Overview of precision capacitance measurements for dielectric-constant gas thermometry

For dielectric-constant gas thermometry, the dielectric constant of rarefied gases has to be determined with a relative uncertainty of one part per million. Due to the very small dielectric susceptibility of the suitable gases, this requires to measure the change of the capacitance of the measuring capacitor containing them on the part per billion level. The technique is described briefly and results are presented together with an outlook.