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When there is a need to monitor a process variable that extends beyond the "big four" (pressure, flow, temperature, and level), some type of process analyzer usually does the job. And this category of instrumentation has its own big four subtypes:
Composition-Detect and measure specific chemical components in the process stream;
Electrochemical-Measure specific ion concentration, most commonly hydrogen (pH);
Spectrophotometric-Use light absorbing characteristics to detect and measure specific components; and,
Physical property-Measure specific gravity, density, viscosity, etc.
Since manufacturing processes are generally designed for specific functions, the composition or characteristics of a given product should fall into relatively narrow bands. For example, while a broad spectrum analyzer that can break down any unknown substance into its component parts may exist in a lab (at least on TV crime dramas), such are not generally practical in a real-life production environment. Gas chromatograph and mass spectrometer...