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Introduction
48iQ Principle of Operation
48iQ Principle of
The 48iQ operates on the principle that carbon monoxide (CO) absorbs
infrared radiation at a wavelength of 4.6 microns. Gas Filter Correlation
Operation
(GFC) is a particular infrared technique that selectively measures light
absorption uniquely due to CO by the ratio of sample-absorbed light to a
filtered reference measurement. Light from a broadband infrared source
passes through a gas filter wheel alternating between N
and CO filled cells
2
and passes through a narrow bandpass interference filter before passing into
the volume containing sample gas. Light that passes through the N
cell is
2
absorbed by CO in the sample gas normally as the sample signal; light that
passes through the CO cell is already blocked where CO absorbs, and so is
unchanged by sample CO as the reference. The ratio of "sample" to
"reference" (S/R) is acquired at high speeds and corrects for light intensity
and other changes to achieve precision measurement. Because the filtering
is achieved with CO gas itself, the GFC technique is specific for CO.
The so-called "spectral resolution" of the technique is such that the
measured ratio is partly but not strictly linear as predicted by the well-
known "Beer's Law." The 48iQ uses an internally stored calibration curve
to accurately linearize the instrument output over any range up to a
concentration of 10,000 ppm.
Gas sample is drawn into the 48iQ through the rear panel of the analyzer
and through the optical detection system using an embedded intake pump,
as shown in
Figure
1–2. Gas may be ambient, or it may be delivered from a
more distant location using standard tubing connected to the rear panel.
The rear panel may be a simple inlet bulkhead, or it could utilize three
inlets controlled with solenoid valves: Conventionally, the three inlet
configuration is used to conduct independent zero and span reference
measurements.
In addition to the inlet configuration, options may be added, such as an
oxygen sensor or a "zero scrubber" that catalyzes CO to CO
to provide a
2
zero reference. Each of these operate within the basic gas intake pathway
described above.
Figure 1–2
shows the simplest case fluid schematic. Gas is introduced into
the "SAMPLE" port at near ambient pressures (we describe a flow bypass
later on that assures ambient pressure sampling). It is pulled by a single
stage pump through the 48 DMC Bench where CO is detected and then
through a constriction (a capillary in the "Flow Pressure DMC") which
regulates the flow intake to approximately 1 SLPM while monitoring and
maintaining ambient pressure on the optical bench side. Note there is also
a filter purge that bypasses the sampling but is also drawn by the pump
downstream of the constriction.
Figure 1–3
shows the example of a three
inlet configuration with zero span capabilities and a zero air scrubber.
Thermo Scientific
48iQ Instruction Manual
1-3
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