Editing Gas constant (section)

== Measurement and replacement with defined value ==
As of 2006, the most precise measurement of ''R'' had been obtained by measuring the [[speed of sound]] ''c''<sub>a</sub>(''P'',&nbsp;''T'') in [[argon]] at the temperature ''T'' of the [[triple point of water]]  at different [[pressure]]s ''P'', and [[extrapolation|extrapolating]] to the zero-pressure limit ''c''<sub>a</sub>(0,&nbsp;''T''). The value of ''R'' is then obtained from the relation
: <math>c_\text{a}(0, T) = \sqrt{\frac{\gamma_0 R T}{A_\text{r}(\text{Ar}) M_\text{u}}},</math>
where
: ''γ''<sub>0</sub> is the [[heat capacity ratio]] (5/3 for monatomic gases such as argon);
: ''T'' is the temperature, ''T''<sub>TPW</sub> = 273.16&nbsp;K by the definition of the kelvin at that time;
: ''A''<sub>r</sub>(Ar) is the relative atomic mass of argon, and ''M''<sub>u</sub>&nbsp;=&nbsp;{{val|e=-3|u=kg⋅mol<sup>−1</sup>}} as defined at the time.

However, following the [[2019 revision of the SI]], ''R'' now has an exact value defined in terms of other exactly defined physical constants.