Ultrasonic flow meters measure the velocity of a fluid by transmitting high-frequency sound waves through the pipe wall or directly into the fluid — and because nothing needs to be inserted into the flow stream, they introduce zero pressure drop, can be clamped onto existing pipes without shutdown, and handle everything from drinking water to slurries to high-pressure natural gas. They are one of the fastest-growing meter technologies in industrial Singapore because they combine the advantages of non-intrusive installation with the accuracy and repeatability needed for energy submetering, PUB water accountability, and process flow control.
How Ultrasonic Flow Meters Work
All ultrasonic flow meters exploit the fact that sound travels faster in a moving fluid when propagating in the direction of flow, and slower when propagating against it. By measuring this difference in transit time — or the frequency shift caused by the moving fluid — the meter calculates fluid velocity. Combined with the pipe's internal cross-sectional area, velocity gives volumetric flow rate.
Transit-Time (Time-of-Flight) Meters
Transit-time meters use two transducers mounted diagonally along the pipe — one upstream, one downstream. Each alternately transmits and receives ultrasonic pulses. The difference in transit time between upstream and downstream signals is proportional to the fluid velocity. This technique works best with clean, homogeneous fluids (water, oils, natural gas) and produces excellent accuracy — typically ±0.5 to ±1.0% of reading for clamp-on meters and ±0.2% or better for wetted in-line meters.
Doppler Meters
Doppler meters transmit a continuous ultrasonic beam into the fluid. Reflectors (particles, bubbles, or entrained gas) moving with the fluid reflect the beam back at a shifted frequency. The frequency shift (Doppler effect) is proportional to fluid velocity. Doppler meters therefore require particulate or bubbles to be present — they are suited to slurries, wastewater, activated sludge, and dirty process liquids. They are generally less accurate than transit-time meters (typically ±1–3%) but excel where transit-time meters would fail.
Clamp-On versus In-Line (Wetted) Ultrasonic Meters
Clamp-On Meters
Clamp-on transducers are attached to the outside of an existing pipe using coupling gel and mechanical clamps — no pipe cutting, no process shutdown, no pressure boundary penetration. This makes clamp-on meters ideal for:
- Retrofit submetering in existing buildings and plants without shutdown
- Temporary or portable flow measurement surveys
- Applications where pipe material or contents make in-line installation impractical
- Verification metering alongside existing installed meters
Clamp-on meters work on most pipe materials — carbon steel, stainless steel, ductile iron, HDPE, PVC, and concrete-lined pipes — provided the pipe wall is not lined with a material (rubber, tar, or epoxy) that attenuates the ultrasonic signal below a usable level. Minimum straight pipe runs are required upstream (typically 10D) and downstream (5D) of the transducers to ensure a stable, developed flow profile.
In-Line (Wetted) Meters
In-line meters have transducers mounted directly through the pipe wall or in spool pieces flanged into the process line. They offer higher accuracy than clamp-on designs because the acoustic path length and transducer coupling are precisely controlled during manufacture. Multi-path in-line meters — using 2, 4, or even 8 acoustic paths across the pipe — achieve the highest accuracy (better than 0.1% of reading) and are used for fiscal metering and custody transfer applications where PUB or EMA metrological requirements apply.
Applications in Singapore Industry
PUB Water Submetering and Accountability
Singapore's PUB Water Conservation Tax and mandatory water efficiency reporting requirements (under the Water Efficient Building scheme) have driven extensive installation of water submeters in commercial buildings, industrial facilities, and residential developments. Ultrasonic clamp-on meters are widely used for department-level submetering because they can be installed on existing pipes in plant rooms without interrupting water supply.
District Cooling and Chilled Water Networks
Singapore's extensive district cooling systems — operated by SP Group and building-level chiller plants — use energy metering that combines ultrasonic flow measurement with temperature difference (ΔT) to calculate cooling energy. The meter pair (flow + ΔT) must be calibrated to provide billing-grade energy measurements. Our Unitest Instruments calibration laboratory provides traceable calibration for flow meters used in energy metering applications.
Compressed Air and Gas Flow
Ultrasonic transit-time meters are used to measure compressed air and natural gas flow in industrial plants. In Singapore's EMA-regulated gas installations, meters used for gas billing must meet the accuracy and certification requirements of the Gas Act. Thermal mass flow meters and ultrasonic meters are both used; the choice depends on gas composition, pressure, and required accuracy. See our article on compressed air leak detection and energy savings for related measurement context.
Wastewater and Effluent Monitoring
PUB's trade effluent discharge licensing under the Sewerage and Drainage Act (administered by PUB) requires some licensees to meter and report discharge volumes. Doppler ultrasonic meters or open-channel flow meters are used in partially filled pipes and channels. Their non-intrusive nature and ability to handle suspended solids make them the preferred choice for wastewater applications.
Process Industries
Petroleum refining, chemical processing, and pharmaceutical manufacturing on Jurong Island and in the Science Hub use ultrasonic flow meters for both process control and inventory management. The ability to handle corrosive, exotic, or high-purity fluids without wetted materials (for clamp-on designs) is a significant advantage in these industries. View flow measurement instruments available from Unitest Instruments.
Accuracy and Performance Factors
Ultrasonic meter performance depends on several installation and process factors:
| Factor | Impact | Mitigation |
|---|---|---|
| Straight pipe run | Disturbed flow profile reduces accuracy | Provide minimum 10D upstream, 5D downstream |
| Pipe scaling or lining | Attenuates signal, may prevent operation | Use multi-path meter; descale before installation |
| Entrained air or gas | Reduces transit-time meter accuracy | Ensure no cavitation; use Doppler for high-void-fraction fluids |
| Temperature variation | Affects acoustic velocity in fluid | Correct with temperature input; verify at operating temperature |
| Fluid composition variation | Affects speed of sound; can cause error | Configure meter for correct fluid type; verify composition assumptions |
| Transducer coupling | Poor coupling with pipe reduces signal | Use appropriate coupling gel; verify signal strength after installation |
Calibration of Ultrasonic Flow Meters
Like all flow instruments, ultrasonic meters require periodic calibration to verify their accuracy against a traceable standard. For billing-grade or custody-transfer meters, calibration is typically required before commissioning and after any significant maintenance event. For process monitoring meters, annual calibration or calibration based on historical drift data is appropriate.
Calibration of in-line flow meters requires either removal to a calibration laboratory with a primary flow rig, or field calibration using a calibrated reference meter. Clamp-on meters can be verified against an installed reference meter or calibrated using pipe-loop test rigs. Unitest Instruments provides flow calibration services accredited under SAC-SINGLAS (LA-2023-0845-C). Read our guide on how often to calibrate instruments for recommended intervals by application. Contact our team for calibration enquiries.
Comparison with Other Flow Meter Technologies
| Technology | Pressure Drop | Dirty Fluid | Accuracy | Retrofit | Cost |
|---|---|---|---|---|---|
| Ultrasonic (clamp-on) | None | Doppler only | ±0.5–3% | Excellent | Medium |
| Electromagnetic | None | Yes (conductive) | ±0.2–0.5% | Poor (cut pipe) | Medium–High |
| Differential pressure (orifice) | High | Limited | ±0.5–2% | Poor | Low |
| Vortex | Low | Limited | ±0.5–1% | Poor | Medium |
| Coriolis | Medium | Yes | ±0.1–0.2% | No | High |
| Turbine | Medium | No | ±0.25–0.5% | Poor | Low–Medium |
Ultrasonic meters excel in retrofit applications and where zero pressure drop is required. For the highest accuracy without pipe cutting, a multi-path in-line ultrasonic meter is competitive with Coriolis at lower cost for larger pipe sizes. For small-pipe high-accuracy mass flow, Coriolis remains the benchmark. Contact Unitest Instruments to discuss the right flow measurement solution for your application.