Preparation of Low HD Contamination Cells for the Measurement of the Triple Point Temperature of n-D2

An effort is under way to produce deuterium triple point sealed cells for a 18.7 K temperature reference point. Stability with time of impurity content in cells, namely HD, was found to be good from measurements made at IMGC since 1978, but nearly all commercially produced “chemical pure” D₂ contains approximately 0.5% HD. To reduce the HD content in the D₂ cell, two steps have been taken: (1) The stainless steel cell was chemically etched and then vacuum baked to remove Fe from the surface and to diffuse protium out of the cell. (2) The cell was flushed and filled to 70 bar with D₂ directly obtained from a thermal diffusion column with HD <70 ppm. Triple point measurements with the cell show an initial drift due to para-ortho conversion of 0.4 mK per hour decreasing to 0.07 mK per hour after 130 hours of conversion. The initial triple point temperature on NBS-IPTS-68 was found to be 18.732 ± 0.001 K; this agrees with the value found in the earlier IMGC cells, filled with commercial D₂, when the latter are corrected for a 0.4 ± 0.1 HD content, but the conversion in this cell was 10 times faster than in 304 stainless steel IMGC cells. Further measurements have been made using the clean n-D₂ in an unetched 304 stainless steel cell which has been flushed with D₂O vapor.