The purpose of a reference PSD based on an easily accessible reference material is twofold: to verify the efficacy of the instrument or method being used, and to correct measurement results by applying a set of correction factors. A procedure in which the mean PSD curve is used to correct measured data obtained using various instruments would work in the following manner: 1) Calculate the correction factor for each size, defined as the ratio between the measured value and the mean value as shown in Table 1 for LAS-W and Table 2 for LAS-D; 2) Multiply measured size data for unknown samples by this correction factor.
This procedure was applied to all available data sets for CCRL 143 and 144, which were provided to the participants of the second round robin study. Figure 3 shows some selected results from this analysis. The complete results are provided in a separate publication [Ferraris et al., 2002b]. It was initially expected by the ASTM committee that a single method and reference PSD could be used to correct all measurements; however, in practice this proved problematic. If the target measurement results themselves (not the calibration curve) contain outliers, (i.e., data points that are more than 5% outside the confidence limits obtained with the bootstrap method), the correction is not sufficient to bring the entire curve within the confidence limits of the calibration curve (Fig. 3). On the other hand, if the data set lies completely within the confidence limits defined by the reference curve, the correction factor will reduce the overall spread of the data. Therefore, the reference curves for SRM 114p could be used in two ways: 1) As part of a validation method, to check that measurements are within the confidence limit range of the reference. This will allow the operator to determine if sample preparation problems or possibly a malfunctioning instrument should be considered; 2) As a calibration curve to correct data from unknown samples, after the method has first been validated as mentioned above. This correction should be used carefully, since the corrected PSD could remain outside the acceptable range.
FIG. 3−Corrections of selected data for cement CCRL 143. A) Data as measured; B) Data corrected. In the legend, 450, 611, and V6 represent coded data sets from participants in the round-robin. Also shown in these graphs are the 95% limits.