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7. PRINCIPLES OF OPERATION
The Model T360 Gas Filter Correlation Carbon Dioxide Analyzer is a
microprocessor-controlled analyzer that determines the concentration of carbon
dioxide (CO
2
) in a sample gas drawn through the instrument. It requires that
sample and calibration gasses be supplied at ambient atmospheric pressure in
order to establish a stable gas flow through the sample chamber where the gases
ability to absorb infrared radiation is measured.
Calibration of the instrument is performed in software and does not require
physical adjustments to the instrument. During calibration the microprocessor
measures the current state of the IR Sensor output and various other physical
parameters of the instrument and stores them in memory.
The microprocessor uses these calibration values, the IR absorption
measurements made on the sample gas along with data regarding the current
temperature and pressure of the gas to calculate a final co
2
concentration.
This concentration value and the original information from which it was
calculated are stored in one of the unit’s internal data acquisition system (DAS -
Section 4.7), as well as reported to the user via a vacuum fluorescent display or a
variety
of digital and analog signal outputs.
7.1. MEASUREMENT METHOD
7.1.1. 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
-
α
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 T360,
carbon dioxide (CO
2
).
α is the absorption coefficient that tells how well CO
2
absorbs light at the specific
wavelength of interest.
07272B DCN6552