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Infrared Technology for
Detecting Combustible Gases
Delphian manufactures a number of systems which incorporate
infrared sensors including the
Determinator and the
Remediator These systems are available with
analog
controllers, digital controllers, as
standalone
systems or attached to our
SAGE system - our
computerized gas monitoring system.
(The following article is reprinted with permission
from
THE GAS MONITORING HANDBOOK
by Gerald L. Anderson & David M. Hadden,
published by Avocet Press Inc in
1999)
For a less
technical explanation:
Radiation in the wavelengths that the human eye can see is very
limited. This radiation is called visible light. There are, however, a number of
elec-tronic detectors that can detect wavelengths much longer and much shorter
than is visible. In the visible range red is the longest visible wavelength and
therefore light just beyond this is called infrared. Violet is the shortest
visible wavelength. Light just shorter than visible is called ultraviolet.
The Nature of Infrared Radiation
Radiation is simply a field in space which has characteristics of frequency,
velocity and power. This radiation is usually described in terms of its
properties, using three models:
Wave Model
Refraction, dispersion,
and similar effects are based on radiation behaving as an oscillating
electromagnetic field which travels through space at the speed of light. Most
methods of measuring or describing light use the wave model.
Particle Model Radiation can also act like a
stream of discrete particles of energy. These particles are called photons. It
its the photon model that accounts for most detector reaction and is the basis
for the radiation’s interaction with matter.
Ray Model Light traveling from a light source
through air, lenses, or other transparent substances and undergoes reflections
at mirrors is completely described as rays. The wave model is unimportant in the
description of imaging by lenses.
When radiation acts as a wave, it can be classified in terms of
wavelength or frequency. The common ways to measure or describe infrared
radiation are:
Wavelengths
Wavelength is the most common measurement for IR radiation. Wavelength is
the distance a wave travels during one period of oscillation, or stated
differently the distance between adjacent crests or adjacent valleys of one
wave. For infrared radiation, this distance is measured in micrometers (microns
or µm) or nanometers (nm). For simplicity we will use wavelengths in microns to
refer to IR energy.
Frequency
Frequency of infrared radiation is often measured in Wavenumbers. A
Wavenumber (u) is the number of wavelengths per centimeter and has reciprocal
centimeters (cm-1) as the unit of measure. For instance, 5.0 microns corresponds
to 2000 reciprocal centimeters. For historical reasons this is the preferred
unit in the science of Spectroscopy which investigates the interaction of light
with matter. Although not used as often, IR radiation can also be measured by
its time-frequency, the number of oscillations in one second, called Hertz (Hz).
For instance, 5.0 microns corresponds to 6 x 1013 Hertz (= 60,000,000,000,000
per second). From this example it can be seen that cycles-per-second can be a
rather large, unwieldy number.
There is a reciprocal relationship between frequency (or wavenumbers) and
wavelength. As the frequency of the wave decreases, the length of the wave
increases. Both are linked by "c," the speed of light, which is a
universal constant of 3 x 108 m. In fact, if we multiply a given wavelength with
its frequency in Hertz we obtain the speed of light.
Energy
Electromagnetic radiation also has energy and power. One
photon carries a very small amount of energy. As an example, one single photon
of 5 micron radiation contains:
0.000,000,000,000,000,000,04 Watt-sec = 4 x 10-20 Ws.
The energy of a photon is proportional to its frequency (wavenumber) and
inversely proportional to its wavelength.
Infrared radiation is part of a broad spectrum of waves called the
electromagnetic spectrum (Figure 7). This spectrum encompasses very short waves
(cosmic rays) to light waves (ultraviolet, visible, and infrared) to very long
(heat, radio waves, and AC electricity). Like visible light, infrared radiation
represents only a very small portion of this electromagnetic spectrum. The
primary area of interest for gas detection is the mid-infrared region which is
generally defined as being 2.0 to 50 microns. Gas detection results from the
interaction of this electromagnetic radiation with chemical matter.
Our environment shows color because of the selective absorption and reflection:
A leaf appears green because much of the blue and red in the sun light that
illuminates the leaf have been absorbed. Absorption of radiation has many
different effects on substances depending on the wavelength of the radiation.
Very short wave radiation, such as X-rays and cosmic rays penetrate into the
core of an atom and can cause ionization or even nuclear changes. Radiation that
is closer to the visible portion is capable of breaking up large molecules into
smaller fragments.
How Infrared Radiation Interacts with Matter
When infrared light strikes a substance, the radiation is
transmitted, reflected or absorbed in varying degrees, depending upon the
substance and the wavelength of the radiation. Since radiation is a form of
energy, absorption of a photon by a molecule results in an increase in the
energy content of the molecule. The absorbed energy causes an increased level of
vibration or rotation. We are all familiar with the transfer of energy from one
form to another, such as excitation of a bell resonance by a hammer strike. At
the atomic and molecular level, however, there are some significant differences
to our everyday experiences: When a molecule is excited by light, the energy of
the light is used up or absorbed by the molecule. Each molecule can vibrate and
rotate in certain patterns, and for each pattern there is an associated amount
of energy of motion. A molecule can only absorb energy from a photon if the
energy matches precisely such an "energy state" of that molecule.
Inert gases [He, Ne, Ar, Kr, Xe, Rn] or diatomic molecules composed of like
atoms such as hydrogen (H2), oxygen (O2), chlorine (Cl2), and nitrogen (N2) only
oscillate in high energy states. Consequently they can not absorb infrared
radiation. They do absorb high energy radiation, such as ultraviolet and X-rays.
They are said to be "transparent to infrared" or "infrared
inactive." More complicated molecules like carbon dioxide (CO2) or methane
(CH4) exhibit oscillation modes that match the energy in infrared radiation. The
oscillation modes might be stretching and bending motions.
It bears to stress again that photons interact with gases only if their
energy matches the energy difference needed to "lift" the oscillation
mode of a molecule from its present state into another proper oscillation mode.
For many gases there are a large number of photon energies in the mid-infrared
range that can be absorbed by the gas molecules: Each molecule can bend,
stretch, or twist in a large number of degrees. Yet, the energy match must be
fulfilled, or the radiation will pass through the gas unattenuated.
Each gas exhibits a very specific set of absorption
wavelengths which depend on the strength of the chemical bonds between the atoms
that make up the molecule. Change one element of the molecule and the absorption
wavelengths will also change. It is this selective absorption of radiation which
forms the basis for detecting a gas and for measuring its concentration. We call
the gas-characteristic wavelength of absorption also the "absorption
band."
Essential Components
The essential components of an IR gas detection
system are: Source of infrared radiation, Detector capable of seeing the
radiation, Path between the detector and source open to the gas to be detected.
Depending upon the design, the IR gas detection system may also need assorted
light filters, choppers, mirrors, lenses, or other optical elements.
Ideally a gas detector can just observe the
light at the wavelength of the absorption band of the gas of interest (target
gas). As long as the air does not contain any of the target gas the light level
is constant. If the target gas enters the light path the light level drops, and
the magnitude of the light level drop serves as a measure of the amount of
target gas in the light beam.
Unfortunately the infrared gas detector world is not ideal. The light emitted
into the gas detection volume varies with the age of the light source, the
electrical supply, and various other influences. Furthermore the light detector
also is prone to errors that need to be compensated. A real world infrared gas
detector is more complicated than the ideal sketched above and it comprises the
following building blocks:
Chopping the Signal
To mitigate electronic drift, the detector electronics measure
the difference between dark (no light hitting the detector) and light (full
energy hitting the detector). To achieve this effect, light between the source
and the detector is chopped so that the detector’s electronics can clearly
differentiate between full light and no light. When gas in the path absorbs
energy from the source, the detector receives less radiation than it normally
would during the "light" phase. This reduction in radiation is used to
measure the gas concentration. Chopping is usually accomplished by a mechanical
device or electronically, such as turning the source on and off. There are
advantages and disadvantages to both approaches.
Reference Signal
Even with a chopped signal, there are a number of factors
which could cause a device to measure incorrectly. Changes in detector
sensitivity or the source strength could cause a device to miscalculate. As a
result, most designs have a reference channel to monitor system integrity. This
reference is often a second detector and/or a second source which verifies the
strength of the full light signal from the source. As a result of chopping the
signal and incorporating a reference, IR devices are continuously checking their
operation and compensating for slow changes that are independent of the target
gas detection.
Path Length
Radiation from the source can be considered a beam of photons.
The Beer-Bouguer Law states that the number of photons absorbed is directly
proportional to the power of the photon beam and to the amount (number of
molecules in the beam) of the gas to be detected. Therefore, the length of the
path between the source and the detector can be a major determiner of the gas
concentration range the instrument can detect. The longer the path length, the
more molecules of target gas will be between the light detector and the source,
and the more molecules, the greater will be the absorption for small
concentrations of gas. Conversely, with a longer path nearly all of the light
may be absorbed before the gas concentration reaches the desired maximum gas
concentration range and the instrument would be saturated. As can be surmised,
the ideal path length is determined by the maximum concentration of gas that the
instrument is designed to detect.
Selective Absorbance
As discussed earlier, one of the chief attributes of infrared
absorption is its outstanding specificity. If only light of the proper frequency
hits the detector, then any absorption will be caused by the gas to be detected.
However, most sources produce radiation over a broad spectrum and most detectors
see energy radiation over a broad spectrum. To be selective, therefore, energy
from the source must be limited so that the detector sees mainly photons which
will be absorbed by the target gas. Filters at the source and/or the detector
are the primary means of selectively limiting the wavelength. This is called
non-dispersive infrared.
Unfortunately, detectable gases do not have only one light frequency that will
cause them to absorb. Most have a great number of absorption peaks, each of
which is of varying strength and varying width. Furthermore, the absorption
peaks are substantially narrower than the best light filters available today. In
many cases there are multiple gases that absorb light within any one light
filter pass band. As a result, designers seek to find an absorption point for
the gas of interest which is strong enough to be seen and which will not also be
shared by a gas which could cause false readings. If the wrong detection channel
is chosen, other gases will interfere and cause erroneous signals. If the wrong
wavelength for the reference channel is chosen, gases in the area could disturb
the calibration of the device and cause it not to misregister the target gas.
Light Detectors
There are a great variety of light detectors which can measure
radiation in the mid-IR range. It goes without saying, that each has its own
problems and benefits. There is no perfect detector. It is how the designer uses
the benefits of the detector and copes with its problems that can cause one
instrument to be more successful than another in any given application.
Detectors can be grouped into several categories depending upon their mechanism
of operation:
Thermal Detectors Thermal detectors such as
thermopiles, thermocouples or pyroelectric detectors operate by changing
temperature when struck by a photon. The change in temperature results in an
alteration of the detector’s electrical properties which can be measured. Such
detectors can be built for very wide spectral ranges.
Photoconductive Quantum Detectors Quantum
detectors such as lead salt (Lead Selenide or Lead Sulphide) detectors are
excited directly when struck by a photon. This excitation can be measured as a
decrease in resistance.
Photovoltaic Quantum Detectors Photons striking
photovoltaic detectors cause a voltage at the detector terminals. A number of
detectors can be used either as photoconductors or as photovoltaic generators,
depending on the electronic amplifier circuit.
Pneumatic (Photoacoustic) The gas to be detected
is trapped in an enclosed chamber. When a photon is absorbed by the gas it
causes a rise in the temperature of the gas and a corresponding increase in gas
pressure. A sensitive microphone is used to pick-up the pressure fluctuation.
Operational Considerations
There are a number of specialized factors which must be taken
into account in designing an infrared gas detector or any gas detector for that
matter. Some of the most important are:
Temperature Most infrared light detectors are
very sensitive to temperature, with cold being the preferred temperature. In the
presence of heat they lose sensitivity and/or drift depending upon the
circumstances.
Humidity Humidity is often a major interference
with infrared systems. Water vapor is transparent to infrared from 3 to 4.6
Micron but shows significant absorption outside this band in the mid-infrared
range. This absorption appears like gas to the detector and can cause false
readings. In addition, water vapor can condense on the optics or in the path and
cause the beam to be deflected or diffracted so that erroneous reading or
instrument failure can occur.
Pressure Infrared systems are also affected by
changes in pressure. As pressure increases more molecules are packed into the
path and therefore more infrared radiation is absorbed. At lower pressures,
therefore, less radiation is absorbed for the same volume of gas. As a result,
when there is a sudden change in barometric pressure, infrared instruments often
produce erroneous absolute partial pressure readings.
Therefore if a weather system comes through which changes the pressure,
temperature and humidity, infrared systems often operate poorly.
Selectivity
The selective absorption of infrared, one of its primary
benefits, can also be one of IR gas detector's primary problems. For instance, a
catalytic sensor can detect all combustible gases, however most IR devices can
only detect the gas it was designed to detect. In most cases this is methane and
a few other combustible (and some not combustible) gases which happen to share
the absorption wavelength band. The only way to solve this selectively problem
is to use multiple detectors in a device and/or use multiple devices, each
intended to detect a particular gas. Of course, if methane (or any other single
gas) is the only gas that can be present at a sensor location, selectivity will
not be a problem.
Open Path vs. Point Sensors
For fixed point products, the ideal path length can be
engineered into the product. As an example, a methane detector designed to
detect 100% LEL, would have a path length of four to seven inches. However, for
open path products, the larger path lengths used often mean that the product is
saturated well before reaching 100% LEL. For instance, with a path of 5 feet, an
IR detector might saturate at 10% LEL of methane before flooding, assuming a
constant methane/air concentration everywhere between the light source and the
light receiver.
In addition, open path detectors can see sources of infrared
other than the one chosen by the manufacturer. For instance, solar radiation,
hydrocarbon flames, even flash bulbs, produce broad spectrum infrared radiation
which could trick the detector, unless countermeasures are provided in the
design.
What To Look For In An
Infrared Detector System
A detector that:
• can detect a much greater variety of combustible gases
• can provide linear responses to a variety of combustible gases rather than
just one
• can detect acetylene. Insist that the detector isn't blocked by acetylene.
• has a light pipe which provides rapid detection and recovery
• is virtually immune to water condensation and temperature/humidity
variations
• has a small footprint.
Questions to ask Manufacturers about
Infrared Detector Systems
1. Does your sensor automatically adjust the scaling factor to
measure more than one hydrocarbon with good accuracy?
2. Can the user see which hydrocarbon has been detected?
3. Can the user read the gas concentration at the transmitter?
4. Does your factory offer sensor upgrades to expand the set of
CHC gases/vapors
that are recognized?
5. Does your sensor deal correctly with acetylene?
6. Can the sensor be adjusted for the prevailing air pressure at high elevation
or in deep mines?
7. How does the sensor handle high CHC concentrations, pure CHC gas?
8. How many alarms levels are built in?
9. Can the alarm levels be disabled or re-programmed in the field?
10. Can your sensor recover from immersion into water?
11. Is there a local alphanumeric display that shows non-fatal warnings?
12. Will the 4-20 mA signal indicate all sensor faults, including microprocessor
failure?
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