ASTM D6749 for pour point of petroleum products (Automatic air pressure method)
13. Precision and Bias
13.1 Precision - The precision of this test method as determined by statistical examination of interlaboratory test results is as follows:
13.1.1 Pour Point at 3°C Testing Intervals (Test Method D6749):
13.1.1.1 Repeatability - The difference between successive test results, obtained by the same operator using the same apparatus under constant operating conditions on identical test material, would in the long run, in the normal and correct operation of this test method, exceed the following, only in one case in twenty.
2.5°C
13.1.1.2 Reproducibility - The difference between two single and independent test results, obtained by different operators working in different laboratories on identical test material, would in the long run, in normal and correct operation of this test method, exceed the following, only in one case in twenty.
3.1°C
13.1.2 Pour Point at 1°C Testing Intervals (Test Method D6749):
13.1.2.1 Repeatability - The difference between successive test results, obtained by the same operator using the same apparatus under constant operating conditions on identical test material, would in the long run, in the normal and correct operation of this test method, exceed the following, only in one case in twenty.
1.1°C
13.1.2.2 Reproducibility - The difference between two single and independent test results, obtained by different operators working in different laboratories on identical test material, would in the long run, in normal and correct operation of this test method, exceed the following, only in one case in twenty.
2.2°C
13.2 Bias - Since there is no accepted reference material suitable for determining the bias for the procedure in this test method, bias has not been determined.
13.3 Relative Bias:
13.3.1 Pour point results at 3°C testing intervals were compared to the pour point results from Test Method D97. Relative bias among certain samples was observed; however, the observed bias does not appear to be of a systematic nature. Biases relative to Test Method D97/IP15 may conceivably occur for sample types not included in the 1998 interlaboratory test program.
NOTE 5 - Large differences in results were observed between methods for one sample in the 1998 interlaboratory test study. The sample was a high-sulfur winter diesel. When cooled during the performance of a test method, this sample formed thin, but very large crystals, that could be described as large plates. These crystals formed wherever sample-glass contact was made as well as covered the top surface of the sample. The entire sample, except for this all encasing thin skin of crystals, remained liquid with apparent low viscosity. When this occurred and the sample was handled gently, the sample did not pour, but with rougher handling, the crust broke and the sample poured readily. Users of this method are advised to be alert for differences in results between test methods when this behavior is observed in the sample being tested.
13.3.2 Pour points results at 1°C testing intervals were examined for bias relative to the pour point results at 3°C testing intervals. A bias of 1.1°C in average was observed in the 1998 interlaboratory test program.
NOTE 6 - It shall be noted that when a specimen is tested at 1°C intervals, statistically the results will be 1°C lower than the results produced by 3°C testing intervals. This is due to test increment and reporting differences. Differences greater than 1°C over a number of samples would be from another cause. In the interlaboratory test program, the tests at 1°C intervals yielded pour points lower than those obtained from the tests at 3°C intervals by 1.1°C in average.
13.4 The precision statements and the relative bias information were derived from a 1998 interlaboratory test program. Participants analyzed two sets of duplicate distillate diesel fuels, five sets of duplicate base oils, three sets of duplicate multigrade lubricating oils, and one set each of duplicate hydraulic oils and automatic transmission fluid in the temperature range of -51 to -11°C. Nine laboratories participated with the automatic apparatus, testing at 1°C and 3°C intervals, and seven laboratories participated with the manual Test Method D97 apparatus. Information on the types of samples and their average pour points are available in the form of a research report.
14. Keywords
14.1 automatic air pressure method; petroleum products; pour point
