An Overview of Hazardous Gas Detection
Robert B. Coulter, P.E.
This two hour online
course provides the student with a basic understanding of the current technology
of hazardous gas detection.
This course includes a multiple choice quiz at the end.
At the conclusion of this course, the student will:
About the Author
Robert B.Coulter, PE, is a provider of safety, process engineering and environmental consulting services including inhouse training on safety and environmental topics. For more information, visit his website at www.rbcoulter.com.
When discussing hazardous gas detection, the first thing that may come to mind is the image of the canary going down into the mine. Fortunately, the current state-of-the-art is much more sophisticated; however, there is yet no "perfect" gas detector. Perfect is defined as responding immediately, extremely accurate, repeatable, specific or nonspecific (as needed), and inexpensive. In this course we will discuss first the theory of gas detection and then the practical application of gas detection.
Detector Technology (Theory):
The following are the gas detectors that are generally available on the market. A description of the theoretical mechanism behind each type of detector is also shown:
Flammable Gas Detectors (FG)
Nearly all of these detectors currently available on the market consists of a catalysis material surrounding a RTD (Resistive-Thermo Device). The sensor basically "burns" the flammable gas and measures the heat output to determine the amount of flammable gas in the atmosphere. Because of this, these detectors WILL NOT work on nonflammable gases. For example, many chlorinated organic compounds are nonflammable and will not respond on this detector. Also, these detectors will not sense a smothering (low oxygen) hazard. Air is typically required for detection. The sensor will not work on a mixture of the flammable gas and an inert gas. These devices are relatively small and inexpensive. The sensors can be fixed in one location or incorporated into a battery powered handheld device. Some detectors have a small pump that forces a small stream of air into the sample chamber.
Flammable gas detectors are high range devices. Don't expect to resolve a reading better then 2% LEL. For example, if Benzene is the gas (technically vapor) that is to be detected, then a 2% LEL reading for this gas is about 300 ppmv. Benzene is a health hazard above 10 ppmv. In other words, these detectors would have little application for toxic gases. Also, if the detector is calibrated for a particular gas then its readings will not be correct when detecting other gases. The manufacturer often provides these "correction" factors for various gases. Still it is very cumbersome to carry around, or program in, a list of all the correction factors if you are dealing with multiple vapors or gases. It is recommended to simply calibrate the device to the LEAST sensitive gas. Specifically, this is the gas that produces the smallest detector reading at a particular %LEL (say 20%). The other gases would then produce false high readings which is much preferable to a false low reading.
To see an example of a flammable gas detector, click here. Remember to click the BACK button on your browser to return to the course.
Electrochemical Detectors (EC)
These detectors have a small electrochemical cell with a membrane entrance for the gas. The gas permeates the membrane and causes a current to flow in the cell. The amount of gas present directly correlates to the current produced. Usually the cell is water based. These detectors have a lagging (slow) response because of the need for the gas to permeate the cell.
Many manufacturers call these "toxic" gas detectors. This is somewhat confusing since there are many toxic gases that it will not detect. The gases detected are generally "acid" gases like carbon monoxide, hydrogen sulfide, and hydrogen chloride. Benzene, which is toxic, would not be detected by this type of sensor.
These are usually low range devices and relatively inexpensive. They are often combined with flammable gas detectors into portable units.
Most oxygen detectors are of this type. Oxygen detection is important when the hazardous gas is not flammable nor toxic. A very large concentration (say greater than 4%) of any gas can reduce oxygen levels below a safe level. This should not be overlooked as gases like carbon dioxide and nitrogen,which are often used to suppress a fire hazard, can cause injury or death because of their ability to reduce oxygen levels. Essentially, any gas can be hazardous under certain conditions.
To see an example of an electrochemical detector, click here. Remember to click the BACK button on your browser to return to the course.
Solid State Detectors (SS)
These detectors usually involve a thin film or matrix embedded on an electronic circuit. The gas sample interacts with the matrix causing a resistance change in the circuit.
Information about these detectors is sketchy. It appears as though they are mostly low cost, high range devices. Depending on how the matrix and circuit is designed, these devices tend to emulate the performance of EC and/or FG detectors.
Infra-red Detectors (IR)
This type of detector works by measuring the adsorption of a particular guess at preselected wavelengths of Infrared (IR) light. Gases can be thought of having an IR "fingerprint" based on it's molecular structure. Different IR wavelengths are adsorbed depending on the types of bonds (C-H, C=H, etc.) present in the molecule. The amount of IR radiation adsorb is directly correlated to the amount of the gas present.
A IR wavelength is chosen that represents the type of gas that is being detected. For example, if hydrocarbons are being monitored, a wavelength that is adsorbed by the C-H bond may be selected. The device would then be configured to direct this wavelength through the sample chamber. For example, methane has four C-H bonds. If methane is in the sample chamber then a response directly related to the these four bonds would be expected. However, many hydrocarbons have more C-H bonds per volume of gas than methane. Butane has ten C-H bonds. The presence of butane in the same sample chamber will cause a much greater response in the detector.
These detectors have a distinct advantage over the previous two detectors discussed (flammable and electrochemical) in that the IR detector can be configured to be much more discerning for the target gas. For example, the flammable gas detector could not distinguish between propane and ethylene. Both of these gases only appear as "flammable" gases. However, an IR detector could be setup only to detect the C=C bond in the ethylene. The propane could then be "invisible" to the detector if one only wanted to know the ethylene concentration.
These detectors can be low or high range and are moderately expensive. This detector also has the unique ability to monitor "fence lines". This is done by setting up an IR beam transmitter at one point and an IR detector as far away as 500 feet.
To see an example of an IR detector, click here.Remember to click the BACK button on your browser to return to the course.
Photoionization Detectors (PI)
Where the IR detector makes use of the Infrared radiation, the photoionization utilizes the ultraviolet spectrum. UV light of an energy less than 11.7 eV (electron volts) is produced and directed into the sample chamber. If a compound is present that is "excited" by the UV light, electrons will be produced and sensed by the detector. The response of the detector is directly related to the number of electrons produced which is related to the type and concentration of gas present. Gases that are ionized above 11.7 eV cannot be detected because of interference with oxygen.
The set of gases that can be detected by this sensor is relatively small (compared to FG, IR, and FID). The gases that are detected by this analyzer tend to be higher molecular weight organic vapors. Methane, some chlorinated organic and many cyanide compounds are not detected. These detectors; however, are not very complicated to operate/maintain and are reliable.
These detectors can be low or high range and are moderately expensive.
Flame Ionization Detectors (FID)
This detector sends the sample over a hydrogen flame where the sample is "ionized". The detector measure the number of these ions as an electrical signal. For all practical purposes, the signal is directly related to the number of carbon atoms present in the sample. Carbon dioxide , CO2, is not detected, but this is actually an advantage since this gas is not usually desired to be detected.
The big advantage of this detector above almost all others is that it is generally not affected by the presence of water. Many applications involve trying to discern hazardous gases in almost 100% relative humidity environments. The disadvantage is that these detectors are somewhat bulky (if portable) and involve burning a hydrogen flame to run the detector.
These detectors can be low or high range and are fairly expensive.
To see an example of a flame ionization detector, click here. Remember to click the BACK button on your browser to return to the course.
Mass Spectrometer (MS)
Because gases have varying molecular weights (masses), this mass (or mass fragments) can be detected to determine the amount and type of gas present. The basic technique is that a gas sample enters into a fragmentation chamber where the gas is split into a distribution of mass sizes. The magnitude and size distribution of these masses is a function of the concentration and composition of the gas sample. This magnitude and size distribution is determined by sending the fragments through a vacuum chamber surrounding by a magnetic field. The magnetic field is varied by electronics to only allow a particular mass fragment to reach the detector at a particular moment. Usually the magnetic field is adjusted following a preprogrammed"path" to detect certain "mass channels" that correspond to the target gases to be detected. The detector can be a high range or low range detector.
Ultrahigh vacuum is needed to run these detectors. Calibration is difficult. It goes without saying that these are among the most expensive detectors. The distinct advantage of these detectors is their discerning ability is very high and are equally suitable for detecting organic and inorganic gases.
Gas Detector Tubes / Paper Tape
Gas detector tubes are small glass tubes containing a mixture of specific chemicals designed to react with a particular target gas. The mix of chemicals is chosen in such a way that a color stain develops in the tube to give a visual indication of the presence of the target gas. In most cases there are markings on the glass tube that indicate the concentration of the target gas. Usually the length of the stain corresponds to the gas concentration.
This is non-electronic technology depending solely on chemistry. The usually procedure is two draw a specified volume of gas through a glass tube with a small hand pump. The glass tubes can be provided to detect a wide variety of chemicals throughout various ranges.
Paper tape technology is similar but the stain occurs on a tape strip. This technology is usually employed on isocyanate compounds.
These gas detection methods are relatively inexpensive. However, they cannot be used as a first warning device. They are almost always used after it is known (by some other means) that a hazardous gas is present. Their use is intended to more specifically identify and quantify the hazardous gas. Even at that, some knowledge of the possible gases present must be known. Each tube is generally made for detecting a specific gas. If the list of possible gases is large (say greater than 10) then it could take over thirty minutes to sample/test for each individual gas (3-4 minutes per gas compound). Tubes and the paper tape cannot be reused.
To see an example of typical gas detector tubes, click here. Remember to click the BACK button on your browser to return to the course.
Gas Chromatography (GC)
This is really not a detector. It is a small tube (a column) with packing that the sample gas is sent through first before being sent to a detector. The idea is to separate different gases that may be present in the sample. Different gases would be indicated by their varying arrival times at the detector. The detector could be of any type but is usually an FID (see above). The GC enhances the ability of a detector to discern the concentrations of individual compounds (species) in a gas mixture. For example, a gas mixture of toluene and benzene sent directly to an FID detector would show one peak. The response of the detector would simply indicate the cumulative presence of the two species with no indication of the "split". If the sample is sent through a properly configured GC then two peaks may be seen at the detector where the magnitude of each peak would directly correlate to the concentration of each species.
The disadvantage of having a GC in a hazardous gas detection system is the longer response time to detect the gas. The cost to add GC to a detection can be high. However, the desire to know the "split" of the gas concentrations may outweigh these disadvantages. For example, one gas in a sample could be very toxic while another gas is only flammable.
Monitors, Multiple Point Monitoring, and Alarms
As with other input devices the output of most gas detectors can be routed to a remote location for monitoring. This can be a computer or specially designed electronic monitor.
Sometimes multiple detection points are required. One can install detectors with transmitters at each point,or install a solitary central detection system with sample tubes running to all desired detection points. Multiple detectors have the advantage of increased response time. A solitary detector is less expensive particularly if it is an FID, IR, or MS; however, a switching sampling system and controls must be added if one wants specific data about a particular sample location.
Usually alarms are desired to indicate a hazardous gas level. Alarms are usually set at some fraction of LEL (say 20% LEL) in the case of flammable gas monitoring, or approximately one half of the permissible exposure limit (PEL) for toxic gases. Oxygen monitoring is usually alarmed at 18% oxygen in air (21% is normal). It is a good idea (in many cases it is required) to have a emergency action plan in place that outlines the response to a hazardous gas alarms.
Examples of Practical Application:
The following are hypothetical examples of how gas detection technology can be applied:
A company stores several nonspecific flammable liquids in storage tanks of various sizes in a room adjacent to a work area. The company is interested in detecting leaks and spills from the storage tanks so that a major spill and potential fire could be avoided. Based on this information, what would be the optimal detector for this system?
In this case the liquids are flammable and nonspecific. A PI detector is likely to be inadequate because some flammable materials do not respond to it. An EC detector is not likely to work on these organic compounds. It is likely that FI, IR, FG, or MS detectors will work in this application but are expensive. The FG detector is the least expensive and should be considered first.
In the example above, where would the best placement of the detectors be -- above the tanks near the ceiling or just above the floor?
Flammable liquids are most likely to generate vapors that will sink. A detector placed nearer the floor will more likely detect a leak sooner.
In example #1, In the example above, a company representative has indicated that toluene is the main constituent in the storage tanks. After a review of the other constituents in the tanks it is decided that the detectors can be calibrated with a span gas consisting of air and toluene. If the span calibration bottle is desired to contain toluene in air at 20 % LEL, what concentration of toluene (in %) does the gas supply company put into the bottle? (Assume the LEL of toluene in air is 1.3 %) Note: The span gas is used to set the upper calibration point on the transmitter.
The answer is 0.26 % Toluene in air. This is obtained by taking 20 % of 1.3.
A company rep is interested in purchasing a "toxic" detector with an alarm to warn of unspecified gas hazards. The company rep does not have any knowledge of the types of gases that could be toxic hazards. The company rep is considering the purchase of a "toxic" detector from a safety catalog. A review of this detector indicates that it is based on the electrochemical (EC) technology. Is the company rep making a good decision in purchasing this detector?
Probably not. This "toxic" detector, as described in many vendor's publications, is only suitable for a specific group (usually inorganic) of toxic gases. Many toxic organic gases would not indicate on this type of detector. The company rep would probably cause more harm than good by installing this detector because of the false sense of security it could bring. Anyone interested in a generic "toxic" sensor should consult a professional who is knowledgeable of multiple gas detection technologies and is capable of understanding the customer's specific needs.
A beverage company is interested in a detector for ethanol vapors which can be flammable and pose a health risk. The company currently has flammable gas (FG) detectors installed but they complain that they are unreliable. A tour of the facility indicates a large amount of water vapor present. What should the company consider next to replace the FG detector?
It appears as though the companies FG detectors are affected by the water vapor. Many inexpensive detectors have this problem. IR could also be negatively affected by water. The best choice is an FID detector.
A refinery/chemical complex is interested in purchasing portable gas detectors for finding leaks from process equipment. The complex consists of a chemical plant that has specific organic chemicals and the refinery that has a variety of nonspecific hydrocarbon compounds. FI and PI detectors are being considered. The customer needs multiple units for both plants. Water vapor is not an issue at either facility. Based on this information, what should be considered first?
In general the PI detector is less expensive, easier to calibrate/operate and to carry. The main problem with the PI detector is that some compounds cannot be detected. A review of the chemical complex target gases indicate that all gases have photoionization potentials less that 11.7 eV. So for the chemical plant the PI would be the best choice. However, the refinery complex has many nonspecific organic compounds that can't all be identified. In fact, methane is present in the refinery which cannot be detected by photoionization (Methane's ionizational potential = 12.98 eV). The best choice for the refinery is FID.
A firm cleans tank trailers in a semi-closed garage. There are hazards from carbon monoxide (CO) and hydrogen sulfide (H2S). The carbon monoxide comes from the engines of the tractors. The hydrogen sulfide emanates from some tank trailers. The firm is interested in an electrochemical detector. However, the sensor is five times more sensitive to hydrogen sulfide than carbon monoxide. In other words, if the sensor is calibrated to read ppms of CO, it will show a reading five times higher than the actual PPM when detecting H2S. Let's assume the PEL for H2S is 10 PPM, and the PEL for CO is 35 PPM. For the CO calibrated sensor, where should the alarm be set?
It would be a good idea to set the toxic alarm at half the PEL. Half of 35 PPM is about 17 PPM. Let's assume we can set the alarm at 17 PPM. This would alarm adequately for CO. H2S needs to be alarmed at 5 PPM of H2S. Because of the five times sensitivity of H2S, this gas would set off the alarm at 17/5 or 3.4 PPM. This is below 5 PPM, so the assumption of setting that alarm at 17 PPM is correct.
A safety engineer is interested in knowing the PEL (permissible exposure limit), LEL (lower explosive limit), and photoionization potential of ammonia in order to configure a gas monitoring system. What are these values?
A good source of these values is provided by NIOSH's Pocket Guide to Chemical Hazards (see the link below). The ammonia entry lists a NIOSH REL (25 PPM) and an OSHA PEL? (50 PPM). Let's use the smaller number, 25 PPM The LEL is listed as 15 %. The ionization potential is 10.18 eV.Course Summary
Current state of hazardous gas detection employs technology that must be well understood to make optimal use of a particular monitoring system. Gas detectors can be low cost to very expensive, and all types have their advantages and disadvantages. One should use caution when selecting a gas detector. If there is any doubt, consult a professional familiar with gas detection for proper advice.
To conclude this course the student should read "Key Concepts in Gas Detection Systems" by clicking the first link below. The second link is NIOSH's guide to chemical hazards. It lists the exposure limits, LELs, photoionization potential and other useful information for many compounds. The student should examine a few chemicals in this database to be familiar with it. After reading the required course material the student should be ready for the test.
For additional technical information related to this subject,
please visit the following websites or web pages:
Scott Instruments - Key Concepts in Gas Detection Systems (PDF reader needed)
NIOSH Pocket Guide to Chemical Hazards
Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.