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Theory of Operation Model GFC7001E Carbon Dioxide Analyzer
Teledyne Analytical Instruments 229
11. THEORY OF OPERATION
The GFC 7001E/EM Gas Filter Correlation Carbon monoxide Analyzer is a microprocessor-controlled analyzer
that determines the concentration of carbon monoxide (CO) in a sample gas drawn through the instrument. It
requires that the sample and calibration gases 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 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 (iDAS - See Sections 7.1) as well as reported to the user via a vacuum
florescent display or a variety of digital and analog signal outputs.
11.1. MEASUREMENT METHOD
11.1.1. 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 11-1
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 GFC 7001E/EM, Carbon Monoxide (CO).
α is the absorption coefficient that tells how well CO absorbs light at the specific wavelength of interest.
11.2. MEASUREMENT FUNDAMENTALS
In the most basic terms, the GFC 7001E/EM 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 a number of times (see Figure 11-1).
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.