Detecting changes in the condition of process instruments

The authors describe a method for detecting and isolating changes in a flow measuring system using a wideband and unconditioned signal taken direct from the flow sensing element. The new concept is based on the hypothesis that the wideband band unconditioned signal taken directly from a sensor represents the response of a linear time-invariant system to both the deterministic and the random components of the excitation from the associated process. It is shown that the output from a process instrument can comprise two components; (i) the measurand and (ii) the condition signals. The emphasis is placed on an approach which can be used to derive the parameters of a condition signal. The logical expressions which describe whether or not the measurement, the condition of the instrument, or the operational status of the plant has changed, can be readily mapped into a digital format representing the condition signal. The concepts have been applied to a differential pressure measurement system