ASTM D664 for acid number of petroleum products
ASTM D664 for acid number of petroleum products by potentiometric titration
13. Quality Control Checks
13.1 Confirm the performance of the test procedure by analyzing a quality control (QC) sample that is, if possible, representative of the samples typically analyzed.
NOTE 18 - Because used oils, particularly used engine oils, are known to change during storage, such samples may not be suitable for this purpose.
13.2 Prior to monitoring the measurement process, the user of the method needs to determine the average value and control limits of the QC sample.
13.3 Record the QC results and analyze by control charts or other statistically equivalent technique to ascertain the statistical control status of the total testing process.4 Any out-of-control data should trigger investigation for root cause(s). The results of this investigation may, but not necessarily, result in instrument recalibration.
13.4 The frequency of QC testing is dependent on the criticality of the quality being measured, the demonstrated stability of the testing process, and customer requirements. Generally, a QC sample should be analyzed each testing day. The QC frequency should be increased if a large number of samples are routinely analyzed. However, when it is demonstrated that the testing is under statistical control, the QC testing frequency may be reduced. The QC precision should be periodically checked against the precision listed in the Precision and Bias section of this test method to ensure data quality.
13.5 It is recommended that, if possible, the type of QC sample that is regularly tested be representative of the samples routinely analyzed. An ample supply of QC sample material should be available for the intended period of use, and must be homogeneous and stable under the anticipated storage conditions. Because the base number can vary while the QC sample is in storage, when an out-of-control situation arises, the stability of the QC sample can be a source of the error.
