Clamp-On Ultrasonic Flow Meter

A clamp-on flow meter exists because of a practical problem: the pipe that most needs measuring is usually the one that cannot be cut. A large existing main, a building’s heating riser, a line feeding a process that runs continuously - each is a candidate for a flow measurement and none can be taken out of service to install one. The ultrasonic clamp-on meter answers that by putting the transducers on the outside of the pipe and measuring through the wall.

The trade for that convenience is a measurement whose quality depends on the pipe itself, which is why the pipe detail matters more here than on any other flow technology.

What the instrument consists of

Two transducers and a host. The host is the instrument: it drives the transducers, times the pulses and computes the flow, and it is the part that changes between the forms in this range. A wall-mounted host lives at one point and keeps a running total. A handheld host is carried between pipes and answers a measurement question without an installation at all. A fixed host shares the same transducers for a permanent point of record.

The transducers divide by how they reach the liquid. Clamp-on types are strapped to the outside of the pipe, and the range in the material covers small, medium and large transducers each spanning a band of bore sizes, plus a bracket-mounted type for confined spaces and small pipes. Insertion types pass through the wall into the flow, which the material describes as installable without stopping production up to a stated wall thickness and to a larger thickness to special order.

Measuring through the pipe wall

The measurement compares the time an ultrasonic pulse takes to cross the pipe with the flow and against it. The difference is small and is calculated from the path length, so the meter has to be told the pipe geometry - and that is where clamp-on measurement parts company with a wetted instrument. The signal crosses the wall twice and depends on the coupling between transducer and wall, so the material the pipe is made of, its thickness, the condition of its outer surface and the quality of the coupling are all part of the measurement rather than accessories to it.

The insertion form removes that dependency by putting the transducer into the liquid, which is why the material offers it up to a large bore where the acoustic path across a thick wall would otherwise be marginal. What it costs is an opening in the pipe.

Installing and using it

Clamp-on transducers are fixed to the outside of the pipe, so the line is not opened and no hole is drilled. Insertion transducers need a shutdown except where the wall is within the stated limit and the fitting is made under pressure. Bracket mounting is offered for small bores and confined spaces, and the material describes it for long-term fixed monitoring.

Because the instrument reads along a path rather than at a point, a straight run is needed upstream and downstream of the measuring position, and the material behind this page does not state a figure for it. That is a question for the enquiry rather than something to be inferred from another technology: the requirement follows from the transducer arrangement and the pipe, and it should be confirmed before the mounting point is fixed.

The forms and what each is for

The wall-mounted host is the one for a permanent point of record, and it is the form that takes the insertion transducer where the pipe cannot be measured from outside. The handheld host carries the same clamp-on transducers and is described for inspection and temporary measurement, which makes it the instrument bought when the duty is not yet confirmed. The fixed host is supplied with the medium transducer for the common range of pipe sizes, and a kit form is offered with a wall-mounted host and a transducer together.

Measurement limitations and selection considerations

The material behind this page is a range summary, and the gaps in it are significant enough to be listed rather than glossed over. It gives no flow velocity range, no repeatability, no output signal, no supply voltage and no communication interface. It states an accuracy of ±1 % of full scale, which is a different basis from a percentage of reading and is not comparable with one. And it describes heat and cooling measurement as part of the host while not listing the matched temperature sensors that such a measurement requires.

The maximum temperature is given in a way that leaves the answer open: a common band of -30 to 90 °C, with a higher figure of 130 °C against one host and 160 °C against another, and no statement of the conditions under which the higher figure applies. Until that is settled, the conservative band is the one to design to.

The other limitation belongs to the technology rather than to this range. Clamp-on measurement needs a pipe that will carry the signal and a pipe whose wall thickness is known, so a heavily corroded, badly coated, concrete or unknown lined pipe may not be measurable from outside at all. That is not a shortcoming of the instrument; it is the geometry refusing to be measured, and the insertion form is the answer where it applies.

Ordering

The items that fix the answer are the pipe material, outside diameter and wall thickness, and whether the pipe is lined; whether the line can be taken out of service, and for how long; the medium and its normal and maximum temperature; the minimum, normal and maximum flow; whether a flow total, a rate, or heat as well as flow is required; whether the installation is permanent or a survey; the output required and what will read it; whether a supply exists at the point of installation; and the straight run available. A pipe detail and a description of what the reading is for are enough to issue a covering quotation. Quantity, destination port and required delivery date complete the enquiry.