Chapter 33. Control
The main cause of control system failure was inadequate specification.—Out of Control (U.K. Health and Safety Executive)
33.1. Instruments that cannot do what we want them to do
33.1.1. Measuring the Wrong Parameter
The pressure of a water supply was normally high enough for it to be used for firefighting. If the supply pressure fell, a low-pressure alarm sounded and an alternative supply of water was then made available. When someone isolated the water supply in error, the trapped pressure in the line prevented the alarm from operating. The instrumentation could do what it was asked to do—detect a low pressure—but not what its designers wanted it to do—that is, detect that the water supply was unavailable (see Figure 33-1 ...
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