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Paddle Wheel Flow Sensors and Turbo Flows: Open-Loop Water Velocity Frequency Monitoring

Quick Answer for Engineers and Buyers

Quick Answer: Paddle wheel flow sensors and turbo flow meters monitor open-loop water velocity by sending a frequency signal proportional to flow. For DN15 to DN200 pipes, Silver Instruments recommends a paddle wheel sensor with Hall effect pulse output or a turbo flow meter with 4-20 mA and pulse output. Send pipe size, water quality, temperature and flow range to get a direct quote with K factor data.

Open-loop water includes cooling water return lines, irrigation, desalination feed water, wastewater lift stations and surface water transfer. Most engineers skip the open-loop part and assume closed-loop calibration. In practice, open-loop velocity profiles are less stable. Frequency output sensors work well here. They show velocity changes in real time and connect to PLC, display or recorder without complex signal conversion.

How Paddle Wheel Sensors and Turbo Flow Meters Measure Open-Loop Water Velocity

A paddle wheel flow sensor uses a small rotor inserted into the pipe. Water pushes the rotor blades. Each blade pass generates a pulse. A turbo flow meter uses a multi-blade turbine rotor and a pickup coil or Hall effect sensor. The output is a square wave frequency. Higher water velocity means higher frequency.

Silver Instruments supplies insertion style paddle wheel sensors for DN15 to DN80 and inline turbo flow meters for DN25 to DN200. We also supply larger insertion units for DN250 to DN600 pipes with a hot tap fitting. In open-loop water, select a sensor with a low minimum velocity threshold. Our standard paddle wheel unit starts at 0.3 m/s. The turbo flow unit starts at 0.35 m/s. Max velocity is 10 m/s for both designs.

Because the signal is frequency, long cable runs do not reduce accuracy. Many customers in Southeast Asia use cable runs of 50 to 120 meters from the sensor to a control cabinet. A 4-20 mA signal can lose accuracy on long runs if loop resistance is wrong. Frequency output does not have that problem. This is one reason water treatment plants in Vietnam and Thailand prefer frequency output paddle wheel sensors for raw water and backwash lines.

Output Frequency, K Factor and Signal Wiring

Each sensor has a K factor. The K factor is the number of pulses per liter or per cubic meter. A typical Silver Instruments paddle wheel for a DN50 pipe has a K factor around 50 pulses per liter. A DN100 turbo flow meter may have a K factor of 4.3 pulses per liter. The actual value depends on pipe ID, material and rotor design. We print the K factor on the body and include it in the calibration sheet.

The standard output is an open collector NPN pulse. You can wire it directly to a PLC high speed counter or a Silver Instruments batch controller. For analog integration, we can add a frequency to 4-20 mA converter. Some models include built in 4-20 mA with HART. Basic paddle wheel units do not include HART. HART is available on our turbo flow meter model SI-TF2200.

If you need totalized flow plus velocity, select a unit with pulse output and a separate display. The display shows flow rate in m3/h and total in m3. A customer in Manila ordered 12 units for cooling water makeup lines. They wanted pulse output to an existing PLC and a local display for operators. We supplied DN80 turbo flow meters with IP65 aluminum housings and 24 VDC power. Total per skid was about 280 m3/h.

Pipe Sizes, Materials and Installation Rules

Choose the sensor body based on water chemistry and pressure. For clean water, PP or PVDF body with ceramic bearings works. For cooling water with slight chlorine, PVDF is safer. For seawater or brine, use 316L or PVDF with titanium pins. Silver Instruments supplied paddle wheel sensors for a desalination plant in Saudi Arabia with 316L