Paddlewheel Flow Sensor Guide: Sizing & Installation

A paddlewheel flow sensor is a simple, repeatable way to measure clean liquids in water treatment, filtration, cooling water and chemical production. Depending on the model, it gives a frequency, pulse or 4–20 mA signal for batch volume control, flow rate indication or a running total. Like any insertion sensor, it’s only as good as its sizing and installation, so here’s what to check before you order.

How It Works

From Spinning Rotor to Flow Rate

The rotor spins as liquid passes, producing pulses whose frequency rises with velocity: a nominal 45 Hz per m/s on the FLS F3.00. A K-factor, which depends on the pipe’s internal diameter, converts that frequency into flow rate. FLS K-factors are referenced to water, so a liquid with a different viscosity or density may need on-site recalibration.

Is It a Good Fit?

Where a Paddlewheel Flow Sensor Works Best

Check your line against these limits from the manufacturers’ datasheets:

Solid-free liquids (the FP-AS510 accepts impurities under 1%)
Velocity of 0.15–8 m/s (F3.00, F6.30) or 0.25–4 m/s (F3.10)
Reynolds number of at least 4,500 (FLS)
Pipe from DN15 to DN600 (F3.00, F6.30)

FIP/FLS tees, clamp saddles, strap-on saddles and weld-on adapters cover that pipe range from DN15 to DN600. If your line falls outside these limits, talk to us before ordering.

Getting the Install Right

Three Mounting Rules From the FLS Manual

These three rules come straight from the FLS insertion sensor instructions.

⚠️ Keep the pipe full: A part-full pipe makes the sensor overestimate flow, even when the rotor stays submerged. Mount it at a low point, with the pipe downstream rising at least 1 × ID higher.

⚠️ Mount at 45° on horizontal runs: A 45° position avoids air bubbles at the top and sediment at the bottom. Never mount at 90°, where shaft friction can affect the reading.

⚠️ Stay away from pumps: Insertion sensors need a uniform velocity across the pipe, and flow is distorted around any disturbance, so maximise the distance between the sensor and any pump.

Model Picker

Which PCSPL Model Should You Pick?

Start with the signal your system needs.

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FIP F3.00

Frequency sensor with Hall, coil or push-pull output, in a C-PVC, PVDF or 316L SS body, rated IP65 or IP68.

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FLS F3.10

Compact, low-cost IP68 sensor for DN15–DN40 on standard FIP tees, with a bearing-free 4-blade rotor.

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FIP F6.30

Blind transmitter with isolated 4–20 mA and a configurable solid-state relay, set up by PC over USB.

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FIP M9.02

Monitor showing rate, total and infinite total on a graphic display, with a 4–20 mA output, two solid-state relays and one mechanical relay.

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Lorric FP-AS510

All-in-one paddle wheel flow meter with LED and LCD displays and 4–20 mA, OCT and Modbus outputs. It also detects flow direction.

Datasheets: F3.00 · F3.10 · F6.30 · M9.02 · FP-AS510 · Fittings. Sensor or transmitter? See Flow Transmitter vs Flow Meter.

Paddlewheel Flow Meters FIP / FLS Brand Page Lorric Brand Page Flow & Batch Controllers

Talk to Us About Paddlewheel Flow Sensors

Send us your liquid, flow range, pipe details and required signal, and our team will match a sensor, fitting and readout to your line.

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Paddlewheel Flow Sensors: How They Work, Where They Fit and Which to Pick