12 - Measurement reliability and safety systems

Abstract

This chapter discusses the reliability of measurement systems that can be quantified as the mean time between faults occurring in the system. The reliability of a measurement system is defined as the ability of the system to perform its required function within specified working conditions for a stated period of time. Measurement systems usually comprise a number of components that are connected together in a series, and hence it is necessary to know how the reliabilities of individual components are aggregated into a reliability figure for the whole system. A measurement system consisting of several components in a series fails when any one of the separate components develops a fault. One way of improving the reliability of a measurement system is to connect two or more instruments in parallel. The most common reason for faults occurring in a measurement system, whereby the error in the measurement goes outside acceptable limits, is the drift in the performance of the instrument away from its specified characteristics. Redundancy means the use of two or more measuring instruments or measurement system components in parallel such that any one can provide the required measurement. As computer processors, and the software within them, are increasingly found in most measurement systems, the issue of the reliability of such components has become very important. Measurement system reliability is usually inexorably linked with safety issues, because measuring instruments used to detect the onset of dangerous situations that may potentially compromise safety are a necessary part of all safety systems implemented.