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The Prandtl number (Pr) is a dimensionless quantity that characterizes the relative importance of momentum diffusion (viscosity) to thermal diffusion (thermal conductivity) in a fluid. It is defined as Pr = μ · cp / λ, where μ is dynamic viscosity [mPa·s], cp is isobaric specific heat capacity [kJ/(kg·K)], and λ is thermal conductivity [W/(m·K)]. Dynamic viscosity itself is computed from Eastman's published kinematic viscosity (ν, ASTM D445) and density (ρ) as μ = ν × ρ.
Because Eastman's kinematic viscosity, specific heat, and thermal conductivity datasets each cover a different temperature span, this calculator is limited to the range where all three overlap for every grade: 100 °F to 210 °F (38 °C to 99 °C). Inputs outside that range return "Out of Range."
Density is the weakest link in this calculation. Skydrol LD-4 and 500B-4 have full density-vs-temperature curves (digitized from The Lee Company, cross-checked against Eastman's 25 °C values to within 0.5% — see the Density page). Skydrol PE-5 and Skydrol 5 have no published curve at all — only a single 25 °C density each. For those two, this page scales the shape of the LD-4 curve to the grade's own Eastman 25 °C density (PE-5 0.9927 g/cc, Skydrol 5 0.9737 g/cc), which is exact at 25 °C and assumes each expands like LD-4 (β ≈ 8.5×10⁻⁴ K⁻¹). Measured β across the grades that do have curves spans roughly 7.0–8.5×10⁻⁴ K⁻¹, so treat PE-5 and Skydrol 5 density as carrying about ±1% over this range, and their Prandtl numbers as correspondingly less certain than LD-4's and 500B-4's.
One clarification, because the names invite it: Skydrol 5 is not Skydrol 7000. Skydrol 7000 is the obsolete 1948 original, a much denser fluid at roughly 1.083 g/cc at 25 °C. Using its curve for Skydrol 5 — the lightest grade Eastman sells, at 0.9737 g/cc — would make it about 11% too dense and push its Prandtl numbers 11% high.