ASTM D6811 Standard Test Method for Measurement of Thermal Stability of Aviation Turbine Fuels under Turbulent Flow Conditions (HiReTS Method)
4. Summary of Test Method
4.1 Fuel is pumped, at pressure, through an electrically heated capillary tube at a constant rate. The heating of the capillary tube is controlled to maintain a constant fuel temperature of 290 +/- 3°C at the exit of the capillary tube. A flow rate of greater than 20 mL/min and the specified capillary bore ofless than 0.300 mm ensures that turbulent flow is maintained (see Appendix X1) within the capillary. The formation of lacquers and fuel degradation products act as a thermal insulator between the cooler fuel and hotter capillary tube, resulting in an increase in temperature of the capillary tube which is measured at a number of positions by a contactless pyrometer. The HiReTS Total (T) number is displayed during and at the end of the test. The HiReTS Peak (P) number can be determined from analysis of the results.

5. Significance and Use
5.1 The thermal stresses experienced by aviation fuel in modern jet engines may lead to the formation of undesirable and possibly harmful insoluble materials, such as lacquers, on heat exchangers and control surfaces, that reduce efficiency and require extra maintenance.

5.2 Aircraft fuel systems operate mainly under turbulent flow conditions. Most large-scale realistic test rigs operate in the turbulent flow regime but fuel volumes are very large and test times are very long.

5.3 This test method tests fuel under turbulent flow (high Reynolds number) conditions, and it gives a quantitative result under standard operating conditions of 65 or 125 min. Continuous analysis of results during the test allows performance of the fuel to be monitored in real time thus enabling the test time to be reduced manually or automatically, if required.

5.4 The results of this test method are not expected to correlate with existing test methods for all fuels, since the test methods and operating conditions are different (see Appendix X2).