Teledyne T100, Instruction Manual, Page: 233 / 404

Teledyne ML - T100 UV Fluorescence SO2 Analyzer Theory Of Operation
06807A(DCNxxxx) 233
9.3. CARBON DIOXIDE (CO
2
) SENSOR THEORY OF OPERATION
The CO
2
sensor probe measures the concentration of carbon dioxde in the sample gas; a
Logic PCA conditions the probe output and issues a 0-5 VDC signal to the analyzer’s
CPU, which computes the CO
2
concentration by scaling the values of the CO
2
_SLOPE
and CO
2
_OFFSET recorded during calibration
The CO
2
sensor assembly itself does not have any serviceable parts and is enclosed in an
insulated canister.
9.3.1. NDIR MEASUREMENT OF CO
2
The optional 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.
FPI
IR
Detector
(NDIR)
Signal
Out
Signal
Out
IR Light
Source
Sample
In
Sample
Out
(Masure Phase)
FPI
(Reference Phase)
Figure 9-8: CO
2
Sensor Theory of Operation
The sensor computes the ratio between the reference signal and the measurement signal
to determine the degree of light absorbed by CO
2
present in the sensor chamber. This
dual wavelength method the CO
2
measurement allows the instrument to compensate for
ancillary effects like sensor aging and contamination.