ASTM D6756 Standard Test Method for Determination of the Red Dye Concentration and Estimation of the ASTM Color of Diesel Fuel and Heating Oil Using a Portable Visible Spectrophotometer
12. Report
12.1 Report the red dye concentration to the nearest 0.1 mg/L as Solvent Red 26 equivalent dye.
NOTE 4 - The number of milligrams per litre of Solvent Red 26 equivalents measured by the DT100C instrument is labeled as ppm on the display. The ppm unit reported by the DT100C instrument is equivalent to mg/L.
12.2 Report the estimated ASTM color value as: estimated ASTM color using ASTM D6756. Report the result to the nearest 0.1 color value.
12.2.1 The reported estimated ASTM color value can be converted to a value that mimics the reporting convention of Test Method D1500. For estimatedASTM color values that are x.0 or x.5, the estimated value is reported unchanged. For estimated color values between x.1 and x.4, round up the value to the nearest x.5 and place an L before the result. For estimated color values between x.6 and x.9, round up the value to the nearest whole number and place an L before the result. If the estimated color value is greater than 8.0, report the estimated result as >8.
13. Precision and Bias
13.1 Interlaboratory tests of the procedure were carried out using sixteen samples covering the red dye concentration range equivalent to 0 mg/Lto 16 mg/L of Solvent Red 26 equivalents and covering the range ofASTM color from 0.5 to 3.0. Seven laboratories participated in the interlaboratory tests. The precision of this procedure as determined by the statistical examination of the interlaboratory test results is as follows:
13.1.1 Repeatability - The difference between successive test results obtained with the same apparatus under constant operating conditions on identical samples would in the long run, in normal and correct operation of the test method, exceed the following value in only one case in twenty:
r = 0.22 mg/L
for Solvent Red 26 equivalent dye concentrations between 0.1 mg/L and 20.0 mg/L, and r = 0.085 ASTM color units
for samples in the ASTM color range of 0 to 5.
13.1.2 Reproducibility - The difference between two single and independent results obtained from different instruments on identical samples, would in the long run, in normal and correct operation of the test method, exceed the following value in only one case in twenty:
R = 0.86 mg/L
for Solvent Red 26 equivalent dye concentrations between 0.1 mg/L and 20.0 mg/L, and R = 0.20 ASTM color units
for samples in the ASTM color range of 0 to 5.
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 (Dye Concentration) - Some relative bias, among certain samples, was observed when the Solvent Red 26 concentration results were compared to the expected Solvent Red 26 concentrations. The bias between the theoretical and measured Solvent Red 26 concentration is observed with the test samples that were made using Solvent Red 164, the chemical used to commercially dye diesel fuel. The observed bias does appear to be of a systematic nature and is not known to be related to the accuracy of this test method, since the Solvent Red 26 equivalency of the Solvent Red 164 used in sample preparation has not been determined, only estimated.
13.4 Relative Bias (ASTM Color) - Relative bias, for one sample, was observed when the color results were compared to the Test Method D1500 results; however, this bias is near the reproducibility of the test methods and is inconclusive. The overall relative bias to Test Method D1500 is not significant.
NOTE 5 - The average bias of the estimated ASTM color relative to Test Method D1500 was -0.17. The largest average biases, -0.4 to -0.5, were observed for four samples that had an ASTM color of 0.5 and red dye concentrations that ranged from 10 mg/L to 20 mg/L.
13.5 Relative Bias (Unfiltered versus Filtered Samples) - A single laboratory analyzed a total of 99 separate split samples (unfiltered versus filtered) to determine the impact that filtering samples can have on reported red dye concentration results. Based on a Practice D6708 analysis of the results, where the Y-method was unfiltered samples and the X-method was filtered samples, a bias correction formula of Y = X - 0.107 was determined. In short, filtering the samples led to a statistically significant bias being determined. As such, to ensure testing consistency between laboratories, samples are not to be filtered as indicated in 11.3. Inclusion of the Test Method D1500 analysis formula is for information only and not intended to be used by laboratories as an official offset to apply to their data if samples were filtered.
13.6 The precision statements in 13.1 were derived from the 1997 interlaboratory test program. Participants analyzed four sets of undyed base fuels and twelve sets of dyed base fuel/color combinations in duplicate in the ASTM color range of 0.5 to 3.0 and Solvent Red 26 equivalent dye concentration from 0 mg/L to 16 mg/L. Seven laboratories participated with the automatic filter apparatus and six laboratories participated with the manual Test Method D1500 apparatus. Information on the types of samples and their dye concentrations and colors are in the research report available at ASTM International Headquarters. Request ASTM RR: RR:D02-1521.
14. Keywords
14.1 ASTM color; diesel fuel, heating oil; red dye concentration; visible spectrophotometry
