Once every day, at a minimum though this could be done more often, a test vector is presented to the PEMS. This represents the process during some normal load condition that matches data we have taken from RATA testing. We know what the emissions should be, so we can evaluate the resulting prediction against the target (RM) value. They should be very close and not change day to day.
This tests the entire prediction cycle and the database such that any change in either would result in a change in the prediction for the test vector (analogous to a calibration drift). In actuality, we do not make such changes between RATAs. Thus, the response from the PEMS to the test vector should not deviate at all. Only when we make adjustments to the model just prior to RATA, will the value change.
This is noted in the QA manual and after the RATA the daily validation is compared to the new target. This allows us to treat this daily validation just like a CEMS calibration. We also present a zero vector (offline) to get zero pollutants and ambient O2 values also. This completes a zero/span calibration record and allows the data acquisition system to process and handle the calibration much like a CEMS daily calibration.