Here - PMOD/WRC

Experiments
The profile (full line, low) given in Figure 1 has been
directly measured by means of the radiation thermometer
(RT), LP-3 8040, by scanning along the radiation shields
placed within the furnace tube in front of the Re-C fixed
point (3S2) in study. The dashed profile (high) has been
constructed so as to represent an upper boundary to the
profile as seen by the cavity.
The experimental results given in the last two columns of
Table 1 show the temperature difference T(3)-T(8)
measured with LP-3 8040 for the cavity bottom of the cell
3S2 (a) without aperture ( 8 mm ) and (b) with a 3 mm
aperture placed in front of the cavity tube, for filter
wavelengths of 650 nm and 950 nm. The measurements
were performed on two consecutive days. Drift of the RT
has been corrected for by simultaneously measuring the
radiance of a second Re-C fixed point (1S1) on both days,
realized in parallel in Nagano VR10-A20, a smaller variant
of VR10-A23 [2]. The quoted standard uncertainties are
based upon the repeatability of the melting temperatures,
associated with the inflection points of the melting curves.
Model versus experiment
As may be appreciated from an inspection of Table 1
experimental and calculated results, shown for T(3)-T(8) in
rows 2 and 3, agree within the combined uncertainties,
when roughly associating the uncertainties in the calculated
results with the difference between the results obtained for
the high and low furnace profiles. Both experimental and
simulated results for T(3)-T(8), rows 2 and 3, are
significantly smaller than (a) the results calculated on the
basis of the ‘cell only’ and (b) those calculated by means of
Eq. (1), rows 4 and 5, respectively.
Correlation of ∆T with the effective total emissivity
Effective emissivities, ε(tot) and ε(λ) -for 650 and 950
nm- have been calculated by means of STEEP-3, a software
package created by Prokhorov [4], assuming diffusely
reflecting grey surfaces with emissivity 0.85. The
calculations have been done for the actual cavity-furnace
combination constituting the radiator, taking into account
the temperature profile T(x) of the furnace tube. The cavity
itself is assumed to be uniform in temperature.
More details are given in an accompanying paper [5]. Here
we confine ourselves to showing (Figure 2) that ∆T and 1-
ε(tot) in their variation with cavity aperture are
clearly correlated. The three pairs of data points
correspond - at increasing values of their coordinates- with
cavity apertures of 3 mm, 5.5 mm (interpolated) and 8 mm,
respectively ; similar correlations are observed between ∆T
and 1-ε(λ). A noticeable dispersion of these correlations
with T(x) is showing up for values of 1-ε(tot) above about
160 ·10 -6 i.e., within the present cavity-furnace
Temperature drop / mK
350
300
250
200
150
100
50
0
0 100 200 300 400 500 600
1-ε(tot) x 10 -6
Figure 2: Temperature drop ∆T vs. 1-ε(tot) for the high (squares)
and low (diamonds) profiles T(x)
configuration,for apertures above 3 mm, as demonstrated
in Figure 2.
Whether correlations such as shown in Figure 2 can be
modeled for a given cavity-furnace geometry and by this
allowing to deduce ∆T from direct measurement of T(x) via
the associated total emissivity ε(tot), or at least correcting
∆T to that corresponding with a reference distribution
T(x,ref), remains to be seen.
Conclusion
From the above it can be concluded that at high
temperatures heat exchange between cavity and furnace
precludes treating the cavity as a separate unit. It has to be
taken as an integral part of the cavity-furnace combination
making up the radiator in question.
Acknowledgements
We wish to thank Dr. Naohiko Sasajima of NMIJ for the
information on the furnace and its temperature distribution. This
study was partly supported via the European Commission Project
HIMERT, contract number: G6RD-CT-2000-00610.
References
[1] Fischer J. and Jung H. J., ‘Determination of the
thermodynamic temperatures of the freezing points of silver
and gold by near-infrared pyrometry’, Metrologia, 26,
1989, pp. 245-252.
[2] Yamada Y., Sasajima N., Gomi H., Sugai T.,
‘High-temperature furnace systems for realizing
metal-carbon eutectic fixed points ‘ TMCSI, 7, 2003,
pp. 965-990.
[3] Jimeno-Largo P., Yamada Y., Bloembergen P.,
Villamanan, M.A., Machin G., ‘Numerical analysis of the
temperature drop across the cavity bottom of hightemperature
fixed points for radiation thermometry’
Proceedings of Tempmeko 2004, to be published.
[4] Prokhorov, A.V., ‘Monte Carlo method in optical radiometry’ ,
Metrologia, 35, 1998, pp. 465-471
[5] Bloembergen P., Khlevnoy B.B., Yamada Y., Jimeno Largo P.,
‘High-temperature fixed-point radiators: The effect of the
heat exchange between cavity and furnace tube on
the effective emissivity of the radiator’. This conference.
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