In this chapter, the following terms have the meanings indicated. Terms Defined. “Calibration drift (CD)”means the difference in the COM output reading from a reference value after a period of operation during which no unscheduled maintenance, repair, or adjustment took place. The reference value may be the simulated zero (or low-level) check or the upscale calibration value. “Calibration error” means the difference between the opacity values indicated by the COM and the known values of a series of calibration attenuators (neutral density filters). “Simulated zero check” means the use of a method or device to provide a simulated zero-opacity (or low-level value between zero and 20 percent of span value) and that provides a system check of the analyzer internal optical surfaces and all electronic circuitry including the lamp and photodetector assembly. “Span value” means the opacity value at which the COM is set to produce the maximum data display output. “Upscale calibration value” means the opacity value at which a calibration check of the COM is performed by simulating an upscale opacity condition using a neutral density filter or other related technique to produce a known obscuration of the light beam as viewed by the receiver.