To install a clamp-on ultrasonic flow meter: (1) Select a straight pipe section with ≥10D upstream and ≥5D downstream clear of any elbows, valves, or pumps. (2) Clean and degrease the pipe surface. (3) Apply ultrasonic couplant gel to the transducer faces. (4) Mount the transducers using the manufacturer's spacing guide for your pipe diameter. (5) Enter pipe parameters (OD, wall thickness, material) into the meter. (6) Verify signal strength ≥60% before commissioning.
What Is a Clamp-On Ultrasonic Flow Meter?
A clamp-on ultrasonic flow meter measures liquid flow rate by clamping two transducers to the outside of an existing pipe — no pipe cutting, no process shutdown, no contamination risk. The transit-time method measures the travel time difference between ultrasonic pulses sent upstream and downstream. This time difference is proportional to the flow velocity of the liquid inside the pipe.
Because clamp-on meters make no physical contact with the fluid, they are ideal for retrofitting to existing piping, measuring aggressive chemicals or high-purity liquids, and permanent monitoring where inline meters cannot be installed without a shutdown. SEGMENsensor clamp-on meters cover pipes from DN15 to DN5000 depending on the model, with ±0.5% accuracy on the U11 and ±1% on the U10 series and U20.
Before You Start: Site Survey Checklist
- Pipe is full: Transit-time meters require 100% full pipe at the measurement point.
- Liquid is clean: Transit-time works on clean liquids. Heavily aerated or slurry flows require Doppler technology instead.
- Pipe material is acoustically transparent: Steel, stainless steel, copper, PVC, PE/HDPE, CPVC, fiberglass — all work. Concrete lining, thick rubber lining (>6 mm), and severely corroded pipes reduce signal strength.
- Straight run is available: Find a section with ≥10D upstream and ≥5D downstream clear of elbows, valves, reducers, and pumps.
- Pipe dimensions confirmed: Measure outer diameter (OD) and wall thickness with calipers — do not rely on nominal pipe specifications which may differ from actual.
- Temperature within range: Standard clamp-on sensors are rated −40 to +80 °C; high-temperature sensors reach +130 °C (H1A) or +180 °C (H2A) on U10, U10S PRO and U11.
Step-by-Step Installation Procedure
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Select and mark the measurement location
Choose a straight pipe run with ≥10× pipe diameter (10D) upstream and ≥5D downstream of the transducer position, clear of all flow disturbances. On a 100 mm (DN100) pipe, this means 1,000 mm upstream and 500 mm downstream. Mark the transducer center-line with a paint marker or tape. Avoid the bottom of the pipe (sediment accumulation) and the top (air pockets) — mount on the side (3 o'clock or 9 o'clock position) for horizontal pipes, or anywhere on vertical pipes with upward flow.
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Prepare the pipe surface
Clean the pipe surface at both transducer positions. Remove paint, scale, rust, grease, and moisture over a 100 × 100 mm area at each transducer location. Use sandpaper (60–80 grit) to remove flaking rust or thick paint. Wipe clean with isopropyl alcohol or acetone and allow to dry completely. A clean, smooth surface is essential for reliable ultrasonic coupling — this step is the single most common cause of poor signal quality in the field.
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Determine transducer spacing (V-mode or Z-mode)
Enter the pipe outer diameter, wall thickness, pipe material, and fluid type into the flow meter display. The meter will calculate and display the required transducer spacing in millimetres — this is the exact center-to-center distance along the pipe axis between the two transducers. For most pipes DN15–DN300, V-mode is used (both transducers on the same side of the pipe, signal bounces off the opposite wall). For larger pipes (DN300+) or heavily scaled pipes, switch to Z-mode (transducers on opposite sides of the pipe, signal passes directly across). Mark the transducer positions precisely with calipers and marker.
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Apply coupling gel and mount the transducers
Apply a 2–3 mm bead of ultrasonic couplant gel (water-soluble silicone-based) to the face of each transducer. Press each transducer firmly against the pipe at the marked positions, squeezing out air bubbles. Secure with the supplied stainless steel strap clamps, tightening evenly on both sides. The transducers must be parallel to each other (not twisted) and at the exact calculated spacing — even a few millimetres of error in spacing will reduce accuracy. For permanent installations, use RTV silicone adhesive couplant instead of gel.
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Connect the signal cable and power
Connect the coaxial signal cables from each transducer to the labeled ports on the flow meter transmitter (upstream and downstream must match the physical installation orientation). Connect the power supply for your model: 10–36 V DC or optional 90–245 V AC (U10 series), 90–245 V AC or 10–36 V DC (U11), 10–24 V DC (U12) or 20–28 V DC (U13). Keep signal cables away from high-voltage power cables and variable frequency drives (VFDs) to avoid electromagnetic interference. On the U10 series and U11 the standard transducer cable is 9 m, with custom lengths up to 274 m; order the length the site needs rather than extending the cable in the field.
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Verify signal strength and commission
Power on the flow meter and navigate to the signal quality / diagnostic screen. Check the signal strength or S/N ratio indicator. A reading of ≥60% signal strength is required for stable measurement — most meters display a bar graph or percentage. If signal strength is below 60%: (a) re-clean the pipe surface and re-apply couplant gel; (b) verify transducer spacing; (c) try Z-mode if V-mode signal is weak. Once signal strength is acceptable, verify that the displayed flow velocity is physically reasonable for your system and run a zero-flow check (close isolation valves and confirm zero reading before re-opening).
Common Installation Mistakes and How to Avoid Them
| Mistake | Symptom | Fix |
|---|---|---|
| Insufficient straight run | Unstable reading, ±5–10% error | Relocate to longer straight section; use multi-path meter |
| Air in pipe at measurement point | No signal or intermittent signal | Move to a point where pipe is always full; bleed air from system |
| Incorrect pipe OD entered | Systematic offset error (e.g., always 5% high) | Re-measure OD with calipers; re-enter accurate dimension |
| Dirty or painted pipe surface | Low signal strength (<40%) | Sand and clean the surface; re-apply fresh couplant gel |
| Transducers not parallel or twisted | Weak signal, erratic readings | Re-mount transducers with ruler to ensure exact alignment |
| VFD interference on cable | Noisy reading, spikes in trend | Route signal cables away from VFD; use shielded cable; add ferrite cores |
| Bottom-of-pipe mounting on horizontal pipe | Low signal due to sediment layer | Relocate to 3 o'clock or 9 o'clock position on horizontal pipe |
Compatible Pipe Materials and Fluids
Transit-time clamp-on meters work on clean, single-phase liquids that fill the pipe, within the velocity range of the selected model. The table below summarises compatibility for the most common industrial piping systems:
| Pipe Material | Compatible | Notes |
|---|---|---|
| Carbon steel | ✅ Yes | Standard installation; clean surface required |
| Stainless steel (304/316) | ✅ Yes | Excellent signal propagation |
| Copper | ✅ Yes | Common in HVAC and CDU cooling loops |
| PVC / CPVC | ✅ Yes | Good; use Z-mode for larger diameters |
| PE / HDPE | ✅ Yes | Some signal loss; 10D+ straight run recommended |
| Cast iron | ⚠️ Caution | Works if no thick graphite layer; verify signal strength |
| Fiberglass (GRP/FRP) | ⚠️ Caution | Works on most; signal loss depends on lay-up thickness |
| Rubber-lined pipe | ❌ No (lining >6 mm) | Signal blocked by thick liner; use inline meter |
| Concrete-lined pipe | ❌ No | Signal attenuated by concrete; use Doppler or inline |
- Transit-time (this guide) requires clean, single-phase liquid with no significant gas bubbles, solids, or suspended particles.
- If your fluid is a slurry, heavily aerated, or contains >2% solids by volume, use a Doppler ultrasonic meter instead — the reflective particles are required for Doppler signal return.
- SEGMENsensor U series are transit-time meters designed for clean water, coolants, oils, chemicals, and similar clean liquids.
V, Z and W Mounting Methods: Which One and How Far Apart
Step 3 above tells you to read the transducer spacing from the meter. The mounting method you choose decides how that spacing is laid out on the pipe and how strong the received signal will be. SEGMENsensor U series clamp-on meters support all three methods below; the transmitter recalculates the spacing L automatically when you change the method in the setup menu.
| Method | Transducer positions | Beam passes | Typical pipe size | When SEGMENsensor recommends it |
|---|---|---|---|---|
| V-mode | Both on the same side of the pipe | 2 (one reflection off the far wall) | DN50–DN300 | Default for most installations. Easiest to align because both transducers sit on one straight line on the same pipe surface. |
| Z-mode | Opposite sides of the pipe, offset by L along the axis | 1 (direct crossing) | DN300 and above; also lined, cast-iron or scaled pipes of any size | Strongest received signal because the beam crosses the wall only twice. Use it whenever V-mode signal strength stays below 60% after cleaning, and on large pipes where the V path becomes too long. |
| W-mode | Both on the same side of the pipe | 4 (three reflections) | DN15–DN50 | The longer path increases the transit-time difference on small pipes, improving resolution at low flow. Only for clean, thin-walled pipes; if the signal is weak, fall back to V-mode. |
Laying out the spacing in practice
- Read L from the meter after entering outer diameter, wall thickness, pipe material, liner (if any) and fluid. The value is in millimetres and changes when any of those inputs change.
- V-mode and W-mode: mark a straight line parallel to the pipe axis with a flexible ruler or a length of angle bar, then mark the two transducer centres L apart on that line.
- Z-mode: mark the first centre, then wrap a paper strip or strap around the pipe, fold it in half to find the exact opposite point (half the circumference), and move that opposite point L along the axis for the second centre. A 2–3 mm error in the opposite point shows up as low signal strength.
- Re-measure after strapping. Clamps can slide 1–2 mm while being tightened; confirm L with calipers or a steel rule before commissioning.
For Z-mode on large circulating water mains, hot condensate lines and the other measuring points in a generating plant, see ultrasonic flow meters in power plants.
Straight-Run Requirements by Upstream Disturbance
The 10D upstream / 5D downstream figure on the SEGMENsensor clamp-on ultrasonic flow meter product page is the minimum for a single elbow with a fully developed flow profile. Fittings that create swirl or a distorted velocity profile need more distance. The table gives the straight-run lengths SEGMENsensor application engineers use when reviewing a customer's pipe layout. D is the pipe inner diameter; measure from the outlet flange of the disturbance to the first transducer centre.
| Upstream disturbance | Upstream straight run | Downstream straight run | Notes |
|---|---|---|---|
| Single 90° elbow or tee (same plane) | 10D | 5D | Product-page minimum; typical for HVAC and CDU coolant loops. |
| Two elbows in different planes (out-of-plane, creates swirl) | 20D | 5D | Swirl decays slowly; if 20D is not available, use Z-mode and mount as far downstream as possible. |
| Reducer or expander | 15D | 5D | Measure on the constant-diameter section, never across the taper. |
| Fully open gate, ball or butterfly valve | 15D | 5D | A butterfly disc that is not fully open behaves like a partially open valve (next row). |
| Partially open or modulating control valve | 30D | 5D | Prefer a location upstream of the valve (5D upstream of the valve inlet) if the pipe stays full there. |
| Pump discharge | 30D | 5D | Measure from the pump discharge flange. Pulsation and entrained air near the pump also reduce signal quality. |
| Vertical pipe, upward flow | Same as above | Same as above | Preferred where available: the pipe stays full and sediment does not collect on one wall. |
Worked example. A DN150 chilled-water line leaving a pump: 30 × 150 mm = 4,500 mm of straight pipe upstream of the transducers and 5 × 150 mm = 750 mm downstream. If only 2,000 mm is available after the pump, look for a location after the next elbow instead (10D = 1,500 mm upstream, 750 mm downstream) or state the layout in your inquiry so the quotation can note the expected accuracy.
Which SEGMENsensor Clamp-On Flow Meter Fits Each Installation?
Choose the model by pipe size first, then by whether the meter stays on the pipe or is moved from site to site. All SEGMENsensor U series meters are transit-time clamp-on meters for full pipes and clean, single-phase liquids, so the procedure above applies to every row. Pipe sizes, accuracy, outputs, power supply, sensor codes and cable lengths are taken from the clamp-on ultrasonic flow meter page, the U20 portable flow meter page and the model datasheets; the exact option set for one meter is fixed in the ordering code.
| Installation case in this guide | SEGMENsensor model | Pipe size | Accuracy and velocity | Output and power | Sensors and cable |
|---|---|---|---|---|---|
| Permanent meter on a small pipe in a tight space (OEM skids, branch lines); clamp tightened with a hex key | U13 compact one-piece | DN15–DN40 | <3% FS (<1.5% FS after calibration); 0.15–10 m/s | 4–20 mA and RS485 Modbus RTU, optional OCT pulse or relay; 20–28 V DC; M12 connector | Liquid −10–65 °C (−10–105 °C option); PP glass-fibre, acid- and alkali-resistant housing |
| Permanent one-piece meter clamped on DN15–DN65 pipe, wired to a PLC or BMS | U12 one-piece | DN15–DN65 (two sizes: DN15–DN40, DN50–DN65) | ±2% of reading (±1% after calibration); 0.03–10 m/s | Any two of 4–20 mA, RS485 Modbus RTU, OCT pulse, 1 relay; 10–24 V DC | Liquid 0–65 °C (0–115 °C option); housing IP65; E version with PT1000 pair for mini heat / cooling metering |
| Fixed wall-mount transmitter with separate transducers in V-, Z- or W-mode (HVAC, water, process lines) | U10, U10S PRO | U10 25–1200 mm; U10S PRO 15–1200 mm | ±1%, repeatability 0.2%; ±0.03 to ±5 m/s | Choose two of 4–20 mA (max. load 750 Ω), pulse, relay, RS232 Modbus, RS485 Modbus; optional HART; 10–36 V DC, optional 90–245 V AC | Transmitter IP65, sensors IP68; sensor cable 9 m standard, custom up to 274 m |
| Large mains in Z-mode, or wherever the tightest accuracy is specified | U11 high-accuracy | 15–5000 mm | ±0.5%, repeatability 0.1%; ±0.01 to ±12 m/s; tolerates intermittent bubbles or impurities up to 5 s | Choose two of 4–20 mA, OCT pulse, relay, RS232 Modbus, RS485 Modbus; optional HART; 90–245 V AC or 10–36 V DC | Sensors IP68; sensor cable 9 m standard, custom up to 274 m |
| Hot pipes: steam condensate, hot water, heated process liquid | U10 series or U11 with H1A or H2A sensors; U20 with P2H sensors | As the selected transmitter | As the selected transmitter | As the selected transmitter | H1A −40 to +130 °C, H2A −40 to +180 °C (U10 series, U11); P2H −40 to +130 °C (U20); DT double mounting rail for A2, H1A and H2A |
| Site survey before a permanent installation, commissioning checks and temporary measurements | U20 handheld | 25–1200 mm (1"–48"); DN15–DN40 with F1 / F1H sensors | ±1%; ±0.01 to ±12 m/s | Choose two of 4–20 mA, RS485 Modbus, OCT pulse, relay; 3000 mAh rechargeable battery, about 16 h per charge; SD card up to 32 GB | P2 −40 to +80 °C, P2H −40 to +130 °C; cable 5 m standard, custom up to 30 m |
Limits of the range. No U series model measures partially filled pipes, slurries or heavily aerated liquids, and the datasheets list no explosion-proof flow meter version. Every model is also available as a heat / cooling (BTU) energy meter with PT1000 temperature sensors. Samples are 1–5 pcs in 5–7 working days; production lead time and MOQ are confirmed with the quotation.
SEGMENsensor Clamp-On Flow Meter Specifications and Datasheet
The figures below are taken from the SEGMENsensor ultrasonic flow meter product page. Model-specific values (transducer type, cable length, enclosure) are listed in the datasheet.
| Parameter | SEGMENsensor clamp-on ultrasonic flow meters |
|---|---|
| Measuring principle | Transit-time, non-invasive clamp-on transducers |
| Pipe size | U13 DN15–DN40 · U12 DN15–DN65 · U10S PRO 15–1200 mm · U10 25–1200 mm · U20 25–1200 mm (1"–48") · U11 15–5000 mm |
| Accuracy | U11 ±0.5% · U10, U10S PRO, U20 ±1% · U12 ±2% (±1% after calibration) · U13 <3% FS (<1.5% FS after calibration) |
| Repeatability | 0.1% (U11) · 0.2% (U10 series) |
| Velocity range | U11, U20 ±0.01 to ±12 m/s · U10 series ±0.03 to ±5 m/s · U12 0.03–10 m/s · U13 0.15–10 m/s |
| Outputs | U10 / U10S PRO / U11: choose two of 4–20 mA, pulse (OCT), relay, RS232 Modbus, RS485 Modbus; optional HART · U12 / U20: choose two of 4–20 mA, RS485 Modbus, OCT pulse, relay · U13: 4–20 mA and RS485 Modbus, optional pulse or relay |
| Minimum straight run | 10D upstream / 5D downstream (single elbow) |
| Sensor temperature | U10 series and U11: −40 to +80 °C standard; +130 °C (H1A) and +180 °C (H2A) high-temperature; DN15–DN40 small-pipe sensors 0–65 °C or 0–115 °C · U20: −40 to +80 °C (P2) or +130 °C (P2H) |
| Protection | U10 series transmitter IP65, all clamp-on sensors IP68; U12 IP65 (per IEC 60529 — Degrees of protection provided by enclosures (IP Code)) |
| Pipe materials | Carbon steel, stainless steel, copper, cast iron, PVC, PE and most plastics |
| Certification | CE, UKCA |
| Commercial terms | OEM volume supply · samples 1–5 pcs in 5–7 days · production lead time confirmed with the quotation · reply within 24 hours |
Download the one-page datasheet (PDF)
Pipe size ranges, accuracy, transducer types and temperature ratings, output options, power supply, enclosure ratings and the ordering code for the SEGMENsensor clamp-on ultrasonic flow meter range. No registration required.
Commissioning Safety, Calibration and References
Isolating the line for the zero-flow check. Clamp-on installation itself needs no shutdown, but the zero-flow check in step 6 means closing isolation valves on a live process line. Follow the site's energy-control procedure for that step; in the United States the applicable regulation is OSHA 29 CFR 1910.147 — The control of hazardous energy (lockout/tagout).
Third-party calibration. If your project specification requires a flow calibration by an accredited laboratory, plan it with that laboratory. National metrology institutes such as the NIST Fluid Metrology Group maintain the primary liquid-flow standards that accredited calibration laboratories reference.
Choosing the right technology first. If the site survey shows aerated liquid or slurry, read transit-time vs Doppler ultrasonic flow meters before ordering. For model selection within the SEGMENsensor range, see the U10 to U20 clamp-on flow meter model comparison; for data-centre coolant loops, see the CDU liquid-cooling sensor guide. For chilled-water and cooling-water pipes reported to a BMS, see data center chilled water flow monitoring with clamp-on meters.
Send your pipe survey for a quotation
Pipe OD and wall thickness, material and liner, liquid and temperature, available straight run, output signal and quantity. OEM volume supply · samples 1–5 pcs in 5–7 days · written quotation within 24 hours. Download the clamp-on flow meter datasheet (PDF) first if you need the ordering code.
Frequently Asked Questions
Clamp-on transit-time ultrasonic flow meters work on most pipe materials that transmit ultrasound: carbon steel, stainless steel, cast iron, copper, PVC, CPVC, PE/HDPE, and fiberglass. They do not work reliably on concrete-lined pipes, heavily corroded pipes, or pipes with rubber lining thicker than 6 mm. The pipe wall must be solid and uniform for the ultrasonic signal to pass cleanly.
Standard installation requires at least 10× pipe diameter (10D) of straight run upstream and 5D downstream of the measurement point, clear of elbows, tees, valves, reducers, or pumps. For single-beam meters near control valves, 15D upstream is recommended. Always measure from the nearest flow disturbance, not from the beginning of the straight section.
Standard couplant is a water-soluble silicone-based gel. For permanent installations, RTV silicone adhesive creates a permanent acoustic bond. For high-temperature applications (pipe surface above 60°C), use a high-temperature couplant rated for the pipe surface temperature. Never use water-based gel on pipes above 60°C — it dries out and loses coupling effectiveness.
No. Transit-time clamp-on meters require the pipe to be completely full of liquid at the measurement point. If the pipe is partially filled or contains air pockets, the signal will be attenuated or lost entirely. For partially filled pipes, use an open-channel flow meter or Doppler ultrasonic meter instead.
V-mode mounts both transducers on the same side of the pipe, with the signal bouncing off the opposite pipe wall. This is standard for most pipe sizes DN15–DN300. Z-mode mounts transducers on opposite sides of the pipe, with the signal passing directly across. Use Z-mode for large pipes (DN300+) or when V-mode signal strength is below 60%.
Check: (1) Signal strength ≥60% on the meter display. (2) Measured velocity is reasonable for your pipe size and pump. (3) Zero-flow check — close isolation valves and confirm zero reading. (4) Cross-check with bucket-and-stopwatch test or a known-accurate reference meter if available.
It depends on the model. SEGMENsensor U11 and U20 meters measure ±0.01 to ±12 m/s, the U10 series ±0.03 to ±5 m/s, U12 0.03–10 m/s and U13 0.15–10 m/s. At very low velocities the transit-time difference becomes very small, so choose a model whose range puts your normal flow well inside it.
Clamp-on meters are factory calibrated. Field calibration is not required if pipe OD, wall thickness, and material are entered correctly. Errors in pipe dimension inputs directly cause systematic flow reading errors. Annual signal quality checks are recommended; full recalibration is only needed if the pipe is modified or replaced.
Yes, with precautions. Remove insulation at the measurement section. SEGMENsensor standard clamp-on sensors for U10, U10S PRO and U11 are rated −40 to +80 °C; the H1A sensor reaches +130 °C and the H2A sensor +180 °C. U20 sensors reach +80 °C (P2) or +130 °C (P2H). Use high-temperature couplant for surfaces above 60°C. Never mount transducers through insulation material — acoustic coupling requires direct contact with the bare pipe surface.
Use W-mode only on small, clean, thin-walled pipes, typically DN15–DN50, where the four-pass path lengthens the transit time and improves resolution at low flow. On larger pipes, scaled pipes or pipes with a liner the extra reflections lose too much signal; use V-mode (DN50–DN300) or Z-mode (DN300 and above, or whenever V-mode signal strength stays below 60%).
Mark the first transducer centre, wrap a paper strip or strap around the pipe at that point and fold it in half to find the exact opposite point (half the circumference). Move that opposite point along the pipe axis by the spacing L shown on the meter and mark the second centre. Re-check L with a steel rule after the clamps are tightened; an error of a few millimetres shows up as low signal strength.
It depends on pipe size and on whether the meter stays on the pipe. Use the U13 (DN15–DN40) or U12 (DN15–DN65) one-piece meters on small pipes, the U10 (25–1200 mm) or U10S PRO (15–1200 mm) wall-mount transmitters at ±1%, the U11 on pipes up to 5000 mm at ±0.5%, and the U20 handheld for surveys. For a quotation, send pipe OD, wall thickness, material, liquid, temperature, output and quantity.