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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.
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.
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.
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 bodies and Hastelloy C rotor pins. Standard brass models failed after six months in that application.
Pressure rating matters. Our standard paddle wheel sensors handle PN10. The turbo flow meter inline body handles PN16. Standard units work up to 70°C. High temperature versions go to 85°C. Above that, magnetic flow meters are often a better choice because moving parts and hot water cause bearing wear.
Installation needs straight run. The rule is 10D upstream and 5D downstream. In tight pump skids, you can use 5D upstream with a flow conditioner. Insertion paddle wheel sensors need a full bore ball valve for hot tap removal. Do not install the sensor upside down. The rotor must face the flow direction shown on the body. For horizontal pipes, side insertion at the 3 o clock or 9 o clock position prevents air bubbles from hitting the rotor.
Model SI-PW1100 is a plastic insertion paddle wheel sensor for DN15 to DN80. It outputs NPN frequency from 0 to 800 Hz, powered by 5 to 24 VDC. Use it for clean water, RO permeate and low conductivity water where magnetic flow meters struggle. Accuracy is plus minus 1 percent of full scale.
Model SI-TF2200 is an inline turbo flow meter for DN25 to DN200. Body material is 316L or brass. It comes with pulse output and optional 4-20 mA with HART. Pressure rating is PN16 standard. Use it for cooling water, irrigation and fire water test lines. Most engineers choose this model when they need total flow and velocity on the same unit.
For open channels or large pipes above DN250, Silver Instruments offers insertion turbo flow sensors with a hot tap assembly. We recently quoted 8 units for a river water intake in Indonesia. The pipe was DN400 carbon steel. We supplied a saddle fitting, 316L insertion sensor and a local flow totalizer. The customer installed the sensors during a short plant shutdown using a wet tap without draining the line.
We have seen this on customer sites many times. A water treatment plant in Ho Chi Minh City replaced faulty paddle wheel sensors from another brand because the bearing swelled in humid air. We supplied PVDF body sensors with ceramic bearings. After 14 months, all 20 units still report stable frequency.
In Australia, a vegetable farm uses open-loop water from a dam for drip irrigation. They needed a simple paddle wheel sensor to stop over watering. The pipe was DN80 HDPE. We supplied SI-PW1100 with a saddle clamp and battery powered flow totalizer. The farmer uses the totalized m3 to track water allocation.
A chemical plant in Dubai uses turbo flow meters on cooling tower makeup water. They requested ATEX Zone 2 certification for the display. We supplied an intrinsically safe version with a remote display. The sensor sits in a safe area. The display is in Zone 2. Total flow is about 150 m3/h per line. Standard 316L turbo flow meters are enough for this open-loop return line.
Question 1: What is the difference between a paddle wheel flow sensor and a turbo flow meter?
A paddle wheel sensor is low cost and insertion style. A turbo flow meter has a multi-blade rotor and is often inline. Both give frequency output. Paddle wheels suit smaller pipes and clean water. Turbo flow meters suit larger pipes and higher total flow.
Question 2: What frequency range should I expect?
Most Silver Instruments units output 0 to 800 Hz or 0 to 1000 Hz. The exact range depends on pipe size and K factor. A DN50 paddle wheel at 10 m/s may output around 700 Hz. A DN100 turbo flow meter at 5 m/s may output around 80 Hz.
Question 3: Can I use a paddle wheel sensor for seawater or brine?
Yes, but avoid brass and standard steel. Select PVDF or 316L body with titanium or Hastelloy C pins. Tell us the chloride level in mg/L and temperature. We will confirm the wetted material list.
Question 4: How do I choose between a paddle wheel and a magnetic flow meter for open-loop water?
Choose a paddle wheel or turbo flow when you need a low cost frequency output and the water is clean enough for moving parts. Choose a magnetic flow meter when the water has sludge, high solids, or the pipe is always full and you need no moving parts.
Question 5: What information do you need for a quote?
Send us pipe size in DN, water type, flow range in m3/h, operating pressure in bar, temperature in °C and output signal. Also tell us the country and port of delivery. We will send a model specific quote within 2 working days.
Contact Silver Instruments for open-loop water velocity frequency monitoring. Tel: +86-25-68650347. Whatsapp: +86-25-52155837. WeChat: +86 15365082610.
