Median selector

One may also construct a median selector with two high selectors and two low selectors as shown in Figure 9.1. The major application has been for auditing multiple flow measurements to a chemical reactor. If either the high or low measurement deviates too far from the median value, an alarm or interlock is activated. So far we have found no applications to distillation, but it is a technique that is worth keeping in mind. 

With a median selector, the selector output is the median of three or more input signals. These devices are useful for situations in which redundant sensors are used to measure a single process variable. By selecting the median value, reliability is improved, because a single sensor failure will not cause the loss of a meaningful feedback signal. 

By using combinations of these basic building blocks it is possible to build other types of selector, such as a median value selection, shown in Figure 6.13. The selector output for a median value selector is a signal that falls between the highest and lowest input. 

An alternate scheme, shown in Figure 6.17, inaplements analysers with a medium selector that will keep the process operating regardless of the failure mode of one of the analysers. The measurement variable to the controller will always be the median transmitter output. If one of the analysers fails, then either upscale or downscale the selector will still choose the median value. 

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