SHB_U14 Chapter 3: Module components 3-7
Sample cell
Location in module
The sample cell is installed between the main frame with modulator and the
infrared detector or calibration unit.
Structure
The cell consists of an aluminum tube available in two versions:
• chemically polished
• and gold plated.
A land divides the cell into a sample chamber and a reference chamber. The
chambers are sealed with infrared-transparent windows on both sides. The
reference chamber is normally filled with N
2
. The sample chamber can receive a
flow of sample gases via two gas ports. The gas ports are designed for stainless
steel or plastic tubing connections. Positioning pins secure the sample cell in the
beam path. For design reasons, with small cell lengths there are additional
chambers in the sample cell sample and reference beam paths. These are filled
with N
2
.
Function
Cell length differs according to the measurement range involved.
Since the infrared detectors are supplied for each component with a fixed signal
yield value, according to the LAMBERT-BEER law, an optimal absorption setting
must be established in the sample cell for the measurement range scale selected.
Infrared radiation absorption depends on the concentration to be measured
(measurement range) and the optical path length.
The optical path length is the distance between the two limiting windows in the
sample and reference chamber.
The two surface treatment options (chemically polished and gold-plated) have the
following functions:
• Optimal beam reflection
• Protection against corrosion of the cell
• Chemically polished cells provide high signal stability, especially when measuring
flue gases
Reflective characteristics are considered and equalized during the optical
alignment and calibration.
For specific applications there are special sample cells:
• Sample cells with flowing reference gas
Here the reference chamber receives a flow of gas (e.g. sample gas without the
sample component).
Result: Absorption in the sample and reference chamber of that portion of the
beam in the interference gas absorption lines. This eliminate the cross-sensitivity
effect.
∆-measurements represent another application.
• Sample cell with integrated filter cell (sample/filter cell)
Here, both filter chambers (sample and reference sides) are filled with the
interference gas component.
Here the portion of the beam in the interference gas line range is absorbed in
order to suppress the interference gas effect (cross-sensitivity).
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