Here - PMOD/WRC

The design of the graphite crucible is shown in Fig. 2.
The blackbody is designed so that limiting apertures could
be placed into the 6 mm diameter opening to increase the
emissivity if desired. The large crucible contains about
1.3 kg of high-purity Au to achieve a uniform temperature
distribution during the freeze and to achieve long-time
duration of the freeze plateau.
Figure 2. The design of two of the Au-freezing temperature
crucibles. The conical cavity has an apex angle of 57 °.
During the experiments, the cavity was fitted with a
2 mm diameter aperture to validate the predicted increase
in the emissivity as well as to verify the emissivity
prediction of the 6 mm diameter cavity with no apertures.
The calculated emissivities are listed in Table 1. The
calculations are performed using a Monte Carlo program
Table 1. Calculated emissivity of the graphite blackbody cavity
with 50 mm depth and 57 ° conical bottom.
Aperture diameter
[ mm ]
Discussion
Emissivity with
graphite emissivity,
ε = 0.86
Emissivity with
graphite emissivity,
ε = 0.80
6.0 0.99964 0.99943
2.0 0.99996 0.99993
1.5 0.99998 0.99996
1.0 0.99999 0.99998
These results demonstrate the long-term stability of the
Au points constructed at NIST and the equivalence
between the three blackbodies. All three of the
blackbodies are still in service after almost 20 years of
intermittent usage after being stored in laboratory
environment without any special care. All three of the
NIST Au-points showed the same shape of the freezing
plateau without noticeable slopes indicating that the
material purity and the temperature uniformity of the
furnaces are equivalent. These results will be compared
against similar measurements using the Ag- and Cu-point
blackbodies at other NMIs to realize the ITS-90.
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