1.3
Non-Dispersive Infrared Sensor for Carbon Dioxide
Alpha Omega Instruments uses a non-dispersive infrared (NDIR) sensor to determine carbon dioxide
concentrations in gases. Infrared gas sensors, which are comprised of solid state devices, do not
chemically react with the gas.
Infrared radiation is a part of the electromagnetic radiation spectrum which also contains visible and
ultraviolet radiation. Since electromagnetic radiation is wavelike, infrared ra diation has wavelengths
ranging from 0.8 to 100 microns. Gas molecules are made up of atoms which are bonded together.
These bonds constantly undergo vibrations and rotations. The frequencies of these vibrational and
rotational motions are a strong function of the size of the atoms and bond strengths. By nature, these
frequencies match with the frequency of the middle portion of the infrared spectrum (called mid IR). When
exposed to a beam of infrared radiation, most gas molecules will absorb IR radiation at their
vibration/rotation frequencies. The unique structure of each compound means that it will have a unique IR
fingerprint which can often be used to identify it with an IR instrument.
The ability of certain gases to absorb infrared radiation has been successfully utilized in developing
instruments for gas sensing. An infrared gas sensor consists of an infrared source (emitting broad band
radiation including the wavelength absorbed by the target gas) and an infrared detector that are separated
by a gas cell. In non-dispersive IR (NDIR) sensors, an IR source and an IR detector are separated by a
gas sample cell. An optical “band pass” filter is placed either in front of the source or the detector to
screen out all radiation except for the wavelength that is absorbed by the target gas.
The characteristic output of the sensor element is called “absorption” and is the percentage loss between
the IR radiation that reaches the detector with and without the target gas in the gas cell. The absorption is
calculated for the infrared signals measured under zero gas (gas that does not have infrared absorption,
i.e. Nitrogen) l
o,
and under the gas of interest l
G
using the relationship:
A
=
l
O
−
l
G
L
O
Absorption increases with increasing gas concentration and increasing optical path length between the
detector and the source. Knowing the dependence of absorption on the gas concentration for a given path
length, a sensor will measure an unknown gas concentration based on the measured absorption.
A simple, single channel sensor interprets that any loss in infrared signal is due to increasing absorption
by the target gas. Therefore, the sensor design should ensure that the signal loss is not due to any other
means such as infrared source or detector deterioration, changes in the optical assembly, or temperature
variations. In practice, a reference channel is added to the sensor to correct for these potential problems.
A reference channel is employed to optically measure change in infrared radiation that are not caused by
the gas being analyzed.
Alpha Omega Instrument’s infrared gas sensor design utilizes a “virtual reference” channel at the
absorbing wavelength that eliminates the need for locating a non absorbing infrared region for the
reference channel. The optical design and the absorption theory are described in detail as follows.
The sensor is comprised of two infrared sources (Lamp 1 and Lamp 2), one infrared detector, and a
narrow band optical filter that passes infrared radiation only at the wavelength absorbed by the gas of
interest. For example, carbon dioxide (CO
2
) is absorbed at a wavelength of 4.2 microns. Lamps 1 and 2
are placed in the gas cell at distances L
1
and L
2
respectively from the detector. When a gas concentration
C is present in the cell, it will absorb the infrared radiation emitted by both the sources according to Beer’s
Law.
.
ALPHA OMEGA INSTRUMENTS CORP.
SERIES 9510 CARBON DIOXIDE ANALYZER INSTRUCTION MANUAL
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