Calibration Considerations for Six Sigma™ Accuracy and Precision in Combustion Pressure Measurement

Precise and accurate internal-combustion engine pressure measurements are typically built upon extremely low level piezoelectric sensor signals acquired in the laboratory. In turn, these minute signals must be accurately conditioned using electronic charge amplifiers and then processed for meaningful analysis. Laboratory standard operating procedures often overlook the importance of timely and rigorous calibration of the equipment, or perform them without using sufficient environmental controls. In some facilities, years have been allowed to pass between metrology-quality calibrations, introducing the potential for significant out-of-tolerance conditions and non-compliance when compared to accredited measurement StandardDetails. Environmental, aging and operator factors influencing calibration metrology and in-use accuracy are presented in this paper, along with a method and example for estimating the calibration interval needed to specify calibration frequency for charge amplifiers. Accurate and timely calibration of combustion analysis or “indicating” equipment supports the validity of using engine incylinder pressure data for engineering design decisions. Quality calibrations, for example those which are done to ISO 17025 StandardDetails, help avoid the complications and costs of development program timing delays. These delays can often be the result when additional unplanned testing becomes necessary due to out-of-tolerance measurement equipment and laboratory non-compliance to requirements.Cross Reference: