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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:
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 m) or nanometers (nm). For simplicity we will use wavelengths in microns to refer to IR energy.
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.

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.

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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