Measuring Tube Construction Affects the Long-Term Stability of Magnetic Flowmeters

Overview:

Electromagnetic flowmeters, also known as mag meters, are popular and proven devices for flow measurement of electrically conductive process fluids and for volumetric filling machine applications. Of prime importance to a mag meter’s accuracy and long term performance is the condition of the metering section of the flow sensor.

Unlike in most processes, mag meters in filling machine applications are frequently subject to widely varying conditions during normal operation. As a result, they are viable candidates for evaluating their long term performance in an accelerated use environment. Therefore PTB, a German research and approvals agency, in association with KROHNE, undertook an extensive project to study the long term measurement stability of mag meters in filling machine applications.

Faraday's law is the basis of a mag meter’s measuring principle. The design generally features an electrical isolating liner on the inner wall of the mag meter measuring tube. Linings such as PTFE, PFA or polypropylene or for hygienic reasons, PFA (perfluoroalkoxy) are used. Pressure bearing ceramic pipes are also used. PFA is known to absorb moisture, it can flow under pressure and temperature which means that it changes structure and shape which, in turn, affects the interior diameter of the measuring tube. Changes in the inner diameter of the measuring tube lead to measurement errors. This can lead to problems, especially when extreme precision or repeatability are at stake. This only takes effect after the devices have been in use for longer periods of time and through the corresponding frequent cleaning processes using liquid or steam as are common in the food industry.

The effect is particularly significant when it comes to mag meters used on filling machines for filling PET bottles ("Filling mag meter"). In this case, an extremely high degree of repeatability is required and the quality of the filling process is directly visible in each individual bottle.

That is why, in a joint research cooperative with the Physikalisch-Technischen Bundesanstalt (National Metrology Institute) (PTB), KROHNE Messtechnik tested the measurement stability of filling mag meters. Filling mag meters with PFA liners and filling mag meters with ceramic measuring tubes were both tested. The PTB was interested in this test because for more than 20 years Magmeters with ceramic measuring tubes have been the norm in the normal PTB measuring systems as well as in many other calibration test stations. Thanks to this test, the PTB was able to gain additional knowledge about the behavior of these devices under difficult conditions.

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Author: Dipl.-Ing. F. Hofmann and Dipl.-Ing. B. Schumacher, KROHNE Messtechnik GmbH Co KG Duisburg, Germany  | File Type: PDF

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