Gas Constant - Measurement

Measurement

As of 2006, the most precise measurement of R is obtained by measuring the speed of sound c_a(p, T) in argon at the temperature T of the triple point of water (used to define the kelvin) at different pressures p, and extrapolating to the zero-pressure limit c_a(0, T). The value of R is then obtained from the relation

where:

• γ₀ is the heat capacity ratio (5/3 for monatomic gases such as argon);
• T is the temperature, T_TPW = 273.16 K by definition of the kelvin;
• A_r(Ar) is the relative atomic mass of argon and M_u = 10−3 kg mol−1.