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