SPC / SQC for Process Analyzer Systems

This chapter separates statistical process control (SPC) and statistical quality control (SQC), even though many of the manufacturing management methods that incorporate statistical methods do not have firmly established definitions. The most fundamental descriptive statistics in SPC/SQC are the average, range, and standard deviation. The distinction between specifications and control limits is very important, especially since newcomers to SPC/SQC tend to confuse the two. The two are not only different, but a calculated number called “capability” provides an objective way to compare the control limits and product, or output, specifications. A nondispersive infrared process analyzer was placed on a gaseous distillation off gas. A daily sample was taken to the laboratory for analysis by gas chromatograph. The laboratory analysis provided not only the concentration of the component that was measured by the process analyzer, but also the concentrations of all the other major stream components.