144 Theory of Operation 083730300A DCN8101
6.1.1 BEER’S LAW
The basic principle by which the analyzer works is called the Beer-Lambert Law or
Beer’s Law. It defines 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
-αLc
Equation 6-1
Where:
I
o
is the intensity of the light if there was no absorption.
I is the intensity with absorption.
e is the mathematical constant (Euler’s number)
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 T300/T300M, Carbon
Monoxide (CO)).
α is the absorption coefficient that tells how well CO absorbs light at the specific
wavelength of interest.
6.2 MEASUREMENT FUNDAMENTALS
In the most basic terms, the T300/T300M 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 a 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 a number of times (see Figure 6-1).
The total length that the reflected light travels is directly related to the intended
sensitivity of the instrument. The lower the concentrations the instrument is designed to
detect, the longer the light path must be in order to create detectable levels of attenuation.
Lengthening the absorption path is accomplished partly by making the physical
dimension of the reaction cell longer, but primarily by adding extra passes back and forth
along the length of the chamber.
Table 13. Absorption Path Length
MODEL
TOTAL NUMBER OF
REFLECTIVE PASSES
DISTANCE BETWEEN
MIRRORS
TOTAL
T300 32 437.5 mm 14 Meters
T300M 8 312.5 mm 2.5 Meters
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