Honeywell ACM 150 Manual To Installation, Operation, And Maintenance page 26

ACM 150
FTIR bench & gas cells
Figure 13. FTIR bench assembly
FTIR optics bench assembly
The Optics Bench is a Perkin Elmer Model Spectrum Two FTIR (Fourier Transform
InfraRed) analyzer. It is the heart of the ACM 150 monitor, providing the analytical
power to identify and quantify a vast array of gases. For more information, refer to
ACM 150 operating
The optics bench always includes a gas cell.
Gas Cells
Sampled air never flows through the FTIR Optics Bench. Instead, the samples flow
through a gas cell. The infrared beam from the FTIR Optics Bench passes through
a window on top of the bench where it is reflected by mirrors and directed into the
gas cell. The infrared beam passes through windows in the gas cell, where the gas
molecules in the sampled air absorb infrared radiation at wavelengths specific to
each gas type. The beam exiting the gas cell is focused by mirrors on the Optics
Bench to a detector, where the signal is collected for analysis.
Most ACM 150 applications are for TLV-TWA level monitoring, which require higher
sensitivity. The level of detection is at the low ppm and, sometimes, sub-ppm levels.
As the pathlength increases, more gas molecules come in contract with the infrared
beam. For this reason, the pathlength of the gas cell is directly proportional to the
detection sensitivity. The standard gas cell used in the ACM 150 monitor has a
pathlength of 5 meters. The infrared beam is focused and directed into the cell and
out to the detector by transfer optics, adjustable mirrors below the cell.
The beam enters and exits the gas cell through windows at the bottom It is reflected
back and forth by the internal mirrors at the top and bottom, making multiple passes
through the gas cell. The mirrors have a highly reflective gold coating.
The gas cells mounts to a plate that bolts to the FTIR Optics Bench.
principle.
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