In fluid mechanics, the Tait equation is an equation of state, used to relate liquid density to hydrostatic pressure. The equation was originally published by Peter Guthrie Tait in 1888 in the form^[1]

${\displaystyle {\frac {V_{0}-V}{PV_{0}}}={\frac {A}{\Pi +P}}}$

where ${\displaystyle P}$ is the hydrostatic pressure in addition to the atmospheric one, ${\displaystyle V_{0}}$ is the volume at atmospheric pressure, ${\displaystyle V}$ is the volume under additional pressure ${\displaystyle P}$, and ${\displaystyle A,\Pi }$ are experimentally determined parameters. A very detailed historical study on the Tait equation with the physical interpretation of the two parameters ${\displaystyle A}$ and ${\displaystyle \Pi }$ is given in reference.^[2]

In 1895,^[3][4] the original isothermal Tait equation was replaced by Tammann with an equation of the form

${\displaystyle {K}=-{V_{0}}\left({\frac {\partial P}{\partial V}}\right)_{T}={V_{0}}{\frac {(B+P)}{C}}}$

where ${\displaystyle K}$ is the isothermal mixed bulk modulus. This above equation is popularly known as the Tait equation. The integrated form is commonly written

${\displaystyle V=V_{0}-C\log \left({\frac {B+P}{B+P_{0}}}\right)}$

where

• ${\displaystyle V\ }$ is the specific volume of the substance (in units of ml/g or m³/kg)
• ${\displaystyle V_{0}}$ is the specific volume at ${\displaystyle P=P_{0}}$
• ${\displaystyle B\ }$ (same units as ${\displaystyle P_{0}}$) and ${\displaystyle C\ }$ (same units as ${\displaystyle V_{0}}$) are functions of temperature

Another popular isothermal equation of state that goes by the name "Tait equation"^[7][8] is the Murnaghan model^[9] which is sometimes expressed as

${\displaystyle {\frac {V}{V_{0}}}=\left[1+{\frac {n}{K_{0}}}\,(P-P_{0})\right]^{-1/n}}$

where ${\displaystyle V}$ is the specific volume at pressure ${\displaystyle P}$, ${\displaystyle V_{0}}$ is the specific volume at pressure ${\displaystyle P_{0}}$, ${\displaystyle K_{0}}$ is the bulk modulus at ${\displaystyle P_{0}}$, and ${\displaystyle n}$ is a material parameter.

A related equation of state that can be used to model liquids is the Tumlirz equation (sometimes called the Tammann equation and originally proposed by Tumlirz in 1909 and Tammann in 1911 for pure water).^[4][10] This relation has the form

${\displaystyle V(P,S,T)=V_{\infty }-K_{1}S+{\frac {\lambda }{P_{0}+K_{2}S+P}}}$

where ${\displaystyle V(P,S,T)}$ is the specific volume, ${\displaystyle P}$ is the pressure, ${\displaystyle S}$ is the salinity, ${\displaystyle T}$ is the temperature, and ${\displaystyle V_{\infty }}$ is the specific volume when ${\displaystyle P=\infty }$, and ${\displaystyle K_{1},K_{2},P_{0}}$ are parameters that can be fit to experimental data.

The Tumlirz–Tammann version of the Tait equation for fresh water, i.e., when ${\displaystyle S=0}$, is

${\displaystyle V=V_{\infty }+{\frac {\lambda }{P_{0}+P}}\,.}$

For pure water, the temperature-dependence of ${\displaystyle V_{\infty },\lambda ,P_{0}}$ are:^[10]

${\displaystyle {\begin{aligned}\lambda &=1788.316+21.55053\,T-0.4695911\,T^{2}+3.096363\times 10^{-3}\,T^{3}-0.7341182\times 10^{-5}\,T^{4}\\P_{0}&=5918.499+58.05267\,T-1.1253317\,T^{2}+6.6123869\times 10^{-3}\,T^{3}-1.4661625\times 10^{-5}\,T^{4}\\V_{\infty }&=0.6980547-0.7435626\times 10^{-3}\,T+0.3704258\times 10^{-4}\,T^{2}-0.6315724\times 10^{-6}\,T^{3}\\&+0.9829576\times 10^{-8}\,T^{4}-0.1197269\times 10^{-9}\,T^{5}+0.1005461\times 10^{-11}\,T^{6}\\&-0.5437898\times 10^{-14}\,T^{7}+0.169946\times 10^{-16}\,T^{8}-0.2295063\times 10^{-19}\,T^{9}\end{aligned}}}$

In the above fits, the temperature ${\displaystyle T}$ is in degrees Celsius, ${\displaystyle P_{0}}$ is in bars, ${\displaystyle V_{\infty }}$ is in cc/gm, and ${\displaystyle \lambda }$ is in bars-cc/gm.

Following in particular the study of underwater explosions and more precisely the shock waves emitted, J.G. Kirkwood proposed in 1965^[11] a more appropriate form of equation of state to describe high pressures (>1 kbar) by expressing the isentropic compressibility coefficient as

${\displaystyle -{\frac {1}{V}}\left({\frac {\partial V}{\partial P}}\right)_{S}={\frac {1}{n(B+P)}}}$

where ${\displaystyle S}$ represents here the entropy. The two empirical parameters ${\displaystyle n}$ and ${\displaystyle B}$ are now function of entropy such that

• ${\displaystyle n\ }$ is dimensionless
• ${\displaystyle B\ }$ has the same units as ${\displaystyle P}$

The integration leads to the following expression for the volume ${\displaystyle V(P,S)}$ along the isentropic ${\displaystyle S}$

${\displaystyle {\frac {V}{V_{0}}}=\left(1+{\frac {P_{0}}{B}}\right)^{1/n}\left(1+{\frac {P}{B}}\right)^{-1/n}}$

where ${\displaystyle V_{0}=V(P_{0},S)}$.

• Equation of state