ISO 6249:2021 pdf download – Petroleum products — Determination of thermal oxidation stability of gas turbine fuels.
3.2.3 abnormal tube-deposit colour that is neither peacock nor like those of the colour standard Note 1 to entry: It refers to deposit colours such as blues and greys that do not match the colour standard [4] . 3.2.4 interferometry technique used for measuring the optical properties of surfaces (refractive index and absorption coefficient) based on studying the pattern of interference created by their superposition Note 1 to entry: In the presence of a thin transparent layer called film, interferometry can also be used to provide film thickness information. 3.2.5 ellipsometry optical technique used for measuring the properties of surfaces (refractive index and absorption coefficient) based on changes in the polarized state of light upon reflection from the surface Note 1 to entry: In the presence of a thin transparent layer, with a known refractive index and absorption coefficient, ellipsometry can also be used to provide film thickness information. 3.2.6 standard spot mean thickness of the six thickest points in a 2,5 mm 2 area Note 1 to entry: The mean thickness is expressed in nm. 4 Principle This test method for measuring the high temperature stability of gas turbine fuels uses an instrument that subjects the test fuel to conditions which can be related to those occurring in gas turbine engine fuel systems. The fuel is pumped under pressure at a fixed volumetric flow rate through a heater, after which it enters a precision stainless-steel filter where fuel degradation products may become trapped. The differential pressure across this filter is continuously monitored and an excess, indicating significant deposition on the filter, will cause a premature shut-down of the apparatus before the end of the normal test period.
At the end of the test period, or after an earlier shut-down, the amount of deposit on the heater tube is rated with reference to a standard colour scale using a visual tube rater (see Annex B) or measured using interferometry or ellipsometry or multi wavelength ellipsometry (see Annexes C, D and E). NOTE These rating procedures are based on manufacturer instructions but are further explained in ASTM D3241 [5] . The final result from this rating procedure is an absolute measurement of the thickness and volume of deposit on the heater tube that provides a basis for judging the thermal oxidative stability of the fuel sample. For aircraft fuel systems performance, deposit thickness and volume are useful parameters. 5 Reagents and materials 5.1 Water, distilled or deionized water in the spent sample reservoir as required for Model 230 excluding Mk III, Mk IV and 240. 5.2 Tri-solvent, consisting of equal mix of acetone, toluene, and isopropanol as a specific solvent to clean internal (working) surface of test section only. 5.3 Cleaning solvent, methyl pentane, 2,2,4-trimethylpentane, or n-heptane (technical grade, 950 mmol/mol minimum purity).