Here - PMOD/WRC

temperature of the cell. The obtained temperatures were
additionally corrected for diffraction loss, occurring at each
of the two precision apertures.
Results
Re-C
# melts average ± u Std. dev.
FD 17 2 2745.55 ± 0.31 0.45
FR 676 6 2746.15 ± 0.38 0.33
FR 800 10 2745.81 ±0.45 0.33
FR 900 3 2745.48 ± 0.51 0.25
TiC-C
# melts average ± u Std. dev.
FD 17 5 3031.22± 0.36 0.16
FR 676 11 3031.13± 0.44 0.22
FR 800 6 3031.00± 0.53 0.14
FR 900 1 3030.53±0.59 -
FD 17
@VNIIOFI
3031.6 ± 0.35 -
ZrC-C
# melts average ± u Std. dev.
FR 676 3 3153.86 ± 0.44 0.16
FR 800 6 3153.78 ± 0.53 0.47
FD 17
@VNIIOFI
3154.28 ± 0.40 -
Table 1: Temperatures in Kelvin for the measurements at
PTB and VNIIOFI.
The results presented in Table 1 are the first
thermodynamic temperature measurements of eutectic
fixed-point cells by absolute radiometry using filter
radiometers in the irradiance mode. The results obtained at
the VNIIOFI and the PTB all agree within their combined
uncertainty, indicating that the reproducibility of the large
irradiance cells in different furnaces measured with
different detectors is very good.
However, all three fixed-point cells showed poor plateaus
compared to fixed-point cells with smaller dimensions and a
3 mm aperture [7]. This is probably due to the larger
dimensions of the large VNIIOFI cells, which require a
uniform furnace temperature over a larger distance than
smaller cells. Additionally a regular ingot formation is more
difficult to achieve for with increasing cell dimensions. The
measurements can therefore not be taken representative for
the materials thermodynamic melting temperature.
Improvements to plateau shape and furnace
systems
In a recent study improvements of the melting and freezing
plateau of the VNIIOFI large area eutectic fixed-point cells
have been shown by enhancing the uniformity of the
furnace [8].
A good agreement in the melting temperature of small and
large eutectic fixed-point cells for an optimized furnace
setup has been shown [3]. Systematic methods to improve
the uniformity of a BB3200/BB3500 furnace have recently
been presented [9].
It was shown that the cavity is up to 10 times more
uniform during melt and freeze [8]. Such cavity uniformity
is important for applications in radiometry as it allows more
accurate spectral irradiance measurements in the ultraviolet.
Additionally, for irradiance measurements the
uncertainty due to diffraction at the apertures can be
reduced with the upcoming of the BB3500 furnaces by the
use of larger precision apertures of up to 6mm in diameter.
The BB3500 furnaces have larger inner diameters of up to
50 mm, and therefore can hold fixed point cells with a cell
aperture of up to 15 mm.
Conclusions
The thermodynamic temperature of Re-C, TiC-C, and
ZrC-C has for the first time be measured using absolute
filter radiometers in the irradiance mode. Such kind
calibrated fixed-points with large radiating area can serve as
absolute standards for radiance and irradiance in radiometry
and photometry. Due to the huge dimensions the
performance of the investigated cells was not ideal, but is
presently under further improvements using an ameliorated
furnace design.
Acknowldgement The authors would wish to thank Emma
Wooliams from the NPL for supplying the 3mm precision aperture.
References
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