| Description: | The lens of the uplooking "IR thermometer" (a 10 micrometer pyrometer)
was replaced on 28 October 1996 at 16:18 GMT. At the time the sky was
heavily overcast (0.1-0.5 mm liquid water path, IR temperature =
281-285 K) with about 4 cm integrated water vapor. A comparison was
carried out against the Atmospheric Emitted Radiance Interferometer
(AERI) both prior to and subsequent to the lens change. The statistics
of the differences (IRT-AERI) are as follows:
Mean (K) Std Dev (K) No.
Before: 0.64 0.24 122
After: 0.35 0.20 89
In addition, the downward step change in the time series plot of the
temperature difference (IRT-AERI) at the time of the lens replacement
is obvious.
This suggests, but is not conclusive evidence, that the primary cause
of the differences between the IRT and AERI reported previously
(PIF no. P960809.2) may have been the weather-worn lens. However,
as this instrument has not been calibrated since it was deployed in
December 1993, there may also be calibration drift to contend with.
Arrangements are being pursued with NREL to check the calibration of
this instrument.
It is preferable to carry out the IRT vs AERI comparison for clear sky
conditions to be certain that both instruments are observing the same
scene at the moment when they make sky measurements. However, it is
not possible at this time of the year to carry out a clear sky
comparison because the integrated water vapor is generally less than
2.0 cm which means that the IR temperature is less than 223 K, the
lower limit of the D/A converter on the IRT.
On the positive side, the AERI and the IRT are within 10 m of each
other and have similar fields of view (about 2 degrees) so their
scenes should be comparable for clear or cloudy skies. |