Teledyne GFC-7000T - Trace CO2 Analyzer - manual - Manual (Page 205)

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer
Teledyne Analytical Instruments 187
9. THEORY OF OPERATION
The Model GFC 7000TA 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 - See Sections 6.7) as well as reported to the user via a
vacuum florescent display or a variety of digital and analog signal outputs.
9.1. Measurement Method
9.1.1. 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
-
αLc
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 GFC 7000TA, carbon
dioxide (CO
2
).
α
is the absorption coefficient that tells how well CO
2
absorbs light at the specific wavelength
of interest.
9.1.2. Measurement Fundamentals
In the most basic terms, the Model GFC 7000TA 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