Electromagnetic flowmeters as a source of diagnostic information

Abstract

Recent studies involving the analysis of process measurement signals have shown that the ‘noise’ components carry potentially useful diagnostic information not only regarding the operating conditions at the point of measurement but also regarding the status of the measurement system. This is particularly true if the primary sensor itself has a wide frequency response and the unconditioned signal is analysed. At first sight, it appears that electromagnetic flowmeters are particularly suitable in this respect because they are essentially non-invasive, their method of sensing does not influence the flow regime and they have no moving parts. However, because of the spurious signals which are developed at the electrodes, the measurement signal has to be obtained by modulating the magnetic field and synchronously demodulating the electrode signal, but this eliminates virtually all the ‘noise’ components of the original signal. For this study, this difficulty was circumvented by holding the excitation of the electromagnet constant for a brief period while the electrode signal was sampled. The ‘noise’ component in the sampled data was then separated from the other components by a high pass filter and analysed to provide diagnostic information regarding both the flow regime as well as the operational status of the associated process equipment. Unfortunately, this mode of operation precludes the ability of the flowmeter to measure the actual flow rate and, in any practical implementation of the concept, it would be necessary to arrange the signal processing system so that it operates in the conventional mode for the majority of the time, but is switched for a brief period to the alternative mode during which the ‘noise’ component of the electrode signal is gathered and then analysed to provide the diagnostic information.