-coded status in the Answer panel. All results assume the switch 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 applies a development correction to the drag force, interpolating between plug flow (very short runs) and a fully-developed turbulent profile (long runs). This correction applies only to undisturbed straight pipe — it cannot account for bends, valves, reducers, tee junctions or other fittings upstream, which introduce asymmetric and swirling flow effects that are three-dimensional and installation-specific. 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.

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Job Details
Optional — for reference and record-keeping on the printed report
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Job / Project No.
Document No.
Revision
Project Name / Description
Equipment Tag
Site / Location
Client Name
Client Reference / PO
Prepared By
Date
Notes / Special Conditions
Input Values
 
Pipe ID
mm
Pipe inside diameter (25–1000 mm)
Process Conn.
Flow switch thread size BSP or NPT
Flow Rate
L/min m³/hr
Enter flow rate in either unit — the other updates automatically
Paddle Length
mm
Paddle length in millimetres (min 10mm, max 22mm)
Paddle length and width refers to the actual section of the paddle that is exposed to the flow
Paddle Width
mm
Paddle width in millimetres (min 10mm, max 19mm)
Media
Process medium (affects density & viscosity)
Water Temp
°C
Water temperature (affects density & viscosity)
Pressure
bar (g)
Operating gauge pressure (density corrected to absolute pressure)
Flow Ref
Flow rate reference — standard flows are converted to actual volume at operating T & P
Pipe Material
Pipe material (affects wall roughness & velocity profile)
Upstream run
× D
Straight pipe diameters upstream of the switch — leave blank to assume a fully-developed velocity profile
Answers
Drag Force on Paddle / Wire Sensor
Kelco Flow Switch Force (g) Force (N) Flow Status
⚠ No Kelco flow switch is suitable for the values entered — please check your inputs and try again.
Too Low — won't actuate
Correct — will actuate
Too High — excessive force
Reynolds number (flow regime)
Roughness ε
0.007
mm
Absolute pipe roughness
Air Density
—
kg/m³
Actual air density at operating temperature & pressure
Actual Flow
—
L/min
Actual volumetric flow at operating conditions
Velocity
—
m/s
Mean flow velocity (Q / A)
Friction f
—
Darcy
Darcy friction factor (Colebrook–White)
V at paddle
—
m/s
Local velocity at sensor (profile + blockage corrected)
Reynolds no.
—
Re
Reynolds number (flow regime)
Paddle in Pipe — Cross Section
KELCO L W Pipe ID 5mm
Kelco Paddles & Trailing Wires

Kelco paddles are made from either Polypropylene or Monel Metal (a corrosion-resistant stainless steel alloy). For switches with a 25mm (1") process connection, the standard Polypropylene paddle measures 29mm wide × 168mm long. For switches with a 20mm (¾") process connection, the paddle is 22mm wide × 168mm long. All Polypropylene paddles are designed to be trimmed — both in length and width — so they can be sized to suit each specific application.

Kelco's all-metal construction flow switches (the FM-D range) are fitted with a Monel Metal paddle measuring 22mm wide × 168mm long as standard. An optional extended Monel Metal paddle — 22mm wide × 300mm long — is also available for use in large-diameter pipes and open channel installations. Contact Kelco for details.

Trailing wire sensors are compatible with most Kelco paddle flow switches and can be retrofitted if needed. These 316 Stainless Steel wire sensors have a 2mm diameter and are available in standard lengths from 50mm to 300mm, in 50mm increments. If a wire needs to be cut to a non-standard length, the cut end must be TIG welded or silver soldered to prevent the wire from fraying.

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Glossary
Abbreviations, symbols and technical terms used in this calculator
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This glossary explains every abbreviation and technical term used in this calculator. If you are unsure about any input field or result, refer here first.

Actuation Range Min–Max force window for switch operation
The range of drag force (in gf) within which a Kelco flow switch will reliably actuate at maximum sensitivity. If the calculated force falls below the minimum, the paddle will not deflect the arm far enough to trip the switch. If the force exceeds the maximum, the paddle arm may be damaged or the switch body may be overstressed.
BSP / NPT British Standard Pipe / National Pipe Taper
The threaded process connection standard used on the flow switch body. BSP is the standard in Australia and most metric countries. NPT is the US/imperial equivalent. The connection size (3/4" or 1") determines which Kelco products are available.
Cd Drag Coefficient
A dimensionless coefficient relating drag force to dynamic pressure. This calculator uses Cd = 1.2 for flat-plate paddles and a Reynolds-number-dependent Cd for cylindrical trailing wire sensors.
F Drag Force on Paddle / Wire
The hydrodynamic force exerted by the flowing fluid on the sensor, expressed in gram-force (gf) and Newtons (N). This force actuates the internal switch mechanism. The calculated force is compared against each product’s actuation range.
f Darcy Friction Factor
A dimensionless coefficient quantifying pipe wall friction, calculated using the Colebrook–White equation. Higher friction (rougher walls) produces a flatter velocity profile; lower friction produces a more peaked profile.
gf Gram-force (unit of force)
A non-SI unit of force equal to the gravitational force on a mass of one gram (1 gf = 0.009807 N). Kelco switch actuation ranges are specified in gf. 100 gf ≈ 0.981 N.
ID Internal Diameter
The internal bore of the pipe or duct. All velocity and Reynolds number calculations use the cross-sectional area derived from the pipe ID, so entering the correct bore — not the nominal size — is important.
Re Reynolds Number
A dimensionless number characterising the flow regime. Re = (ρ × V̄ × D) ÷ μ. Below ~2300 the flow is laminar; above ~4000 it is turbulent. Most industrial applications are well into the turbulent regime.
ρ Fluid Density (rho)
Mass per unit volume (kg/m³). Density directly scales the drag force. Water density decreases with temperature. Air density varies strongly with both temperature and pressure.
μ Dynamic Viscosity (mu)
A measure of the fluid’s resistance to deformation (Pa·s). Viscosity affects the Reynolds number and velocity profile. Water becomes less viscous as temperature increases; air becomes slightly more viscous.
Trailing Wire Cylindrical wire flow sensing element
A 2mm diameter 316 stainless steel wire that replaces the flat paddle. Wire sensors produce less drag than paddles and are suited to higher-velocity or larger-pipe applications. Available in 50–300mm lengths.
Upstream Run Straight pipe diameters upstream of the switch
The length of unobstructed straight pipe before the flow switch, expressed in pipe diameters (D). More upstream straight pipe allows the velocity profile to develop more fully, increasing force on the sensor. Fully-developed turbulent flow typically requires 20–30 D.
V̄ Mean Flow Velocity
Average fluid velocity across the pipe cross-section, calculated as Q/A (volumetric flow rate divided by pipe area). The actual local velocity at the sensor differs from the mean due to the velocity profile shape.
Profile Correction Velocity profile adjustment factor
The ratio of the average velocity across the sensor face to the mean pipe velocity. Accounts for the non-uniform velocity profile shape (peaked in the centre, zero at the walls). Combined with the blockage factor to determine the effective velocity used in the force calculation.
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.