IP 389: Determination of wax appearance temperature (WAT) of middle distillate fuels by differential thermal analysis (DTA) or differential scanning calorimetry (DSC)

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  • Method adopted/last revised: 1993
  • Method reapproved: 2004
  • REF/ISBN: IP389-2934869
  • Status: Current
  • First printed in STM books: January 2004


This standard describes methods for the determination of the temperature at which waxy solids form when middle distillate fuels are cooled. The wax appearance temperature (WAT) of a fuel is related to the lowest temperature of its utility for certain applications. Two procedures are presented in this method, designated method A (rapid procedure) and method B (slow procedure). The rapid procedure is convenient; the slow procedure yields results closer to the equilibrium value, with improved discrimination, and is expected to be applicable to fuels containing additives such as cloud point depressants, cold flow improvers and detergents. Both methods require only a very small sample (less than 50 ul).

NOTE 1 - For the relation to the wax appearance point (WAP) and cloud point (CP); see A.1 and A.2. Results from one laboratory on 25 base fuels indicate a direct correlation between WAT and either WAP (see A. I), or CP (see A.2) with correlation coefficients greater than 0.95.

Results in the literature (see A.3) where a similar DSC method was used on 95 middle distillates, indicate a correlation between the onset of crystallization by DSC and CP by IP 219 BS 2000: Part 219/ASTM D-2500 with correlation coefficients greater than 0.997. The slow procedure, method B, is based on work in the literature (see A.4) demonstrating improved precision with low cooling rates.

NOTE 2 - Limited work indicates that the slow procedure, method B, can be applied to crude oils with the modification of an increased sample size (at least 50 u1, or more, if the pan size allows), but precision has not been determined.

NOTE 3 - If a laboratory is set up for maximum sample throughput, one determination by method A takes about 10 min, and one determination by method B takes about 20 min.

For method A, precision has been determined within the range -26 °C to + 16 °C; for method B, precision has been determined within the range - 50 °C to + 15 °C. The precision samples were diesel, kerosine, light and heavy gas oil fuels.

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