Theory and Method
The freezing point of an aviation fuel is the lowest temperature at which it stays free of solid hydrocarbon crystals. Jet fuel is a blend of many hydrocarbons, so it does not freeze at one sharp temperature. As it cools, the heaviest n-paraffins crystallize first and form wax that can block filters and fuel lines. Aircraft fuel temperatures fall during long flights at altitude, so the freezing point must sit below the lowest expected fuel temperature.
ASTM D5972 measures this property optically. The method cools the specimen at about 15 ± 5°C/min while a light source illuminates it continuously. An array of optical detectors watches for light scattering from the first crystals that form. Once crystals appear, the specimen warms at 10 ± 0.5°C/min, and the detectors track the crystals as they melt back into the liquid.
The temperature at which the last crystal disappears becomes the reported freezing point. Reading the transition on the warming side gives a more repeatable value, since crystal formation on cooling depends on supercooling while melting does not. The QualiRFPT™ D5972 uses an auto-cascade refrigeration system with cooling capacity down to −90°C, below the −80°C lower measurement limit. Its software logs the full temperature curve and identifies the transition automatically.
