The Isotech Nitrogen Boiling Point Apparatus is designed specifically to realise and maintain the liquid/vapor equilibrium, or the boiling point, of nitrogen or argon for the calibration of thermometers on the International Temperature Scale of 1990.

The Boiling Point Apparatus is self-contained, and refrigerated by liquid nitrogen or liquid argon, which the user must supply.

The Boiling Point Apparatus will maintain the liquid-vapor equilibrium, or the boiling point, of nitrogen (-195.198°C) or of argon (-185.8468°C) indefinitely, provided boiled-off gas is replenished.

ITS-90 specifies the triple point of argon (-189.3442°C) as the low end of the long-stem Standard Platinum Resistance Thermometer range. Practically speaking, realization of this triple point is a costly (in equipment and time) and complicated process.

Most laboratories will choose to calibrate this end of the platinum range by comparison of the thermometer under test with a thermometer of known calibration. National Laboratories themselves will invariably calibrate thermometers submitted to them by comparison, realizing the actual argon triple point only infrequently for calibration of their own reference thermometers. The National Physical Laboratory of England makes this statement:
"Most thermometers (submitted for calibration) will involve measurement (by) ... comparison with NPL standards in a bath of liquid nitrogen (about -196°C)."

The Model 18205 Comparator is designed for precisely such comparison calibration. It comprises a stainless steel dewar, an inner equalizing block having wells for 3 thermometers, top connection for filling and monitoring the level of liquid coolant, a pressure safety blow-off, and a manifold which may be used to thermally tie the thermometers under test to the equalizing block with helium gas (optional).

Since the slopes (dR/dT) of Standard Platinum Resistance Thermometers are very similar at any temperature, calibration uncertainties not larger than 0.002K can be obtained at a small fraction of the cost of an absolute calibration.