In industry verifying flowmeter accuracy is an ongoing requirement to ensure that the meter used is within the stated accuracy of the manufacturer and meets with the statutory requirements for fiscal metering (billing) or proving flowmeter readings that are reliable for measuring process applications. Clamp-on meters are often used to ‘check’ if another meter is working or to verify the device. Clamp-on meters are being used more and more in industry as technology improves and understanding of the devices and their potential application spreads.
Abstract
Clamp-on ultrasonic type meters are available as temporary and permanent types for application on clean liquids containing no more than 2% entrained gas and no more than 5% suspended solids. This paper covers transit time (TT) or time of flight devices (TOF) and does not include Doppler type ultrasonic meters although some of the practices apply to Doppler types as well.
Temporary applications are mainly offered using portable type meters and permanent types are mainly designed for fixed installations. Portable types generally have a memory log or data-logging capability to record data and the types designed for permanent applications do not always have this capability. The enclosures are also quite different from each other. The portable types are supplied in a carry case with battery power, whereas the semi-permanent and permanent types are supplied with industrial type enclosures with different IP ratings and utilise mains power supply.
Disturbing trends
Some large water authorities and distributors/resellers of bulk potable water have moved away from using accurate calibrated traceable meters and resorted to the use of clamp-on meters as their primary measuring devices for billing purposes, primarily due to low initial cost of procurement and ease of installation of clamp-on TOF or TT flow versus higher accuracy devices. Using single track/channel/path meters for fiscal metering would be affected by a number of factors and the achieved uncertainty of the measurement would depend on the on-site installation on the actual existing pipe and the real liquid running through the pipe.
A further complication is that some users are not familiar with clamp-on meters and their intended applications. Be careful of persons or organisations claiming to be able to check and adjust the accuracy of electromagnetic flowmeters and other metering applications using clamp-on meters.
On large pipe applications where acquisition cost may be high for traditional meters such as electromagnetic or EMF meters, users should consider the use of multibeam ultrasonic meters of the wetted type as they would be lower in cost than electromagnetic flow meters while offering excellent accuracies.
According to the author the facts in this article should provide reassurance and reference for end users.
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