01 January 2004
Infrared dusts canary on a stick
Altering the landscape of industrial safety gas detection.
By Claudio C. Groppetti
Since industries used the infamous canary on a stick to alert workers of the presence of hazardous gases, we have been on a quest to find the perfect gas detector. Although we've made significant progress over the past twenty to thirty years, the perfect gas detector still eludes us.
Yet a recent technological advancement brings light to a high-efficiency nondispersive infrared combustible gas-sensing technology with the potential to dramatically alter the landscape of industrial safety gas detection. The technology ranking index (a tool to assist in comparing the strengths and weaknesses of various gas-sensing technologies) highlights inherent advantages of conventional nondispersive infrared (NDIR) technology and identifies lower operating power and cost. Initial applications include a new class of portable and fixed gas detectors and systems.
The primary benefit of the breakthrough is the technology's low power consumption and reduced cost, enabling design of 100% intrinsically safe combustible gas detectors. It could also promote the design of long battery life (five hundred to one thousand hours), portable combustible gas detectors, and true two-wire 4-20 mA loop-powered combustible gas-detection systems. It has all the inherent advantages of NDIR gas-detection technology that include 100% poison immunity, fail-safe operation, self-calibration, extended sensor life, and accurate measurements up to 100% by volume. It also does not require oxygen to operate accurately.
Until now, the availability of a low-power, low-cost, highly reliable NDIR sensing platform has not existed. Based on a proprietary technology, the first high-efficiency NDIR (HE-NDIR) platform is going commercial. Power consumption for this new technology can be as low as 2 milliwatts for the measurement of methane, which is 75 times lower than conventional NDIR systems and more than 150 times lower than catalytic bead sensors. With lower power consumption and cost, the technology ranking index (TRI) for HE-NDIR increases from 58 for conventional NDIR to 62. The cost is now similar to catalytic bead sensor technology, and power consumption is significantly less.
This significant improvement is possible because of a newly developed sensor-able to create and measure infrared energy more effectively than previously possible with conventional technology.
Common technologies for industrial safety gas detection
The four most frequently used technologies for industrial safety gas detection today include NDIR, catalytic bead, metal oxide semiconductor (MOS), and electrochemical sensors.
The industry commonly uses NDIR and catalytic bead sensors to detect combustible gases. You can also technologically use NDIR to detect other gases such as carbon dioxide and carbon monoxide, but it will not detect monotonic gases such as hydrogen. Catalytic sensors are able to detect hydrogen. Although you can use metal oxide semiconductor sensors to measure toxic and combustible gases, their lack of specificity (ability to measure a single gas of interest) typically limits their use to niche applications.
Nondispersive infrared technology is based on the ability of a gas to absorb infrared energy. Combustible hydrocarbon gases absorb infrared energy. A typical NDIR system consists of an infrared emitter and sensor tuned to a particular wavelength-matched to the absorption of characteristics of the gas of interest. A gas collection chamber separates the emitter and the sensor.
The relationship between the absorption characteristics of a gas and the measurement of a specific gas concentration is threefold. It is related to the absorption coefficient of the measured gas, the length of the gas chamber, and the concentration of the measured gas. Beer's Law relates these variables as follows:
I = I0ABC, where I0 is the initial intensity of the infrared radiation, A is the absorption coefficient of the measured gas, B is the distance or path of the measured gas, and C is the concentration of the gas.
Catalytic bead sensors typically consist of a set of heated platinum coils that a porous ceramic material encapsulates. One of the elements is coated with a catalytic reactive material that enables it to respond to combustible gases, while the other element renders it nonresponsive to combustible gases and serves as a compensating element. They typically appear in a wheatstone-bridge type configuration. This configuration has proven very effective in measuring combustible gas up to 100% of its lower explosive limit (LEL).
Initially the output of the sensor increases as the gas concentration increases. The increase continues until the catalytic reaction becomes oxygen deficient, at which time the output from the sensor begins to decrease. This can create an unsafe condition where you can no longer determine whether the measured gas concentration is above or below its LEL.
MOS sensors consist of a set of heated electrodes encapsulated or surrounded by a metal oxide such as tin oxide. As gases absorb into the heated oxide's surface, the electrical conductivity of the oxide changes. You can relate this change in conductivity to a gas concentration. The lack of specificity to a particular gas or class of gases is perhaps the greatest limitation of MOS sensors.
Electrochemical sensors consist of two or three electrodes positioned in an electrolyte and packaged so that gas can enter the sensor. When gas enters the sensor, an electrochemical reaction occurs that produces a signal proportional to the gas concentration. The sensors generally have good specificity and are the detector of choice when measuring oxygen or toxic gases such as hydrogen sulfide, carbon monoxide, sulfur dioxide, and chlorine.
It is important to assess the strength and limitations of the current gas-detection technologies to understand where the industry requires technological advances. Creating a TRI is a simple yet effective means to do this. Use the index to qualitatively compare each technology. The higher the score, the more fundamentally sound the technology. The ranking system uses a 1, 3, 7 scoring system-where 7 is the highest, most favorable score.
Be careful when using the TRI system to evaluate competing technologies. Comparing NDIR to electrochemical technology would be inappropriate if your primary interest is detecting combustible gases. For combustible gas detection, a more valid comparison would be between NDIR and catalytic technologies.
The cost and power requirements of NDIR technology are its greatest detractors. In fact, these two parameters prevent more widespread use of this technology. Despite these facts, NDIR technology is making significant strides in replacing catalytic sensors used in fixed-area detectors. Recent cost reductions in fixed NDIR hydrocarbon gas detectors, improved packaging, and decreased operation and maintenance costs have been primary economic drivers.
CONVENTIONAL VS. NEW
You typically conduct NDIR gas detection in the 2-5 micron region of the electromagnetic spectrum. Conventional NDIR devices use infrared sources that have very broad bandwidth emissions similar to those of a black body. You actually only use a small fraction of the energy produced for the gas measurement. For hydrocarbon gases you only need a narrow band of energy-less than 0.3 microns-to measure combustible gases. A source/sensor combination capable of emitting and measuring a narrow band of infrared energy at low power levels would represent a significant improvement for NDIR detection. The result of creating such a source/sensor combination is creating HE-NDIR detection.
The HE-NDIR technology now enables NDIR technology applications previously limited to other technologies. Because of HE-NDIR's inherently low power requirements, we can design the first truly 100% intrinsically safe NDIR gas detectors. Previously, we would have to design combustible gas-detection products for use in hazardous locations, at best, using a combination of intrinsically safe and explosionproof components. Portable gas detectors with a long battery life were limited to detectors using electrochemical sensors.
Units using electrochemical sensors could operate two thousand hours or more on a single set of two AA batteries. Portable gas detectors using conventional combustible gas detectors, either catalytic bead or NDIR, have battery lives in the order of ten to fifteen hours on a set of two AA batteries. HE-NDIR will enable battery life extensions in the range of five hundred to one thousand hours-especially important in emergency or crisis situations requiring extended use beyond the typical eight to twelve hours.
Combustible gas-detection systems are three- or four-wire systems because of the combustible gas detector's power requirements. HE-NDIR now enables the creation of a true two-wire, self-powered, 4-20 mA loop system to detect combustible and toxic gases. IT
Behind the byline
Claudio C. Groppetti is director of marketing for gas detection with Honeywell International in St. Charles, Ill.
INTERPRETING THE TRI SCORE CARD
Direct measurement refers to whether the technology measures a fundamental property of a gas directly, such as energy absorption with NDIR, or whether the technology uses a secondary or indirect measurement, as does MOS technology.
Specificity or susceptibility to cross interferences refers to the propensity of the technology to measure a single gas of interest and not other gases or vapors that are not of interest but may be present in the environment. MOS technology is known for its ability to detect a wide range of gases and distinguish between the gas of interest and compounds routinely present in the ambient environment (water vapor, cleaning agents). Electrochemical sensors generally have good specificity, while catalytic sensors are very specific to combustible gases. NDIR sensors have the highest specificity of these technologies.
The ability to resist poisoning is important when a substance in the ambient environment permanently renders the sensor unable to detect gas. Organic silicon compound in common silicone sealants and hydrogen sulfide are two common poisons for catalytic and MOS sensors. Inhibition, unlike poisoning, is a temporary loss of the sensor's ability to detect gas. Organic solvents inhibit electrochemical sensors. NDIR technology is completely immune to poisoning and inhibition, which makes it suitable for use in harsh environments that would dramatically shorten the life of other sensors.
Oxygen requirement refers to the need for oxygen to be present for an accurate and stable reading.
Susceptible to overrange reading error refers to whether a technology is vulnerable to erroneous readings after it is exposed to gas concentrations exceeding the sensor's upper measurement range.
Power consumption is the amount of electrical energy required to operate the gas sensor. Electrochemical sensors require very little power whereas catalytic, NDIR, and MOS require significantly higher power.
Fail safe refers to whether the detector or system can interrogate the sensor to identify all conditions that would render the sensor unable to detect gas. This would include any failure modes that would go undetected until the sensor comes in contact with gas and is nonresponsive. NDIR is the only technology we can design into a detector to operate in a truly fail-safe manner.