Cryogenic fluids density reference system :

The measurement capability of the density reference system (DRS) of the National Bureau of Standards, Cryogenics Division, is described.This system measures density, pressure, and temperature of LNG mixtures for the purpose of testing densimeters which are contained in the liquid sample.Sample composition is determined by weighing the gas samples separately before condensing them into the sample.The DRS measures density by weighing a single-crystal of silicon immersed in the LNG mixture.This process is described and the equations used in the computation of density are discussed.At this time the estimate of sample standard deviation for a single density measurement made on this system is + 0.062% (at 0.4 g/cc).Using three times this standard deviation as a limit for random error and adding + 0.026% as an upper bound for known sources of possible systematic error, the uncertainty of a single determination by this system is + 0.21%.This statement of accu- racy applies for the density range 380 to 430 kg/m^and 1200 to 1400 kg/m\ pressures to 7 bar, and temperatures from 80 K to 140 K, This statement is expected to be correct in the intermediate density range and for all temperatures up to 300 K.Measurement uncertainties for temperature, pressure, and composition are discussed.Comparison of measurements for liquid argon densities with the results of other laboratories is given.