What does the difference in the CEMS output reading from a reference valve indicate?

The difference in the Continuous Emission Monitoring System (CEMS) output reading from a reference valve indicates drift. Drift refers to the gradual deviation of the measured values from the true value over time. In the context of CEMS, this occurs when the instrumentation or sensors experience changes due to factors such as environmental conditions, aging components, or contamination, leading to inaccurate readings.

When a reference valve is used, it serves as a standard for comparison. If the readings from the CEMS are not aligning with the reference valve measurements, it suggests a drift in the system's accuracy. Monitoring drift is crucial as it can affect the reliability of emissions reporting and compliance with regulatory requirements. Regular calibration and maintenance are necessary to ensure that the system remains accurate and that any drift is promptly corrected.