Teledyne GFC-7001E - Trace CO Analyzer - Manual (Page 230)

Theory of Operation Model GFC7001E Carbon Dioxide Analyzer
Teledyne Analytical Instruments 230
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 11-1: Absorption Path Lengths for GFC 7001E and GFC 7001EM
MODEL
TOTAL NUMBER OF
REFLECTIVE PASSES
DISTANCE BETWEEN MIRRORS
TOTAL
ABSORPTION LIGHT
PATH
GFC 7001E 32 437.5 mm 14 Meters
GFC 7001EM 8 312.5 mm 2.5 Meters
IR
Source
IR Beam
Sample Chamber
Band-Pass Filter
Photo-Detector
Figure 11-1: Measurement Fundamentals
Upon exiting the sample cell, the beam shines through a band-pass filter that allows only light at a wavelength of
4.7 µm to pass. Finally, the beam strikes a solid-state photo-detector that converts the light signal into a
modulated voltage signal representing the attenuated intensity of the beam.
11.2.1. GAS FILTER CORRELATION
Unfortunately, water vapor absorbs light at 4.7 µm too. To overcome the interfering effects of water vapor the
GFC 7001E/EM adds another component to the IR light path called a Gas Filter Correlation (GFC) Wheel.
Measurement Cell
(Pure N
2
)
Reference Cell
(N
2
with CO)
Figure 11-2: GFC Wheel