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Thermal diffusivity (α) describes how quickly a material conducts heat relative to its ability to store thermal energy. It is derived, not measured: α = k / (ρ · cp), where k is thermal conductivity [W/(m·K)], ρ is density [kg/m³], and cp is isobaric specific heat capacity [J/(kg·K)]. NADC-79248-60 (Conte & Hammond, 1980, DTIC ADA081597) does not report thermal diffusivity; the inputs are its MIL-H-5606 columns of Table 2 (density), Table 7 (specific heat) and Table 8 (thermal conductivity), tabulated at 38, 93, 149 and 204 °C at atmospheric pressure.
The smooth curve is the ratio of three fits, with T in °C: ρ = 869.517 − 0.71295·T kg/m³; cp = 1786.96 + 4.3331·T J/(kg·K); k = 0.13822 − 0.000077493·T W/(m·K). The curve is used only between 38 °C and 204 °C. That is the density fit's range: the specific heat and conductivity fits extend down to −18 °C, but no density below 38 °C was found in the sources, so the derived diffusivity is not extended there. The curve departs from the four tabulated values by about 0.3 % at most.
MIL-PRF-5606 has a thermal diffusivity of roughly 8.2×10⁻⁸ m²/s at 38 °C, falling to about 6.3×10⁻⁸ m²/s at 204 °C. Over 38–70 °C it is about 2–3 % higher than MIL-PRF-23699 (comparison below). The two fluids come from different sources and pressures, so that small difference should not be over-interpreted. The NADC report gives no measurement uncertainty, and the derived values inherit the uncertainty of its Tables 2, 7 and 8.