Isotech<\/a> 18205 Nitrogen Boiling Point Apparatus provides a simple, reliable and highly stable way to realize the liquid\u2013vapour equilibrium of liquid nitrogen or liquid argon. This equilibrium forms a well\u2011defined reference point at \u2013195.798 \u00b0C (liquid nitrogen) or \u2013185.8468 \u00b0C (liquid argon) and is widely used for thermometer calibration under the International Temperature Scale of 1990 (ITS\u201190). The apparatus is completely self\u2011contained, requires no electrical power, and is cooled solely by the liquid cryogen supplied by the user. Liquid nitrogen is typically preferred due to its low cost and easy availability.<\/p>\nA practical alternative to the Argon Triple Point<\/h3>\n
Although ITS\u201190 defines the Argon Triple Point (\u2013189.3442 \u00b0C) as the lowest fixed point for long\u2011stem SPRTs, realizing this point is expensive, time\u2011consuming and technically demanding. For this reason, most laboratories calibrate thermometers in this region by comparison with a reference thermometer immersed in a liquid nitrogen bath.<\/p>\n
Even national metrology institutes routinely use this method. As the National Physical Laboratory (NPL) notes:
\n\u201cMost thermometers submitted for calibration will involve measurements by comparison with NPL standards in a bath of liquid nitrogen (about \u2013196 \u00b0C).\u201d<\/p>\n
The Isotech 18205 is specifically designed for this type of comparison calibration.<\/p>\n
Design and operation<\/h3>\n
The apparatus consists of a stainless steel dewar containing an internal equalizing block with wells for three thermometers. The block provides a highly uniform temperature field, with a distribution of less than 2 mK across the copper mass. Top\u2011mounted connections allow the user to fill the dewar, monitor the liquid level and ensure safe operation via a pressure blow\u2011off. An optional helium gas manifold can be used to thermally couple the thermometers to the block, improving stability during comparison measurements. As long as the boiled\u2011off gas is replenished, the liquid\u2013vapour equilibrium remains indefinitely stable, enabling long, drift\u2011free calibration sessions.<\/p>\n
Accuracy and performance<\/h3>\n
Because SPRTs have very similar dR\/dT slopes at these temperatures, comparison calibrations can achieve uncertainties of \u00b10.002 K, at only a fraction of the cost of an absolute triple\u2011point realization.<\/p>\n
The final uncertainty depends on:<\/p>\n