Specific gravity, also referred to as relative density, is the ratio of the densities of one fluid at a known temperature and pressure to a reference fluid at a specific temperature and pressure. In most cases, the reference fluid is water at atmospheric pressure. The most common reference temperatures in the standards and literature are
The values for specific gravity of water based on these common reference temperatures are listed in the table below.
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Enthalpy H =
Entropy S =
The uncertainty in density of the equation of state is 0.0001% at 1 atm in the liquid phase, and 0.001% at other liquid states at pressures up to
The uncertainty in pressure in the critical region is 0.1%.
The uncertainty of the speed of sound is 0.15% in the vapor and 0.1% or less in the liquid, and increases near the critical region and at high temperatures and pressures.
The uncertainty in isobaric heat capacity is 0.2% in the vapor and 0.1% in the liquid, with increasing values in the critical region and at high pressures.
The uncertainties of saturation conditions are 0.025% in vapor pressure, 0.0025% in saturated liquid density, and 0.1% in saturated vapor density. The uncertainties in the saturated densities increase substantially as the critical region is approached.
Eric W. Lemmon, Mark O. McLinden and Daniel G. Friend, "Thermophysical Properties of Fluid Systems" in NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Eds. P.J. Linstrom and W.G. Mallard, National Institute of Standards and Technology, Gaithersburg MD, 20899, https://doi.org/10.18434/T4D303, (retrieved February 19, 2019).
Last updated on May 12th, 2020