LI-COR LI7200RS, Инструкция по эксплуатации

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Section 8. Theory of Operation
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The instrument’s grammar also makes the individual values ( S
c
and S
w
) available
for output.
Delta Signal Strength
Why Delta Signal Strength? The concern when checking a source strength dia-
gnostic is “Is the optical cell clean?” However, the bigger underlying concern is
“Has the calibration drifted enough to matter?” Signal Strength addresses the first
question, but can say nothing about the second.
Calibration shifts will occur due to contamination if the contaminants are not spec-
trally neutral. That is, if they scatter radiation more (or less) at the reference
wavelength than the measuring wavelength. If contamination is spectrally neutral,
you could block nearly all the signal (e.g. Signal Strength drops to 10) without
affecting the calibration. However, most (all?) contamination is not spectrally neut-
ral, and even worse, if it is highly spectral, very little of it (i.e., small change in Sig-
nal Strength) can have a large effect on your calibration.
Delta Signal Strength, since it is the difference in signal strength between the two
reference wavelengths, provides an indicator of how spectrally non-neutral con-
tamination is. In fact, if the contamination’s scattering function is linear over the
range of wavelengths used by the LI-7200/RS (2.4 to 4.2 microns), then Delta Sig-
nal Strength would directly predict the calibration shift. If it is non-linear, however,
it would over- or under-predict. As an approximate guide, however, the following
relations are offered:
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Note: these assume contamination with a linear spectral response. As a practical
matter, you should experiment at your installation to see if there is a consistent rela-
tion between Delta Signal Strength and calibration shift.
8-10 LI-7200RS Enclosed CO₂/H₂O Analyzer