Perhaps more valuable is that this level of agreement is achieved between compressibility measurements made by the traditional PVT method and by the more recent speed of sound techniques [Trusler et al. 1997]. Based on this information, a relative standard uncertainty of 10 × 10-6 will be used for the experimental measurements of compressibility. This uncertainty is for a pressure of 100 kPa and it scales with density. The residuals of the equation fitting process to the experimental data are negligibly small (