Clamp-on ultrasonic flow meter: measuring flow without process interruption

→ Ultrasonic flow measurement

In many industrial plants, the flow rate of a liquid needs to be measured without cutting into the pipe, stopping the process or installing a sensor directly in the medium. This is exactly where clamp-on ultrasonic flow meters are used.

The sensors are mounted externally on the pipe and measure the flow through the pipe wall without contact with the medium. This makes clamp-on systems particularly suitable for retrofits, temporary measurements, service work, comparative measurements, energy analyses and applications where intervention in the pipeline is undesirable or only possible with considerable effort.

This article explains how clamp-on flow measurement works, what advantages non-invasive measurement offers, which pipe and medium data must be known in advance and which typical errors during installation, sensor positioning and coupling should be avoided.

You can find an overview of suitable devices in our category
Ultrasonic flow meters.
For stationary clamp-on measurements, the
SITRANS FS220 clamp-on ultrasonic flow meter
is particularly relevant. For temporary measurements, check metering and test measurements, the
SITRANS FS290 ultrasonic flow measurement system
can be a suitable solution.

What is a clamp-on ultrasonic flow meter?

A clamp-on ultrasonic flow meter is a flow measuring device whose sensors are mounted externally on the pipe. The pipeline does not have to be opened for this. The sensor does not come into contact with the medium and does not cause any additional pressure loss in the process.

This measuring method is particularly interesting when existing pipelines are to be retrofitted, processes are to be checked temporarily or flow values are to be determined without mechanical intervention. Depending on the device, the measurement can be permanently installed as a stationary solution or used as a mobile system for service and test purposes.

Feature	Clamp-on ultrasound	Practical meaning
Installation	Sensors are attached externally to the pipe.	No pipe cutting and no process connection required.
Medium contact	No direct contact with the medium.	Good for corrosive, hygienic or difficult-to-access media.
Pressure loss	No additional installation resistance.	The process is not hydraulically influenced.
Retrofitting	Possible on existing pipelines.	Ideal for plants where flow data is required at a later stage.
Type of use	Stationary or mobile possible.	Suitable for continuous monitoring, service, check metering and temporary measurements.

Clamp-on flow meters are frequently used for water, wastewater, process liquids, cooling circuits, heating systems, energy analyses, chemical processes and temporary comparative measurements.

Why measure flow without cutting into the pipe?

Cutting into a pipeline involves considerable effort in many plants. The plant may have to be shut down, drained, cleaned, flushed or resealed. Additional costs arise for pipework, welding, leak testing, approvals and recommissioning.

In existing plants, it is often also unclear whether there is enough space for an inline measuring device at all. Especially with large pipe sizes, aggressive media, high operating pressures or older plants, non-invasive measurement can be significantly easier.

Problem with inline retrofitting	Effect	Advantage of clamp-on
Pipeline must be cut.	Downtime, installation effort and possible leakage risks.	Sensors are mounted externally.
Process must be interrupted.	Production downtime or restricted operation.	Measurement can often be retrofitted without process interruption.
Medium is critical or aggressive.	Material selection and sealing become complex.	No direct medium contact of the sensor.
Pipe size is large.	Inline devices and installation become expensive.	Clamp-on can be economically interesting for large pipelines.
Measurement is only required temporarily.	Permanent installation is not worthwhile.	Mobile clamp-on devices enable temporary measurements.

Clamp-on is therefore particularly useful when flow data is required without structurally modifying the plant.

Operating principle: How does ultrasound measure externally through the pipe?

In clamp-on ultrasonic measurement, ultrasonic sensors send and receive signals through the pipe wall and the medium. For clean or slightly loaded liquids, the transit-time method is often used. This compares the transit time of an ultrasonic signal with and against the flow direction.

When the medium flows, the transit time of the ultrasonic signals changes. From this transit time difference, the measuring device calculates the flow velocity. Together with the internal pipe diameter, the volume flow can then be determined.

Term	Meaning	Why important?
Transit-time method	Compares ultrasonic transit times with and against the flow.	Suitable for many clean or slightly loaded liquids.
Sound path	Path of the ultrasound through pipe wall and medium.	Sensor distance and sensor arrangement must be calculated correctly.
Internal pipe diameter	Derived from outside diameter, wall thickness and liner.	Basis for calculating the volume flow.
Sound velocity	Propagation velocity of ultrasound in the medium.	Influences signal quality and calculation.
Signal quality	Stability and strength of the received signal.	Decisive for reliable measured values.

For the measurement to work, pipe data, medium, sensor position and acoustic coupling must match. A good measuring device can only compensate for incorrect pipe parameters or poor sensor installation to a limited extent.

Advantages of non-invasive flow measurement

The greatest advantage of clamp-on measurement is that no intervention in the pipeline is required. This makes the measuring method particularly suitable for existing plants, temporary measurements and applications where contact with the medium should be avoided.

• No pipe cutting: The pipeline remains closed.
• No process interruption: Retrofitting is often possible without shutdown.
• No pressure loss: No component is installed in the flow.
• No medium contact: Sensors do not come into contact with the medium.
• Lower installation effort: Particularly interesting for large pipelines.
• Flexible application: Can be used stationary, mobile or temporarily.
• Low maintenance: No moving parts in the medium.

Application goal	Why clamp-on is suitable	Typical example
Retrofitting	Existing pipeline does not have to be modified.	Flow monitoring in an existing cooling water line.
Temporary measurement	Device can be installed only for a test measurement.	Checking pump performance or assessing plant condition.
Service / troubleshooting	Measuring device can be used at different measuring points.	Comparison of different pipe sections or consumers.
Critical medium	No contact with medium or process pressure.	Chemical liquid or hygienically sensitive process.
Large pipelines	Inline conversion can be very expensive.	Water, wastewater or process lines with large nominal sizes.

Limits of clamp-on measurement

Clamp-on ultrasound is very versatile, but it is not automatically suitable for every application. The measurement depends heavily on pipe data, pipe condition, medium, signal quality and flow profile. If these factors are not checked, incorrect or unstable measured values can occur.

Limit / influence	Possible effect	Practical solution
Incorrect pipe data	Volume flow is calculated incorrectly.	Verify outside diameter, wall thickness and liner.
Deposits or corrosion	Signal and effective internal pipe diameter are influenced.	Check pipe condition and deliberately choose measuring point.
Air bubbles or solids	Transit-time signal can be disturbed.	Assess medium and, if necessary, check another measuring principle.
Poor coupling	Signal becomes weak or unstable.	Clean surface, use coupling medium and fasten sensors correctly.
Unfavorable flow profile	Measured value is not representative.	Observe inlet runs and avoid disturbances.
Multilayer or lined pipes	Sound path can be difficult to determine.	Clarify pipe material, liner and wall structure in advance.

Clamp-on is therefore not a pure “clamp on and done” measurement. For good results, the measuring point, parameters and installation must be carefully prepared.

Which pipe data is required?

Accurate pipe data is decisive for reliable clamp-on measurement. The measuring device calculates the sensor distance and volume flow based on this information. Small errors in wall thickness, liner or pipe diameter can lead to significantly deviating measured values.

Pipe data	Why important?	Typical error
Outside diameter	Basis for sensor position and pipe geometry.	Nominal size is confused with actual outside diameter.
Wall thickness	Determines the internal pipe diameter and sound path.	Wall thickness is estimated or taken from old documents.
Pipe material	Influences sound velocity and damping.	Material is not reliably identified.
Liner	Changes internal diameter and acoustic properties.	Cement, rubber or plastic liner is not considered.
Medium	Sound velocity and signal quality depend on the medium.	Medium data, temperature or concentration are unclear.
Temperature	Influences medium and sensor/coupling material.	High-temperature application is planned with standard sensors.

Especially in existing plants, pipe data should not simply be taken from drawings. Pipelines may deviate, may have been modified or may have internal deposits.

Which media are suitable?

Clamp-on ultrasonic flow meters are frequently used for liquids. Clean or only slightly loaded liquids are particularly suitable, provided that a stable ultrasonic signal can be transmitted through the medium.

For media with many air bubbles, a high solids content, very high viscosity or strongly changing properties, the application must be checked more carefully. In such cases, signal quality may be limited or a different ultrasonic measuring principle may be more suitable.

Medium / application	Suitability	Note
Water / cooling water	Very often suitable	Typical application for HVAC, industry and energy.
Wastewater	Depends on solids and air bubbles	Check medium condition and measuring principle.
Process liquids	Often suitable	Consider pipe material, temperature and chemical properties.
Oils	Often possible	Check viscosity, temperature and signal quality.
Chemical media	Advantage due to no medium contact	Observe pipe material and acoustic properties.
Media with air bubbles	Can be critical	Check signal stability; choose alternative solution if necessary.

Sensor position, inlet run and outlet run

The sensor position has a major influence on measurement quality. An ultrasonic flow meter should preferably be installed at a location where the flow profile is stable and representative. Directly downstream of pumps, bends, valves, tees or restrictions, turbulence can occur and influence the measured value.

In practice, the measuring point is therefore selected with sufficient inlet and outlet run wherever possible. If this is not possible, another measuring method, another sensor position or a multi-path system may be useful.

Influencing point	Possible problem	Practical recommendation
Pump	Pulsation, turbulence or asymmetrical flow profile.	Select measuring point at a distance from the pump.
Pipe bend	Flow is distorted and accelerated on one side.	Use a straight pipe section wherever possible.
Valve / damper	Restriction and strong vortex formation possible.	Do not install sensor directly downstream of fittings.
Tee	Mixing and uneven flow.	Select measuring point after sufficient calming section.
Partially filled pipe	Ultrasonic signal and volume flow calculation become unreliable.	Ensure fully filled pipeline.
Air accumulation at top of pipe	Signal can be disturbed.	Check sensor position and pipe orientation.

The mounting position around the pipe circumference is also important. Depending on pipe orientation and medium, lateral installation can be more favorable than mounting directly at the top or bottom of the pipe.

Sensor coupling: Why gel, surface and contact pressure are important

With clamp-on systems, the ultrasound must be transmitted from the sensor into the pipe wall and further into the medium. Good acoustic coupling is required for this. Dirty surfaces, rust, paint layers, unevenness, missing coupling medium or insufficient contact pressure can significantly degrade the signal.

Installation point	Why important?	Typical error
Clean pipe surface	Dirt, paint or rust dampens the signal.	Sensor is mounted directly on a dirty surface.
Use coupling medium	Avoids air gap between sensor and pipe.	Too little or unsuitable gel is used.
Maintain sensor distance	The sound path must match the pipe geometry.	Sensors are installed by eye.
Secure contact pressure	Sensor must rest stably on the pipe.	Belt, holder or rail is too loose.
Observe temperature	Coupling medium and sensor must match the temperature.	Standard gel is used at too high a temperature.

Good signal quality therefore does not begin in the transmitter, but already during the mechanical preparation of the measuring point.

Stationary or mobile: Which version fits?

Clamp-on ultrasonic flow meters are available as stationary and mobile versions. The correct choice depends on whether the flow is to be monitored permanently or only checked temporarily.

Version	Typical application	Advantage
Stationary clamp-on flow meter	Permanent flow monitoring in process, water, HVAC or energy.	Fixed integration into plant control, control system or energy management.
Mobile clamp-on flow meter	Service, check metering, test measurement, pump control, troubleshooting.	One device can be used successively at several measuring points.
Temporary measurement	Analysis over hours, days or weeks.	Record flow data without permanent installation.
Comparative measurement	Checking existing flow meters.	Plausibility check without intervention in the pipeline.

For permanent measuring tasks, a stationary system such as the
SITRANS FS220
can be suitable. For mobile and temporary measurements, devices such as the
SITRANS FS290
or the
Portaflow C
are particularly interesting.

Typical errors with clamp-on flow meters

Many problems with clamp-on measurements are not caused by the device itself, but by incorrect input data, unfavorable measuring points or installation errors. Incorrect pipe data, poor coupling, inlet runs that are too short or partially filled pipelines are particularly common.

Fault pattern	Possible cause	Practical solution
Measured value is significantly too high or too low	Incorrect internal pipe diameter, incorrect wall thickness or liner not considered.	Check pipe data and correct parameters.
Signal is unstable	Poor coupling, air bubbles or unfavorable sensor position.	Clean surface, renew coupling medium, check measuring point.
Device finds no signal	Wrong sensor type, wrong sensor distance or strongly damping pipe material.	Check sensor selection, distance and pipe data.
Measured value fluctuates strongly	Turbulence, pump, valve, partial filling or air accumulation.	Relocate measuring point or assess flow profile.
Zero point is implausible	Residual flow, incorrectly set zero point or unstable conditions.	Check zero point only when standstill is ensured.
Measurement does not match plant meter	Different measuring principles, installation locations or pipe parameters.	Document comparative measurement cleanly and observe boundary conditions.

Which ultrasonic flow meters are suitable?

The suitable device selection depends on whether a stationary measurement, a mobile test measurement, a particularly simple retrofit, a large pipe size, high accuracy or data transmission to PLC, control system or energy management is required.

Product	Especially relevant for	Note
SITRANS FS220 clamp-on ultrasonic flow meter	Stationary clamp-on flow measurement of liquid processes	Suitable when permanently installed, non-invasive flow monitoring with simple operation is required.
SITRANS FS290 ultrasonic flow measurement system	Temporary measurements, check metering and test measurements	Interesting for service, plant testing, comparative measurements and flexible measuring tasks.
Portaflow C	Portable flow measurement from outside on pipelines	Useful when a compact mobile device is required for service and temporary measurements.
Time Delta C – ultrasonic flow meter	Stationary ultrasonic measurement with non-invasive setup	Suitable for applications where low-maintenance measurement without moving parts and with flexible communication options is required.
WIKA Type FLC-CS4 clamp-on ultrasonic flow meter	Non-invasive process liquid measurement with two independent streams	Relevant when two streams need to be measured, data logged or a convenient user interface is desired.

You can find a complete overview in the category
Ultrasonic flow meters.

Practical examples from industry, water, HVAC and service

Example 1: Retrofitting on a cooling water line

In an existing plant, the cooling water flow is to be monitored. An inline device would require plant shutdown and pipework. With a clamp-on ultrasonic flow meter, the sensors can be mounted externally on the pipe. This allows the flow to be monitored without cutting into the line.

Example 2: Checking pump performance

A pump apparently no longer delivers the expected performance. With a mobile ultrasonic flow meter, the actual volume flow can be checked at different pipe sections. This makes it easier to assess whether the problem is caused by the pump, a restriction, air in the system or a consumer.

Example 3: Comparative measurement against an existing meter

An existing flow meter displays implausible values. A temporary clamp-on measurement can serve as an independent comparative measurement. However, pipe data, measuring point, flow profile and operating conditions must be documented cleanly so that the comparison is meaningful.

Example 4: Energy analysis in heating or cooling systems

For an energy analysis, the flow in a heating or cooling circuit is required. A clamp-on measurement can help determine volume flows without intervention in the pipeline. In combination with temperature measurements, energy flows can be assessed more effectively.

Example 5: Temporary measurement in a production plant

In a production plant, the actual flow through a process consumer is to be checked for a few days. A mobile device such as the
SITRANS FS290
or the
Portaflow C
can be used for this without permanently intervening in the plant.

Checklist for selection and commissioning

This checklist helps to better prepare and assess a clamp-on flow measurement.

Check question	Why important?	Practical recommendation
Is non-invasive measurement useful?	Clamp-on is particularly worthwhile when the pipeline should not be opened.	Assess process interruption, medium contact and pipework effort.
Which pipe data is known?	Pipe diameter, wall thickness and material influence measurement and sensor distance.	Check and measure pipe data; do not rely only on old plans.
Is the pipe fully filled?	Partial filling leads to unreliable measured values.	Select measuring point so that a fully filled line is ensured.
Is the medium suitable?	Air bubbles, solids or heavy loading can influence signal quality.	Check medium, temperature, viscosity and gas content.
Is there sufficient inlet and outlet run?	Disturbances influence the flow profile.	Select measuring point with flow as calm as possible.
Is the pipe surface suitable?	Rust, paint, dirt or unevenness worsen coupling.	Clean surface and use coupling medium correctly.
Is stationary or mobile better?	Continuous monitoring and temporary testing require different devices.	Compare measuring task, duration of use and documentation requirements.
Which outputs or interfaces are required?	Integration into PLC, control system or data logger must be planned.	Check 4 … 20 mA, pulse, relay, Modbus or other interfaces.
Has the measurement been checked for plausibility?	Clamp-on values should be checked in the plant context.	Use pump data, meter values, energy balance or comparative measurement.

Conclusion: Clamp-on measures flow without intervention in the pipeline

Clamp-on ultrasonic flow meters are a very practical solution when flow data is required without cutting into the pipeline or interrupting the process. They are particularly suitable for retrofits, temporary measurements, service work, comparative measurements and applications with critical media or large pipelines.

For reliable results, however, accurate pipe data, a suitable measuring point, a stable flow profile, good acoustic coupling and a suitable medium are decisive. Many measurement errors are caused by incorrect parameters, unsuitable sensor positions or poor installation conditions.

Depending on the application, stationary systems such as the
SITRANS FS220,
mobile systems such as the
SITRANS FS290
or portable devices such as the
Portaflow C
may be suitable. You can find a complete preselection in our category
Ultrasonic flow meters.

FAQ: Frequently asked questions about clamp-on ultrasonic flow meters

What is a clamp-on ultrasonic flow meter?

A clamp-on ultrasonic flow meter measures flow using sensors that are mounted externally on the pipe. The pipeline does not have to be opened and the sensor does not come into contact with the medium.

Can flow be measured without cutting into the pipe?

Yes, this is possible with a clamp-on ultrasonic flow meter. The sensors are attached externally to the pipe and measure the flow through the pipe wall. This means measurement can often be retrofitted without process interruption.

For which media is clamp-on ultrasound suitable?

Clamp-on ultrasound is particularly suitable for many liquids such as water, cooling water, process liquids, oils or chemical media. For heavily loaded media, air bubbles or very difficult media, the application must be checked more closely.

Which pipe data do I need for a clamp-on flow meter?

Important data includes outside diameter, wall thickness, pipe material, possible liner, medium, temperature and, if applicable, pipe condition. This data influences sensor distance, sound path and volume flow calculation.

How accurate is a clamp-on ultrasonic flow meter?

Accuracy depends on the device, pipe data, installation, signal quality, medium and flow profile. Good results require a correctly prepared measuring point and clean parameterization.

Why are inlet runs important?

Inlet runs help achieve a stable flow profile. Directly downstream of pumps, bends, valves or tees, the flow can be strongly turbulent. This can influence the measured value.

Why does a clamp-on sensor need coupling gel?

The coupling gel improves the acoustic connection between sensor and pipe. Without suitable coupling, air gaps occur, which can strongly dampen or interrupt the ultrasonic signal.

When is a mobile ultrasonic flow meter useful?

A mobile device is useful for service, troubleshooting, pump testing, comparative measurements, temporary measurements or check metering. It can be used successively at different measuring points.

When is a stationary clamp-on flow meter useful?

A stationary system is useful when flow is to be monitored continuously and connected to a PLC, control system, energy management system or process monitoring system.

Which clamp-on ultrasonic flow meters are suitable?

Depending on the application, suitable devices include, for example, the
SITRANS FS220
for stationary clamp-on measurements, the
SITRANS FS290
for temporary measurements and check metering, the
Portaflow C
for portable measuring tasks or the
WIKA Type FLC-CS4
for non-invasive process liquid measurement. You can find an overview in the category
Ultrasonic flow meters.