Thermo Scientific 410iQ, Instruction Manual

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Introduction
410iQ Principle of Operation
Thermo Scientific 410iQ Instruction Manual 1-3
The 410iQ operates on the principle that carbon dioxide (CO
2
) absorbs
infrared radiation at a wavelength of 4.2 microns. Interference filters are
selected to measure specifically: 1) the 4.2 micron region where CO
2
strongly absorbs (the sample measurement); and 2) a nearby non-absorbing
wavelength where there is no absorbance by CO
2
or other interfering gases
(the reference measurement). Coupled with a broadband light source, the
filters are arranged within a rotating filter wheel assembly so that sample
and reference filtered light passes through a sampling volume (the “optical
bench”) and are then detected. Where CO
2
is present in the sample
volume, the sample light will be absorbed and attenuated, while the
reference light is unchanged. The ratio of sample light to reference light
(“S/R”) is acquired at high speeds and corrects for light intensity and other
changes to achieve a precision measurement.
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 410iQ uses an internally stored calibration curve
to accurately linearize the instrument output over any range up to a
concentration of 10,000 ppm (Standard) or 25% (High Level).
Gas sample is drawn into the 410iQ through the rear panel of the analyzer
and through the optical bench 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 delivered 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, the 410iQ also has a bulkhead port for introducing CO
2
free
air or N
2
to purge the housing that contains the light source and filter
wheel – this filter wheel purge feature is necessary to remove interfering
CO
2
absorption due to ambient air, which carries CO
2
concentration close
to 400 ppm.
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 410iQ Bench DMC where CO
2
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 also the filter purge for CO
2
-free gas to be also introduced from the
rear panel. Figure 1–3 shows the example of a three inlet configuration
associated with the zero span valve option.
410iQ Principle
of Operation