THEORY OF OPERATION
Model 300E Carbon Monoxide Analyzer
Instruction Manual
156
P/N 04288 Rev: A
10.1. Measurement Method
10.1.1.
Beer’s Law
The basic principle by which the analyzer works is called Beer’s Law. It defines the
how light of a specific wavelength is absorbed by a particular gas molecule over a
certain distance. The mathematical relationship between these three parameters
is:
I = I
o
e
-
a
Lc
Where
:
I
o
is the intensity of the light if there was no absorption.
I
is the intensity with absorption.
L
is the absorption path, or the distance the light travels as it is being absorbed.
C
is the concentration of the absorbing gas. In the case of the Model 300E, carbon
monoxide (CO).
α
is the absorption coefficient that tells how well CO absorbs light at the specific wavelength
of interest.
10.1.2.
Measurement Fundamentals
In the most basic terms, the Model 300E uses a high energy heated element to
generate a beam of broad-band IR light with a known intensity (measured during
Instrument calibration. This beam is directed through multi-pass cell filled with
sample gas. The sample cell uses mirrors at each end to reflect the IR beam back
and forth through the sample gas to generate a 14 meter absorption path (see
Figure 10–1). This length was chosen to give the analyzer maximum sensitivity to
fluctuations in CO density.