$-
'''
Many flammable and toxic gases have absorption bands of infrared light.
Optical detectors for combustible gas detection provide an increasingly
popular alternative to traditional Pellistor-based combustible gas detectors.
This technology has undoubted
advantages such as:
-being virtually immune to
temperature and humidity variations;
-being immune to poisoning;
-offering a very high selectivity.
In some circumstances, like carbon
dioxide detection, the use of infrared
sensor is in practice irreplaceable.
IR absorption frequency or wave(Doc. MicroParts)
length corresponds to molecular
links resonance among different
atoms.
98
H2O
96
94
CO2
92
CO
90
4.4
4.5
4.6
CO
4.7
4.8
4.9
5.0
54
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'''
),)
In a specific wavelength, optical sensors take the attenuation difference
between a reference signal and the signal made by the light passing in the
substance to survey.
A source of light gives out a ray of light that gets to an optical receiver, through
a path fixed in advance and after being filtered. The source of light may be a
tungsten bulb, a diode LED or a IR source.
According to the type of substance to survey, the optical path will have a
different length as far as to use a multireflection system made with adequate
mirrors.
The sensor may be open or closed in a cuvette, with inlets and outlets for the
gas or the mixture to survey.
55
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'''
),)
The source of light is usually pulsating
as sensors require a modulated signal.
The reference sensor is usually
positioned in a IR band not influenced
by the presence of the gas.
The sensor may be photovoltaic,
photoconductive or pyroelectric.
Some optical element may be inserted
at the end of the optical path to protect
the sensitive part from corrosion.
(Doc. Draeger)
56
$-
'''
)),'
Infrared sensors may be used to survey the most of
combustible gas, except hydrogen, in every
concentration range up to 100% V/V. Oxygen has
no influence on the signal.
By selecting adequate wavelength and optical path
length, we can obtain apparatus for:
•
•
•
•
hydrocarbons total measure
selective measure of a single component in a mixture
ppm measure of small parts
measure up to 100% V/V
Sensors ranges and characteristics must be fixed on the basis of the application.
Compensation of environmental conditions, self-diagnosis and self-calibration
qualify the sensor and reduce the maintenance.
57
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'''
$,
Pressure variations have no
effects on the zero of the signal
but the sensitivity is directly
proportional to the pressure.
Knowing this sensitivity, it is
necessary to avoid pressure
alterations at gas entrance.
Fast temperature changes may
also causes unbalances but,
through a proper design, the
effect may be minimised. Other
gases interfering on the same
waveband may cause a
sensitivity worsening.
Absorptionbands of some gases.
Please use all this values for discussion only
Gas
Absorption in µm and cm-1
(Maxlmum or Range)
HC (Hydrocarbons)
3.4
2940
* 6–8
1666 – 1250
CO2 (Carbon dloxide)
4.22
2370
4.26
2347
15
670
N2O(Nitrous oxide)
4.45
2247
4.50
2222
* 7.7 - 7.9
1299 – 1266
CO (Carbon monoxide)
4.6
2174
4.7
2128
H2O (Water)
2.5 - 2.9
4000 – 3450
5–8
2000 – 1250
NH3 (Ammonia)
3.0
3333
* 6.15
1626
9 – 12
1111 - 633
SO2 (Sulfur dioxide)
4.0
2500
* 7.25
1380
* 7.40
1350
8.6
1163
8.8
1136
18 – 20
555 – 500
NOX (Nitric oxide)
* 5.3
1887
* 5.4
1852
NO2 (Nitrogen dioxlde) and others, all are not stable under common
conditions and can not be measured using simply equipments!
!
Estimated
Intensity
strong
middle
strong
strong
middle
middle
middle
weak
middle
middle
middle
strong
weak
middle
strong
very weak
strong
strong
middle
middle
weak
weak
weak
(*) Measurement disturbed by water vapour (!)
(Doc. MicroParts)
58