What are non-invasive flowmeters? An expert view.

This type of flowmeter specifically doesn’t ‘invade’ the pipeline. By definition then, there is no obstruction to the flow which gives this type of metering a significant advantage over many others. (Sign up to the Litre Meter Newsletter FlowSight)

Clamp-on meters are typical of the non-invasive type thereby utilising the pipe itself for measurement and therefore offering no extra pressure drop. Additionally, they may be repurposed on other pipelines and some versions are ‘portable’ for just this purpose. It is for this reason that the most common flowmeter for hire is the clamp-on ultrasonic.

There are other meters that don’t reduce the diameter of the existing pipe. These include electromagnetic and inline ultrasonic. They are classified as non-intrusive rather than non-invasive.

Insertion flowmeters, such as pitot tubes and insertion thermal, do ‘invade’ the pipe so wouldn’t qualify as non-invasive but could be useful flow measurement problem solvers.  Some electromags inserted and are designed to sit just proud of the pipe wall and provide, strictly speaking, a flow velocity output, soon transformed to a volumetric flow rate with the right parameters. They have minimal pressure drop, particularly with large pipe diameters.

So, what are the downsides of the clamp-on? They won’t measure very well if the two transducers aren’t lined up. Sometimes this event is mitigated by mounting the transducers on rails. Depending on the type, they probably won’t handle a certain degree of turbidity, air entrainment or pulsating flow.  The pipe walls have to be consistent and solid, no pipe rouge build-up, for example. and the flow profile has to be reasonable; pipe wall thickness might be an issue. They have a specific low velocity cut-off.  Most pipe systems are designed with larger pipe diameters so this low velocity might be near the actual flow rate – check the specifications; accuracy might be reduced.

And what are the other benefits?  Potentially wide flow range flow measurement (particularly if the pipe can be used at high velocity), independent of pressure and with

Ultrasonic flowmeter, principle
The principles of ultrasonic flow measurement

good temperature acceptance.


For further application advice, contact the specialists.

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