Function waterDensityKgPerM3

Density of air-free water at one atmosphere, kg/m³.

Kell's equation (G. S. Kell, "Density, thermal expansivity, and compressibility of liquid water from 0° to 150 °C", J. Chem. Eng. Data 20(1), 1975, 97–105):

       999.83952 + 16.945176·t − 7.9870401e-3·t² − 46.170461e-6·t³
                 + 105.56302e-9·t⁴ − 280.54253e-12·t⁵
ρ(t) = ───────────────────────────────────────────────────────────
                       1 + 16.879850e-3·t
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Fitted over 0–150 °C, it reproduces tabulated densities to a few parts per million. It returns 998.204 kg/m³ at 20 °C and 958.364 kg/m³ at 100 °C. We only use ratios of it, so even that small error mostly cancels.

It also reproduces water's density maximum at 3.98 °C (not 0 °C), which is an easy check that the coefficients were transcribed correctly.