Internal UV Ozone Photometer Page 9-23 Start out with the Derivative term set to zero. It ispossible (and simpler) to tune the servo loop using only theProportional and Integral terms. If there is a slightovershoot, however, the Derivative term can sometimes beincreased slightly to eliminate the overshoot, rather thandecreasing the Integral term, resulting in an improvedresponse time. Start out by selecting the factory default Proportional andIntegral terms (about .4 and .2, respectively). With theProportional constant remaining fixed, increase the Integralconstant if the photometer takes too long to reach thesetpoint value and decrease it if there is overshoot oroscillation. The best Proportional constant value can befound by trying different values, but for each newProportional constant, the Integral constant bust bereadjusted to compensate for undershoot or overshoot. After setting the constants using the Fake Hardware mode,resume normal mode (by entering ALT-H) and run tests with thereal Photometer and Ozone Generator. Be sure to restore theoriginal Sample Hold and Measure times. The response may besomewhat different than that while using the simulatedPhotometer, requiring additional adjustment of the PIDconstants. Test that the servo control operates properly evenwhen the photometer calibration is high or low.Routine MaintenanceThe Model 2010D's Photometer is designed to operate with minimalmaintenance, however periodic maintenance will ensure that thePhotometer continues to operate in the optimum condition. Inaddition, there are several components in the Photometer thatshould be considered expendable and that will need periodicreplacement. This section describes the common maintenanceitems, beginning with those requiring the most frequentattention.CalibrationIn order to ensure the accuracy of the 2010D's Photometer, itshould be calibrated periodically by comparing it to an ozonereference standard. This will not only ensure that thePhotometer's measurements are accurate, but will also pointout when there is a problem and maintenance is needed. Thesub-section entitled "Photometer Calibration" beginning onPage 9-17 describes how to calibrate the 2010D's Photometer.
