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Radiometric Temperature Comparisons of Three NIST Gold Freezing-point<br />
Blackbodies<br />
N. Sasajima,<br />
NMIJ, Tsukuba, Japan<br />
C. E. Gibson, V. Khromchenko, R. D. Saunders, H. W. Yoon<br />
NIST, Gaithersburg, MD, USA<br />
Abstract.<br />
We describe the radiometric temperature comparisons of<br />
three NIST gold freezing-temperature blackbodies. The<br />
respective blackbodies are used in different facilities for<br />
the radiance temperature and spectral radiance scales and<br />
the comparisons are needed to determine the equivalence<br />
of the gold-point based realizations. The temperatures<br />
of each gold-point blackbody are compared against each<br />
other using a detector-based radiation thermometer which<br />
is calibrated using the cryogenic electrical substitution<br />
radiometer. These measurements were performed over a<br />
month period in 2004 and compared to the previous<br />
measurements of two of the same blackbodies in 1991.<br />
The freezing temperatures of all three blackbodies are<br />
found to be in agreement to < 10 mK (k=2) and the<br />
differences are within the comparison uncertainties.<br />
These differences are in agreement, within the combined<br />
uncertainties, with the < 16 mK (k=1) differences found in<br />
1991.<br />
Introduction<br />
In the International Temperature Scale of 1990 (ITS-90),<br />
the temperatures above the freezing temperature of Ag can<br />
be derived from the use of Ag, Au, or Cu<br />
freezing-temperature blackbodies and radiances from<br />
Planck’s radiation law. Although in ideal situations, the<br />
freezing temperatures of the respective metals are<br />
presumed to be equivalent, the transition temperatures will<br />
be depend upon many factors such as the material purity,<br />
cavity design, furnace design and operational parameters.<br />
The long-term use might also be compromised by the<br />
possible contaminations of the pure metal during use.<br />
Thus, to determine whether there are intrinsic differences<br />
in the temperature scales at the different national<br />
measurement institutes (NMI) related to the ITS-90<br />
realizations, the fixed-points should be directly compared.<br />
Direct fixed-point blackbody comparisons are difficult<br />
due to the need to transport the furnaces and the power<br />
supplies between the institutes. Furthermore, the cost of<br />
the quantity of the pure metal required can be a barrier to<br />
constructing the number of fixed-points needed for<br />
comparisons, and due to the expense of acquiring the<br />
quantity of pure Au, many NMIs utilize fixed-points made<br />
from either Ag or Cu.<br />
We compared three Au fixed-point blackbodies with<br />
three different furnaces utilizing Na-heatpipe liners. Two<br />
of the blackbodies are constructed with same physical<br />
dimensions and the other blackbody was slightly smaller.<br />
The blackbodies were operated in two furnaces with<br />
different physical dimensions but with a similar design.<br />
Measurements of radiance temperatures were performed<br />
with and without a graphite aperture to determine the<br />
change in the cavity emissivity. The emissivity was also<br />
modeled, and the results compared with the experimental<br />
change in the radiance temperatures.<br />
Experimental Setup<br />
The furnace and the fixed-point cell are shown in Fig. 1.<br />
The purified Au (99.9999 %) metal is placed in a purified<br />
graphite crucible. The long alumina or fused silica tube is<br />
placed at the center of a cylindrical, Na-heatpipe liner, and<br />
the furnace is electrically heated from the outside of the<br />
liner with two, semi-cylindrical heaters which enclose the<br />
heatpipe liner. The crucible is physically separated from<br />
the heating elements by the alumina or the silica tube, and<br />
argon is flowed through from the rear of the furnace to be<br />
vented at the front opening. The use of the heatpipe liner<br />
Figure 1. The schematic of the gold-point furnace.<br />
enables simple two-level control of the electrical heaters<br />
for melting or freezing of the metal.<br />
The radiance temperature measurements were<br />
performed using the NIST Absolute Pyrometer 1 (AP1).<br />
The design of the AP1 has been described in previous<br />
publications. The AP1 has been measured for absolute<br />
spectral radiance responsivity in the NIST Spectral<br />
Irradiance and Radiance Responsivity Calibrations using<br />
Uniform Sources (SIRCUS) facility. Since these<br />
measurements are radiance temperature comparisons of<br />
blackbodies at almost equal temperatures, the spectral<br />
responsivity and other sources of uncertainties such<br />
linearity, size-of-source effect are do not contribute to<br />
these comparisons.<br />
Blackbody Design and Emissivity Calculation<br />
Proceedings NEWRAD, 17-19 October 2005, Davos, Switzerland 327