WMA-5 Operation Manual V. 1.01
9
Summary of System Design
Overview and Theory
The WMA-5 is designed to function as a self-contained instrument for continuous measurement of CO
2
in
air. Its open-path design allows for continuous, unattended air sampling, as the pump introduces fresh
sample gas to the essential component, the IRGA (infrared gas analyzer).
The IRGA forms the core of gas analysis systems that measure CO
2
. Non-dispersive infra-red (NDIR)
refers to the transmission of a broad-band infra-red wavelength from the IRGA source lamps. A single
IRGA consists of four basic components:
•
Infra-red source
•
Sample cell of known path length and volume
•
Optical interference filter
•
Infra-red detector
The theory itself is quite simple – light from mid-infra-red wavelengths is produced by the source and
pulsed through a gold plated cell. The interference filter narrows the bandwidth of the IR source received
by the detector to the signature wavelength absorbed by the target gas molecule, e.g. CO
2
. The CO
2
cell
employs a unique optical filter. As the sample gas fills the cell, the target gas molecule absorbs IR
energy at the particular wavelength, and the reduction in IR energy reaching the detector is measured.
The
higher
the target gas concentration, the
lower
the infra-red signal received at the detector, as defined
by the Lambert-Beer Law of Attenuation.
CO
2
molecules have a discrete absorption band at 4.26 µm that has very little overlap with any other
molecule’s absorption band, so that wavelength provides good sensitivity and selectivity. The WMA-5
electronics could be considered another major component, which processes raw analog-to-digital (A/D)
information from the IRGA detector, accurately translating this information into gas concentrations.
