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In the 1970s, industry frequently presented quite valid arguments that the performance of continuous monitoring systems was questionable. Two basic principles of CEM technology were soon learned:

1 There is no one “best” type of system for all applications.

2 A CEM system must be maintained if it is to operate.

During this period, aggressive CEM system vendors frequently sold their systems to anyone who could be convinced to buy their product. This resulted in misapplications of both in‐situ and extractive systems. The resulting poor performance led to unfortunate perceptions about the reliability of the technology and bankruptcy and absorption of several companies. This is a process that continues still today. From this experience, formal procedures for specifying and evaluating CEM systems have been developed and should be used by companies planning major CEM system purchases.

Errors in application have not been the only reason for poor CEM system performance. It is often assumed that after a CEM system is installed, it can generate data as routinely as a thermocouple or pressure gauge. It must be realized that routine maintenance programs are necessary for the continuing operation of extractive system plumbing and electro‐optical systems. Although this necessity is now well understood, awareness of this need did not develop in the United States until the early 1980s. A CEM specialty conference of the Air Pollution Control Association held in Denver in 1981 pointed out the need for established and effective CEM system quality assurance (QA) programs. By the time of a subsequent conference held in Baltimore in 1985, the U.S. Environmental Protection Agency proposed CEM system quality assurance requirements and many companies were reporting the success of their own QA programs in improving CEM system performance. In 1989, the U.S. EPA promulgated quality assurance procedures for CEM systems used for compliance determinations, specifying requirements for calibration and periodic audits. With this lesson learned, when new performance specifications are promulgated, quality assurance procedures specific to the pollutant are published concurrently. This can be seen in the almost concurrent publication of Appendix F quality assurance procedures for particulate monitoring systems, mercury monitoring systems, and hydrochloric acid monitoring systems with the publication of their respective performance specifications in Appendix B of Part 60.

Like an automobile, where the oil must be changed and the tires rotated, a CEM system requires routine checks and replacements. QA programs incorporating daily and weekly checks, periodic audits, and preventive maintenance procedures have been found to be the key to continuing CEM system operation. Systems with such programs today show better than 98% data availability.