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12. PRINCIPLES OF OPERATION
The T801 analyzer is a microprocessor-controlled analyzer that determines the
percent concentration of carbon dioxde (CO
2
) in a sample gas drawn through the
instrument. The carbon dioxide
measurement is achieved using infrared
absorption technology.
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 sensor outputs and various other physical
parameters of the instrument and stores them in memory. The microprocessor
uses these calibration values, measurements made on the sample gas along with
data regarding the current temperature and pressure of the gas to calculate the
final concentrations.
12.1. NDIR MEASUREMENT OF CO
2
The CO
2
sensor is a silicon based Non-Dispersive Infrared (NDIR) sensor. It uses
a single-beam, dual wavelength measurement method.
An infrared source at one end of the measurement chamber emits IR radiation
into the sensor’s measurement chamber where light at the 4.3 μm wavelength is
partially absorbed by any CO
2
present. A special light filter called a Fabry-Perot
Interferometer (FPI) is electronically tuned so that only light at the absorption
wavelength of CO
2
is allowed to pass and be detected by the sensor’s IR detector.
A reference measurement is made by electronically shifting the filter band pass
wavelength so that no IR at the CO
2
absorption wavelength is let through.
07274B DCN6418