ITL-M-18205 Nitrogen Boiling Point Apparatus.
The Nitrogen Boiling Point Apparatus is designed specifically to realize and maintain the liquid-vapor
equilibrium (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 must
be supplied by the user. Either liquid is suitable for the purpose. Liquid nitrogen is generally less costly and
more readily available than liquid argon. The Apparatus does not require electrical power for its operation.

Nitrogen Boiling Point Apparatus
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Nitrogen Boiling Point Apparatus

Model  Description 
ITL-M-18205  Nitrogen Boiling Point Apparatus.

The Boiling Point Apparatus will maintain the liquid-vapor equilibrium of nitrogen (-195.798°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.
As a practical matter, realization of this triple point is a costly (in equipment and time) and complicated

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
The National Physical Laboratory of England makes this statement:
"Most thermometers (submitted for calibration) will involve measurements (by) ... comparison with
NPL standards in a bath of liquid nitrogen (about -196°C)"

Model 18205 Comparator is designed for precisely such comparison calibration.
It comprises a stainless steel debar, an inner equalizing block having wells for 3 thermometers, top
connections 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.
The Nitrogen point apparatus may be adapted for use with liquid argon.

Nitrogen point apparatus specification
Model  ITL-M-18205 
Temperature Range  -185°C or -196°C 
Accuracy  ±0.002°C 
Uncertainties  The temperature distribution across the copper block is less
than 2mK. To this must be added the uncertainty issued
with the calibration certificate. Extra uncertainties will also
exist if dissimilar probes are compared. 

Dimensions  Below flange 430mmDiameter below flange 127mm
Flange diameter 165mm 

Weight  30.9 lbs (14.0 kg)