Here - PMOD/WRC

As the correction for atmospheric absorption is so
significant, and it was not clear if the accuracy of the
LOWTRAN calculations was sufficient for metrology
purposes, experiments were conducted to confirm its
predictions. Ideally one would compare the total detected
flux a distance from a blackbody source through normal
atmospheric and inert atmospheres. However, two
difficulties with purging the total path were anticipated:
firstly, the heavy oxide coating on the Inconel heatpipe is
not stable in a neutral atmosphere, and secondly, the
thermopile sensitivity may be slightly different under
different atmospheres. We arranged to be able replace a
part of 830 mm atmospheric path by a dry nitrogen
atmosphere, leaving the heatpipe and thermopile in air. A
450 mm purge tube between the blackbody and thermopile
with 15 mm apertures at either end could be flushed with
dry nitrogen, whilst at either end small fans were used to
ensure nitrogen escaping through the apertures was blown
out of the optical path. Automatic cycling with a 5 minute
period between dry N 2 and air in to tube modulated the
detected thermopile signal allowing a clear measure of the
absorption. Figure 3 shows a comparison of the measured
and calculated absorption. The error band on the calculated
data is due to uncertainties in the knowledge of the length
of the effective air path, due to the hot air region in the
heatpipe mouth and the ends of the purged tube (10 mm at
each end). The measured and computed values agree
within the measurement uncertainties.
Uncertainty analysis
The dominant uncertainty sources are the blackbody
temperature measurement and uniformity, the temperature
rise of the aperture plane and the uncertainty in effective
atmospheric path length. Diffraction corrections are
negligible, 0.04% at 20 µm (assuming a 20 mm aperture
200 mm from a 50 mm blackbody). Together these give a
2σ uncertainty of 0.3% over 500-1700 o C. Experiments
using aperture-thermopile spacings of 300-600 mm, and
with 10, 15 and 20 mm apertures give results agreeing well
within these bounds.
Conclusions
A new facility for characterizing the response of heat-flux
sensors to blackbody irradiance at a range of source
temperatures has been established, with uncertainties as
low as 0.3%. The new facility will be used to examine IR
spectral sensitivity effects in heat-flux sensors.
Acknowledgments The author would like to acknowledge Mr
Chris Freund, who designed the experimental apparatus, and Mr
Mark Darlow, who performed to precision machining and
construction.
References
[1] Grosshandler, W.L., Heat Flux Transducer Calibration:
Summary of the 2 nd Workshop, NISTIR 6424, NIST
Washington, 1999
[2] CSIRO report RS22350, KB33/NML-1/13, 1992
[3] Ballico, M.J.,The CSIRO-NML Radiation thermometer
Calibration Facility, in proc, Tempmeko 2004, Dubrovnik
[4] M.Ballico, A simple technique for measuring the infrared
emissivity of blackbody radiators, Metrologia, 2000, 37,
295-300
[5] M.Ballico,Modelling the effective emissivity of a
graphite tube blackbody, Metrologia 1995/6,32, 259-265
Figure 1. Cross section and image of the water cooled low
reflectance plate for stray IR control and holding the precision
aperture and annular light trap.
Transmission
100%
95%
90%
85%
80%
75%
2000 C
1500 C
800 C
500 C
0 0.5 1 1.5 2 2.5 3
(Path length /m )^0.5
Figure 2. Calculated integrated absorption as a function of
distance for 50%RH at 21 o C and a range of source radiance
temperatures.
atmospheric absorption
4.00%
3.00%
2.00%
1.00%
0 C
1000 C
0.00%
400 600 800 1000 1200 1400 1600 1800
blackbody temperature deg.C
measured
LOTRAN
Figure 3. Comparison of measured and calculated absorption
between over 450 mm of a total 830 mm air path (50% RH,
21 o C)
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