EXPLANATION
Hydrogen content represents a fundamental quality of a petroleum distillate that has been correlated with many of the performance characteristics of that product. Combustion properties of gas turbine fuels are related primarily to hydrogen content. As hydrogen content of these fuels decreases, soot deposits, exhaust smoke, and thermal radiation increase. Soot deposits and thermal radiation can increase to the point that combustor liner burnout will occur. Hydrogen content is a procurement requirement for military fuels Jp-5 specified in MIL-DTL-5624U, JP-8 specified in MIL-DTL-83133E, and Naval Distillate specified in MIL-PRF-16884K. This test method provides a simple and precise alternative to existing Test Methods ASTM D3701, ASTM D4808, and ASTM D5291 for determining the hydrogen content of petroleum middle distillate products. It uses a low-resolution pulsed nuclear magnetic resonance (NMR) spectrometer. The boiling range of the distillates covered by the test method is 150 to 390° C. While the test method may be applicable to middle distillates outside this boiling range, in such cases the precision given in the method may not apply.
The test method is generally based on Test Methods ASTM D3701 and ASTM D4808, with a major difference being the use of a pulsed NMR spectrometer instead of a continuous wave NMR spectrometer.
TEST SUMMARY
A test specimen is analyzed in a pulsed NMR spectrometer calibrated with standard reference materials. The analyzer records in a nondestructive fashion the total NMR signal, which arises from the absolute amount of hydrogen atoms in the reference standards and the test sample. The absolute signal intensity is normalized by the corresponding sample mass. The resulting signal-per-gram ratios are used as a means of comparing theoretical hydrogen contents of the standards with that of the sample. To ensure an accurate measure of the absolute hydrogen content of the reference standards and sample, it is necessary to ensure that the measured hydrogen signal intensity is always directly proportional to the absolute hydrogen content of the standards and sample. Undercounting of the reference standard with respect to the sample is avoided by a number of strategies, including accurate filling into the linear response region of the sample compartment so that the mass recorded for the sample represents the true amount measured by NMR, and use of a recycle delay considerably greater than the longest relaxation time constant for the sample.
TEST PRECISION
The precision of this test method was determined at two temperatures: 35 and 40° C.
Where X is the mass % hydrogen content.
Since there are no accepted reference materials for determining the bias of this test method, no statement on bias is being made.