Information on selecting the proper gas meter for entering confined spaces.
GAS MONITORING
How to Select a Proper Detector, Part 1
By Jason Barlow
Gas detection technologies have improved dramatically in recent years. These days, portable gas detection for recording and monitoring exposure may include microprocessor-driven electronics, next generation signal processing, state-of-the-art battery choices and enhanced data manipulation and storage. Faced with all of these improvements, how should safety professionals select the product that’s best for their workplace? I’m not a scientist, but I’ve spent a lot of time researching the options and identifying their pros and cons. Let me tell you what I’ve found.
Selecting Proper Gas Detectors
Without proper gas detection, hazardous atmospheres may significantly affect the health and safety of workers. Many airborne contaminants cannot be detected by smell or vision and can only be measured with special equipment. Depending on its sensor configuration, proper gas detection equipment can help identify the hazard and protect your workers.
Selecting a gas detector should be based on the hazard encountered. For example, in confined space work, it is necessary to monitor for oxygen deficiency/enrichment, combustible gases and toxics. Therefore, it’s necessary to choose an instrument capable of dealing with these issues.
Unfortunately, too many purchasers make large and crucial equipment expenditures without really understanding what they are buying. Sensors and their capabilities are the single most important factor when choosing a gas detector; yet more often than not, decisions are based on size, price and other features that have nothing to do with the instrument’s detecting abilities.
The Three Categories of Gas Monitors
There are many federal, provincial/ state and local safety and environmental regulations requiring hazardous gas and vapor monitoring. Most (but not all) requirements fall into three categories:
Personal exposure monitoring;
Confined space entry; and
Ambient air monitoring
Let’s take a brief look at each of these.
1. Personal Exposure Monitoring
Personal exposure monitoring is the detecting of toxic gases in an individual worker’s breathing zone. Alarm settings on monitoring instruments generally relate to the physical and toxic characteristics of a specific gas and/or regulated threshold limits.
The American Conference of Governmental Industrial Hygienists (ACGIH) recommends measuring exposures by threshold limit values (TLV), such as the 8-hour time-weighted average (TWA) and the 15-minute short-term exposure limit (STEL).
Occupational Safety & Health Administration (OSHA) measures with permissible exposure limits (PELs), some of which are based on TLVs. In Canada, PELs are set by provincial Occupational Health and Safety (OHS) regulations. The provincial PELs are similar to those adopted by OSHA in the U.S.
Some monitors are equipped with alarms that measure yet another set of limits called "immediately dangerous to life or health" (IDLH).
2. Confined Space Entry
In general, confined space regulations require that air in a confined space be evaluated for the presence of combustible gases before any worker enters the area. Gases detected must be in the lower explosive limit (LEL) range, with less than 10 percent LEL typically considered an acceptable concentration. This percentage may change depending on whether hot work is to be performed within the confined space.
The space must also be measured for oxygen deficiency or enrichment: 20.9 percent oxygen volume in air is generally considered clean air; 19.5 percent is low (deficient); and 23 percent is too high (enriched).
Finally, the space must be tested for the presence of specific toxic gases, which must be within defined applicable concentration ranges, typically parts per million (PPM) or parts per billion (PPB).
Note: Before setting any sensor alarms, refer to your applicable legislation and verify legal requirements.
3. Ambient Air Monitoring
Ambient air monitoring involves continuous measurement of any workplace air to which workers are exposed – indoors or out. Normally, permanent monitoring systems are used for ambient air monitoring, although it can also be done with portable instrumentation set up to detect specific gases.
The Two Types of Gas Detection
There are two categories of gas detection instruments:
Indirect reading, where samples must be sent to the laboratory for analysis; and
Direct reading, which provides information at the time of sampling.
Direct reading instruments are the only safe source of sampling information in operations where the primary objective of gas detection is to immediately warn a worker of adverse atmospheric changes.
Conclusion
There are a number of direct reading portable gas detector manufacturers in the market today. Their product lines come in a variety of sizes, shapes, colors and sensor configurations. Next week, we’ll look at various combustible gas sensors and discuss their pros and cons.
SELECTING GAS DETECTORS
Sensor Technologies, Part 2 of 4
By Jason Barlow
Proper gas detection equipment can help identify hazardous atmospheres and protect your workers. When choosing a gas detector, the single most important factors to consider are the sensors and their capabilities. I’ve done a bit of research on this topic and this week I’ll share my findings on the types of combustible gas sensors available and compare the pros and cons of each.
Instrument Flexibility
Gas detection manufacturers produce instruments with a variety of sensor configurations. These days, it’s not uncommon to find units with one to six gas monitoring options with interchangeable sensors. But they’re not one-size-fits-all. You need to be fully aware of the different sensor types and the capabilities of each. Before using a gas monitoring instrument, there are two things you need to know:
1. The hazards that are likely to be present in the particular space you’re testing; and
2. The capability of the instrument you’re going to use to detect such hazards.
Understanding Sensor Technology
Sensor technology is the foundation of any instrumentation — whether portable or fixed. It’s also the cornerstone of accurate compliance reporting. Remember, though, that every sensor has its limitations. If your sensor selection is inadequate or inappropriate for the application, then everything downstream of the sensor will be compromised. So you must match the capability of the sensor with your requirements; otherwise, you’ll get inaccurate data and prematurely wear out your sensor.
Combustible Gas Sensor Options
In confined space work, you must monitor for combustible gases. There are three types of combustible gas sensors available for you to choose from: catalytic, metallic oxide semiconductor and infra-red. Let’s take a look at their advantages and disadvantages:
1. Catalytic combustible gas sensors detect combustible gases by causing an actual combustion of gases within the sensor chamber.
Catalytic sensors consist of a flame arresting material encasing two chambers, each of which contains a coiled wire filament. One chamber, whose coil is typically coated with platinum or palladium, is designed to allow air to enter. The other chamber, whose coil is not coated, is sealed to prevent air from entering.
Both coils are heated, typically to 500ºF or higher. When combustible gases are exposed to the coil, they will ignite and raise the temperature even higher. This temperature increase and the change of the coil’s electrical resistance are displayed as “percent LEL.”
Pros
Offer good linearity
Can react to most combustible gases
Cons
Work best in concentrations between 1,000 and 50,000 PPM
Don’t measure trace amounts of gas (under 200 PPM) and therefore are of no use determining toxic levels
Require a minimum of 16% oxygen content in the air to work accurately
Sensor can be damaged by lead or silicone
Readings can be affected by humidity and water vapor condensation
Tend to lose their linearity after a year or so
Not recommended for use in an acetylene atmosphere
Note: The flame arrestor will prevent ignition of most gases outside the sensor, except acetylene. It is extremely important to check the approvals to determine the types of hazardous locations the detector can function within.
2. Metallic oxide semiconductor (MOS or “Solid State”) combustible gas sensors consist of a housing (either a stainless steel sintered cup or plastic) containing an electric conductor. This conductor is made up of a heating element, typically operating between 150ºF and 350ºF, and a bead that contains a mixture of metal oxides.
As the electrical current travels through the bead when exposed to clean air, a base resistance is established. When a gas comes into contact with the sensor surface, a change in sensor resistance occurs. The sensor resistance can change significantly even with small quantities of gases (less than 200 PPM).
Pros
Long operation life (typically 3 to 5 years)
Very rugged with capacity to recover from high concentrations of a gas that could damage other types of sensors
Cons
Require oxygen to work accurately, although not as much as the catalytic
Some sensors’ heating elements have a high demand for power that requires large battery packs
Readings may be affected by humidity and water vapor condensation
3. Infra-red combustible sensors have recently begun appearing in some instruments. These sensors work by reflecting light off a mirror and measuring the amount of light adsorbed during refraction.
Pros
Work well in low oxygen levels or acetylene atmospheres
Cons
Quite expensive
Typically require a constant flow across the sensing assembly and may be slow to clear from alarm
Unable to detect hydrogen
Conclusion
Depending on what you need, there is quite a selection for combustible gas sensor technology. Next week, we’ll look at various toxic sensors and discuss their pros and cons.
SELECTING A GAS DETECTOR
A Look at Toxic Sensors, Part 3 of 4
By Jason Barlow
When choosing a gas detector, the single most important factor to consider is the sensor and its capabilities. Last week, we compared the pros and cons of combustible gas sensors. This week, we’ll look at toxic sensors.
The Two Kinds of Toxic Sensors
When choosing a toxic sensor, you have two options: a wet chem toxic sensor and an MOS toxic broad range gas sensor. Let’s look at each option:
1. Electrochemical (wet chem) toxic sensors react to specific chemical substances such as chlorine, ammonia, carbon monoxide, carbon dioxide, nitrogen dioxide, nitric oxide, hydrogen cyanide, hydrogen sulfide, sulfur dioxide and hydrochloric acid. Read the technical information supplied by the detector’s manufacturer to learn what sensors are available for the unit.
The electrochemical sensor housing contains two, sometimes three, electrodes sitting in a liquid solution, either a base or alkali, depending on what the sensor is “looking for.” The housing is covered by a Teflon membrane, which keeps the fluid in the housing but allows in air. As air molecules enter through the thin Teflon membrane, the fluid reacts with a specific substance if it’s present.
When the detector is working, a small electrical current passes between the two electrodes. Any change in the fluid’s density caused by a reaction to the substance in the air will affect the density of the fluid and change the amount of current passing between the two electrodes. The current then passes through a temperature compensating circuit and the electron flow is read as a specific amount of the substance.
The sensor’s ability to detect specific types of gases is based on:
The choice of membrane;
The number of electrodes;
The alloy of the electrodes (gold, lead, etc.); and
The type of electrolyte fluid.
Pros
Very good linearity, which makes them very accurate for the substance to which they’ll react.
Can measure either large or small quantities.
Cons
Have a typical life span of approximately one year.
Fluid can freeze when left in environments having temperatures lower than 0°C.
Adversely affected by altitude. (Air pressure at sea level (14.73 psi absolute) is the force required to induce the air into the sensor. An increase in altitude means less force is available to push the air into the sensor, thus reducing the accuracy of the reading.)
Some substances, such as moisture, affect the sensor by changing the make-up of the fluid. This reduces the amount of electrical resistance and impacts the reading. Check the manufacturer’s instructions to see which substances will affect the sensor.
May generate readings that are abnormal or don’t make sense. (Note: This problem can be minimized if you know the hazards in your workplace, have a basic understanding of chemistry, know what interference gases adversely affect your unit and follow strict testing protocols.)
2. Metallic oxide semiconductor (MOS) toxic broad range gas sensors are just one of many MOS sensors on the market. The MOS sensor specifically developed for detecting toxic gases is similar in concentration and operation to those used for the detection of combustible gases.
Pros
Capable of reacting to a wide range of toxic gases including carbon monoxide, hydrogen sulfide, ammonia, styrene, toluene, gasoline and many other hydrocarbons and solvents.
Cons
Cannot detect carbon dioxide or sulfur dioxide.
Incapable of telling you what gas you have encountered or the concentration, only that the atmosphere may be hazardous to your health.
Last word on toxic gas sensors
It’s important to note that you should never use an oxygen sensor to detect toxic gases. It is true that a toxic gas will displace the oxygen in a confined space. However, it takes 60,000 PPM of any gas to lower the oxygen from 20.9% (normal) to 19.5% (alarm point). More importantly, 60,000 PPM of any toxic gas will kill you.
Conclusion
There are several types of oxygen sensors available and we’ll look at those next week.
SELECTING A GAS DETECTORA Look at Oxygen Sensors, Part 4 of 4 September 5, 2006
In this series, we’ve compared the pros and cons of the various combustible gas sensors and toxic sensors available in today’s market. This week, I’ll wrap up the series by sharing my findings on oxygen sensors.
The 5 Types of Oxygen Sensors
There are five common types of oxygen sensors. Let’s look closely at the merits of each.
1. Ambient Temperature Electrochemical Oxygen Sensors
Ambient temperature electrochemical oxygen sensors, often referred to as galvanic sensors, operate much like a battery. Oxygen gas flows past an electrode and becomes a negatively charged hydroxyl ion. The ions move through electrolytes in the oxygen sensors to positively charged electrodes, typically made of lead, react with the lead and releases electrons. The electron flow is measured and the measurement can be mathematically converted to an oxygen concentration.
Pros
The only true chemically specific sensor (similar to the electrochemical toxic sensor described last week).
Cons
Susceptible to freezing
Affected by altitude
Nominal operational life of one year.
Susceptible to damage when used with samples containing acid gas species such as hydrogen sulfide, hydrogen chloride, sulfur dioxide, etc. Unless the offending gas constituent is scrubbed prior to analysis, their presence will greatly shorten the life of the sensor.
2. Paramagnetic Oxygen Sensors
Oxygen has a relatively high magnetic susceptibility as compared to other gases such as nitrogen, helium and argon; and it displays a paramagnetic behavior. The paramagnetic oxygen sensor is based on these qualities. It typically consists of a cylindrical shaped container in which a small glass dumbbell is placed. The dumbbell is filled with an inert gas such as nitrogen and suspended on a taut platinum wire within a non-uniform magnetic field. The dumbbell is designed to move freely as it is suspended from the wire. When oxygen is processed through the sensor, the oxygen molecules are attracted to the stronger of the two magnetic fields created by each side of the dumbbell. This causes a displacement of the dumbbell and causes it to rotate. A precision optical system consisting of a light source, photodiode and amplifier circuit is used to measure the degree of rotation of the dumbbell.
In some sensor designs, an opposing current is applied to restore the dumbbell to its normal position. The current required to maintain the dumbbell in its normal state is directly proportional to the partial pressure of oxygen and is represented electronically in percent oxygen.
The magnetodynamic or dumbbell type of design is the predominate sensor type of paramagnetic oxygen sensors. But design variations are available, depending on the manufacturer. Also, other types of sensors have been developed that use the susceptibility of oxygen to a magnetic field, which include the thermomagnetic or “magnetic wind” type and the magnetopneumatic sensor.
Pros
Offer very good response time characteristics
Use no consumable parts, making sensor life (under normal conditions) quite good
Offer excellent precision over a range of 1% to 100% oxygen
Cons
Quite delicate and sensitive to vibration and position
Not recommended for trace oxygen measurements in general, due to the loss in measurement sensitivity
Other gases that exhibit a magnetic susceptibility can produce sizeable measurement errors. Manufactures of paramagnetic oxygen sensors and analyzers should provide details on these interfering gases.
3. Polarographic Oxygen Sensors
Polarographic oxygen sensors are often referred to as a Clark Cell. In this type of sensor, both the anode (typically silver) and cathode (typically gold) are immersed in an aqueous electrolyte of potassium chloride. The electrodes are separated from the sample by a semi-permeable membrane that diffuses oxygen into the sensor. The silver anode is typically held at a potential of 0.8V (polarizing voltage) with respect to the gold cathode. Molecular oxygen is consumed electrochemically with an accompanying flow of electrical current directly proportional to the oxygen concentration (based on Faraday’s law). The current output generated from the sensor is measured and amplified electronically to provide a percent oxygen measurement.
Pros
While inoperative, there is no consumption of the electrode (anode)
Almost indefinite storage times
Not position sensitive
The sensor of choice for dissolved oxygen measurements in liquids
Cons
Relatively high frequency of sensor replacement
Sensor membrane and electrolyte require maintenance
For gas phase oxygen measurements, the sensor is suitable for percent level oxygen measurements only
4. Non-Depleting Coulometric Oxygen Sensors
Non-depleting coulometric sensors are a variant of the polarographic oxygen sensor in which two similar electrodes are immersed in an electrolyte consisting of potassium hydroxide. Typically, an external EMF of 1.3 VDC is applied across both electrodes which act as the driving mechanism for reduction/oxidation reaction. The electrical current resulting from this reaction is directly proportional to the oxygen concentration in the sample gas. As with other sensor types, the signal derived from the sensor is amplified and conditioned prior to displaying.
Pros
Can be used for both percent and trace oxygen measurements
Can measure parts per billion levels of oxygen
Cons
One sensor cannot be used to measure both high percentage levels as well as trace concentrations of oxygen
Sensors are position sensitive and replacement costs are quite expensive, in some cases, paralleling that of an entire analyzer of another sensor type
Not recommended for applications where oxygen concentrations exceed 25%.
5. Zirconium Oxide Oxygen Sensors
Zirconium oxide oxygen sensors are occasionally referred to as the “high temperature” electrochemical sensor and are based on the Nernst principle.
Zirconium oxide sensors use a solid state electrolyte typically fabricated from zirconium oxide stabilized with yttrium oxide. The zirconium oxide probe is plated on opposing sides with platinum, which serves as the sensor electrodes.
For a zirconium oxide sensor to operate properly, it must be heated to approximately 650° C. At this temperature, on a molecular basis, the zirconium lattice becomes porous, allowing the movement of oxygen ions from a higher concentration of oxygen to a lower one, based on the partial pressure of oxygen. To create this partial pressure differential, one electrode is usually exposed to air (20.9% oxygen) while the other electrode is exposed to the sample gas. The movement of oxygen ions across the zirconium oxide produces a voltage between the two electrodes, the magnitude of which is based on the oxygen partial pressure differential created by the reference gas and sample gas.
Pros
Can be utilized in high temperature environments
Excellent response time characteristics
Can be used to measure 100% oxygen, as well as parts per billion concentrations
The “de facto standard” for in-situ combustion control applications
Cons
Due to the high temperatures (typically) of operation, the life of the sensor can be shortened by on/off operation
Constant heating and cooling often causes “sensor fatigue”
Unsuitable for trace oxygen measurements when reducing gases (hydrocarbons of any species, hydrogen and carbon monoxide) are present in the sample gas
At operating temperatures of 650°C, the reducing gases will react with the oxygen, consuming it prior to measurement thus producing a lower than actual oxygen reading. The magnitude of the error is proportional to the concentration of reducing gas.
Conclusion
Selecting the right gas detector and attendant sensor is a crucial safety decision. I hope this and the previous installments help you make the right decision for your workplace and workforce.
How to Select a Proper Detector, Part 1
By Jason Barlow
Gas detection technologies have improved dramatically in recent years. These days, portable gas detection for recording and monitoring exposure may include microprocessor-driven electronics, next generation signal processing, state-of-the-art battery choices and enhanced data manipulation and storage. Faced with all of these improvements, how should safety professionals select the product that’s best for their workplace? I’m not a scientist, but I’ve spent a lot of time researching the options and identifying their pros and cons. Let me tell you what I’ve found.
Selecting Proper Gas Detectors
Without proper gas detection, hazardous atmospheres may significantly affect the health and safety of workers. Many airborne contaminants cannot be detected by smell or vision and can only be measured with special equipment. Depending on its sensor configuration, proper gas detection equipment can help identify the hazard and protect your workers.
Selecting a gas detector should be based on the hazard encountered. For example, in confined space work, it is necessary to monitor for oxygen deficiency/enrichment, combustible gases and toxics. Therefore, it’s necessary to choose an instrument capable of dealing with these issues.
Unfortunately, too many purchasers make large and crucial equipment expenditures without really understanding what they are buying. Sensors and their capabilities are the single most important factor when choosing a gas detector; yet more often than not, decisions are based on size, price and other features that have nothing to do with the instrument’s detecting abilities.
The Three Categories of Gas Monitors
There are many federal, provincial/ state and local safety and environmental regulations requiring hazardous gas and vapor monitoring. Most (but not all) requirements fall into three categories:
Personal exposure monitoring;
Confined space entry; and
Ambient air monitoring
Let’s take a brief look at each of these.
1. Personal Exposure Monitoring
Personal exposure monitoring is the detecting of toxic gases in an individual worker’s breathing zone. Alarm settings on monitoring instruments generally relate to the physical and toxic characteristics of a specific gas and/or regulated threshold limits.
The American Conference of Governmental Industrial Hygienists (ACGIH) recommends measuring exposures by threshold limit values (TLV), such as the 8-hour time-weighted average (TWA) and the 15-minute short-term exposure limit (STEL).
Occupational Safety & Health Administration (OSHA) measures with permissible exposure limits (PELs), some of which are based on TLVs. In Canada, PELs are set by provincial Occupational Health and Safety (OHS) regulations. The provincial PELs are similar to those adopted by OSHA in the U.S.
Some monitors are equipped with alarms that measure yet another set of limits called "immediately dangerous to life or health" (IDLH).
2. Confined Space Entry
In general, confined space regulations require that air in a confined space be evaluated for the presence of combustible gases before any worker enters the area. Gases detected must be in the lower explosive limit (LEL) range, with less than 10 percent LEL typically considered an acceptable concentration. This percentage may change depending on whether hot work is to be performed within the confined space.
The space must also be measured for oxygen deficiency or enrichment: 20.9 percent oxygen volume in air is generally considered clean air; 19.5 percent is low (deficient); and 23 percent is too high (enriched).
Finally, the space must be tested for the presence of specific toxic gases, which must be within defined applicable concentration ranges, typically parts per million (PPM) or parts per billion (PPB).
Note: Before setting any sensor alarms, refer to your applicable legislation and verify legal requirements.
3. Ambient Air Monitoring
Ambient air monitoring involves continuous measurement of any workplace air to which workers are exposed – indoors or out. Normally, permanent monitoring systems are used for ambient air monitoring, although it can also be done with portable instrumentation set up to detect specific gases.
The Two Types of Gas Detection
There are two categories of gas detection instruments:
Indirect reading, where samples must be sent to the laboratory for analysis; and
Direct reading, which provides information at the time of sampling.
Direct reading instruments are the only safe source of sampling information in operations where the primary objective of gas detection is to immediately warn a worker of adverse atmospheric changes.
Conclusion
There are a number of direct reading portable gas detector manufacturers in the market today. Their product lines come in a variety of sizes, shapes, colors and sensor configurations. Next week, we’ll look at various combustible gas sensors and discuss their pros and cons.
SELECTING GAS DETECTORS
Sensor Technologies, Part 2 of 4
By Jason Barlow
Proper gas detection equipment can help identify hazardous atmospheres and protect your workers. When choosing a gas detector, the single most important factors to consider are the sensors and their capabilities. I’ve done a bit of research on this topic and this week I’ll share my findings on the types of combustible gas sensors available and compare the pros and cons of each.
Instrument Flexibility
Gas detection manufacturers produce instruments with a variety of sensor configurations. These days, it’s not uncommon to find units with one to six gas monitoring options with interchangeable sensors. But they’re not one-size-fits-all. You need to be fully aware of the different sensor types and the capabilities of each. Before using a gas monitoring instrument, there are two things you need to know:
1. The hazards that are likely to be present in the particular space you’re testing; and
2. The capability of the instrument you’re going to use to detect such hazards.
Understanding Sensor Technology
Sensor technology is the foundation of any instrumentation — whether portable or fixed. It’s also the cornerstone of accurate compliance reporting. Remember, though, that every sensor has its limitations. If your sensor selection is inadequate or inappropriate for the application, then everything downstream of the sensor will be compromised. So you must match the capability of the sensor with your requirements; otherwise, you’ll get inaccurate data and prematurely wear out your sensor.
Combustible Gas Sensor Options
In confined space work, you must monitor for combustible gases. There are three types of combustible gas sensors available for you to choose from: catalytic, metallic oxide semiconductor and infra-red. Let’s take a look at their advantages and disadvantages:
1. Catalytic combustible gas sensors detect combustible gases by causing an actual combustion of gases within the sensor chamber.
Catalytic sensors consist of a flame arresting material encasing two chambers, each of which contains a coiled wire filament. One chamber, whose coil is typically coated with platinum or palladium, is designed to allow air to enter. The other chamber, whose coil is not coated, is sealed to prevent air from entering.
Both coils are heated, typically to 500ºF or higher. When combustible gases are exposed to the coil, they will ignite and raise the temperature even higher. This temperature increase and the change of the coil’s electrical resistance are displayed as “percent LEL.”
Pros
Offer good linearity
Can react to most combustible gases
Cons
Work best in concentrations between 1,000 and 50,000 PPM
Don’t measure trace amounts of gas (under 200 PPM) and therefore are of no use determining toxic levels
Require a minimum of 16% oxygen content in the air to work accurately
Sensor can be damaged by lead or silicone
Readings can be affected by humidity and water vapor condensation
Tend to lose their linearity after a year or so
Not recommended for use in an acetylene atmosphere
Note: The flame arrestor will prevent ignition of most gases outside the sensor, except acetylene. It is extremely important to check the approvals to determine the types of hazardous locations the detector can function within.
2. Metallic oxide semiconductor (MOS or “Solid State”) combustible gas sensors consist of a housing (either a stainless steel sintered cup or plastic) containing an electric conductor. This conductor is made up of a heating element, typically operating between 150ºF and 350ºF, and a bead that contains a mixture of metal oxides.
As the electrical current travels through the bead when exposed to clean air, a base resistance is established. When a gas comes into contact with the sensor surface, a change in sensor resistance occurs. The sensor resistance can change significantly even with small quantities of gases (less than 200 PPM).
Pros
Long operation life (typically 3 to 5 years)
Very rugged with capacity to recover from high concentrations of a gas that could damage other types of sensors
Cons
Require oxygen to work accurately, although not as much as the catalytic
Some sensors’ heating elements have a high demand for power that requires large battery packs
Readings may be affected by humidity and water vapor condensation
3. Infra-red combustible sensors have recently begun appearing in some instruments. These sensors work by reflecting light off a mirror and measuring the amount of light adsorbed during refraction.
Pros
Work well in low oxygen levels or acetylene atmospheres
Cons
Quite expensive
Typically require a constant flow across the sensing assembly and may be slow to clear from alarm
Unable to detect hydrogen
Conclusion
Depending on what you need, there is quite a selection for combustible gas sensor technology. Next week, we’ll look at various toxic sensors and discuss their pros and cons.
SELECTING A GAS DETECTOR
A Look at Toxic Sensors, Part 3 of 4
By Jason Barlow
When choosing a gas detector, the single most important factor to consider is the sensor and its capabilities. Last week, we compared the pros and cons of combustible gas sensors. This week, we’ll look at toxic sensors.
The Two Kinds of Toxic Sensors
When choosing a toxic sensor, you have two options: a wet chem toxic sensor and an MOS toxic broad range gas sensor. Let’s look at each option:
1. Electrochemical (wet chem) toxic sensors react to specific chemical substances such as chlorine, ammonia, carbon monoxide, carbon dioxide, nitrogen dioxide, nitric oxide, hydrogen cyanide, hydrogen sulfide, sulfur dioxide and hydrochloric acid. Read the technical information supplied by the detector’s manufacturer to learn what sensors are available for the unit.
The electrochemical sensor housing contains two, sometimes three, electrodes sitting in a liquid solution, either a base or alkali, depending on what the sensor is “looking for.” The housing is covered by a Teflon membrane, which keeps the fluid in the housing but allows in air. As air molecules enter through the thin Teflon membrane, the fluid reacts with a specific substance if it’s present.
When the detector is working, a small electrical current passes between the two electrodes. Any change in the fluid’s density caused by a reaction to the substance in the air will affect the density of the fluid and change the amount of current passing between the two electrodes. The current then passes through a temperature compensating circuit and the electron flow is read as a specific amount of the substance.
The sensor’s ability to detect specific types of gases is based on:
The choice of membrane;
The number of electrodes;
The alloy of the electrodes (gold, lead, etc.); and
The type of electrolyte fluid.
Pros
Very good linearity, which makes them very accurate for the substance to which they’ll react.
Can measure either large or small quantities.
Cons
Have a typical life span of approximately one year.
Fluid can freeze when left in environments having temperatures lower than 0°C.
Adversely affected by altitude. (Air pressure at sea level (14.73 psi absolute) is the force required to induce the air into the sensor. An increase in altitude means less force is available to push the air into the sensor, thus reducing the accuracy of the reading.)
Some substances, such as moisture, affect the sensor by changing the make-up of the fluid. This reduces the amount of electrical resistance and impacts the reading. Check the manufacturer’s instructions to see which substances will affect the sensor.
May generate readings that are abnormal or don’t make sense. (Note: This problem can be minimized if you know the hazards in your workplace, have a basic understanding of chemistry, know what interference gases adversely affect your unit and follow strict testing protocols.)
2. Metallic oxide semiconductor (MOS) toxic broad range gas sensors are just one of many MOS sensors on the market. The MOS sensor specifically developed for detecting toxic gases is similar in concentration and operation to those used for the detection of combustible gases.
Pros
Capable of reacting to a wide range of toxic gases including carbon monoxide, hydrogen sulfide, ammonia, styrene, toluene, gasoline and many other hydrocarbons and solvents.
Cons
Cannot detect carbon dioxide or sulfur dioxide.
Incapable of telling you what gas you have encountered or the concentration, only that the atmosphere may be hazardous to your health.
Last word on toxic gas sensors
It’s important to note that you should never use an oxygen sensor to detect toxic gases. It is true that a toxic gas will displace the oxygen in a confined space. However, it takes 60,000 PPM of any gas to lower the oxygen from 20.9% (normal) to 19.5% (alarm point). More importantly, 60,000 PPM of any toxic gas will kill you.
Conclusion
There are several types of oxygen sensors available and we’ll look at those next week.
SELECTING A GAS DETECTORA Look at Oxygen Sensors, Part 4 of 4 September 5, 2006
In this series, we’ve compared the pros and cons of the various combustible gas sensors and toxic sensors available in today’s market. This week, I’ll wrap up the series by sharing my findings on oxygen sensors.
The 5 Types of Oxygen Sensors
There are five common types of oxygen sensors. Let’s look closely at the merits of each.
1. Ambient Temperature Electrochemical Oxygen Sensors
Ambient temperature electrochemical oxygen sensors, often referred to as galvanic sensors, operate much like a battery. Oxygen gas flows past an electrode and becomes a negatively charged hydroxyl ion. The ions move through electrolytes in the oxygen sensors to positively charged electrodes, typically made of lead, react with the lead and releases electrons. The electron flow is measured and the measurement can be mathematically converted to an oxygen concentration.
Pros
The only true chemically specific sensor (similar to the electrochemical toxic sensor described last week).
Cons
Susceptible to freezing
Affected by altitude
Nominal operational life of one year.
Susceptible to damage when used with samples containing acid gas species such as hydrogen sulfide, hydrogen chloride, sulfur dioxide, etc. Unless the offending gas constituent is scrubbed prior to analysis, their presence will greatly shorten the life of the sensor.
2. Paramagnetic Oxygen Sensors
Oxygen has a relatively high magnetic susceptibility as compared to other gases such as nitrogen, helium and argon; and it displays a paramagnetic behavior. The paramagnetic oxygen sensor is based on these qualities. It typically consists of a cylindrical shaped container in which a small glass dumbbell is placed. The dumbbell is filled with an inert gas such as nitrogen and suspended on a taut platinum wire within a non-uniform magnetic field. The dumbbell is designed to move freely as it is suspended from the wire. When oxygen is processed through the sensor, the oxygen molecules are attracted to the stronger of the two magnetic fields created by each side of the dumbbell. This causes a displacement of the dumbbell and causes it to rotate. A precision optical system consisting of a light source, photodiode and amplifier circuit is used to measure the degree of rotation of the dumbbell.
In some sensor designs, an opposing current is applied to restore the dumbbell to its normal position. The current required to maintain the dumbbell in its normal state is directly proportional to the partial pressure of oxygen and is represented electronically in percent oxygen.
The magnetodynamic or dumbbell type of design is the predominate sensor type of paramagnetic oxygen sensors. But design variations are available, depending on the manufacturer. Also, other types of sensors have been developed that use the susceptibility of oxygen to a magnetic field, which include the thermomagnetic or “magnetic wind” type and the magnetopneumatic sensor.
Pros
Offer very good response time characteristics
Use no consumable parts, making sensor life (under normal conditions) quite good
Offer excellent precision over a range of 1% to 100% oxygen
Cons
Quite delicate and sensitive to vibration and position
Not recommended for trace oxygen measurements in general, due to the loss in measurement sensitivity
Other gases that exhibit a magnetic susceptibility can produce sizeable measurement errors. Manufactures of paramagnetic oxygen sensors and analyzers should provide details on these interfering gases.
3. Polarographic Oxygen Sensors
Polarographic oxygen sensors are often referred to as a Clark Cell. In this type of sensor, both the anode (typically silver) and cathode (typically gold) are immersed in an aqueous electrolyte of potassium chloride. The electrodes are separated from the sample by a semi-permeable membrane that diffuses oxygen into the sensor. The silver anode is typically held at a potential of 0.8V (polarizing voltage) with respect to the gold cathode. Molecular oxygen is consumed electrochemically with an accompanying flow of electrical current directly proportional to the oxygen concentration (based on Faraday’s law). The current output generated from the sensor is measured and amplified electronically to provide a percent oxygen measurement.
Pros
While inoperative, there is no consumption of the electrode (anode)
Almost indefinite storage times
Not position sensitive
The sensor of choice for dissolved oxygen measurements in liquids
Cons
Relatively high frequency of sensor replacement
Sensor membrane and electrolyte require maintenance
For gas phase oxygen measurements, the sensor is suitable for percent level oxygen measurements only
4. Non-Depleting Coulometric Oxygen Sensors
Non-depleting coulometric sensors are a variant of the polarographic oxygen sensor in which two similar electrodes are immersed in an electrolyte consisting of potassium hydroxide. Typically, an external EMF of 1.3 VDC is applied across both electrodes which act as the driving mechanism for reduction/oxidation reaction. The electrical current resulting from this reaction is directly proportional to the oxygen concentration in the sample gas. As with other sensor types, the signal derived from the sensor is amplified and conditioned prior to displaying.
Pros
Can be used for both percent and trace oxygen measurements
Can measure parts per billion levels of oxygen
Cons
One sensor cannot be used to measure both high percentage levels as well as trace concentrations of oxygen
Sensors are position sensitive and replacement costs are quite expensive, in some cases, paralleling that of an entire analyzer of another sensor type
Not recommended for applications where oxygen concentrations exceed 25%.
5. Zirconium Oxide Oxygen Sensors
Zirconium oxide oxygen sensors are occasionally referred to as the “high temperature” electrochemical sensor and are based on the Nernst principle.
Zirconium oxide sensors use a solid state electrolyte typically fabricated from zirconium oxide stabilized with yttrium oxide. The zirconium oxide probe is plated on opposing sides with platinum, which serves as the sensor electrodes.
For a zirconium oxide sensor to operate properly, it must be heated to approximately 650° C. At this temperature, on a molecular basis, the zirconium lattice becomes porous, allowing the movement of oxygen ions from a higher concentration of oxygen to a lower one, based on the partial pressure of oxygen. To create this partial pressure differential, one electrode is usually exposed to air (20.9% oxygen) while the other electrode is exposed to the sample gas. The movement of oxygen ions across the zirconium oxide produces a voltage between the two electrodes, the magnitude of which is based on the oxygen partial pressure differential created by the reference gas and sample gas.
Pros
Can be utilized in high temperature environments
Excellent response time characteristics
Can be used to measure 100% oxygen, as well as parts per billion concentrations
The “de facto standard” for in-situ combustion control applications
Cons
Due to the high temperatures (typically) of operation, the life of the sensor can be shortened by on/off operation
Constant heating and cooling often causes “sensor fatigue”
Unsuitable for trace oxygen measurements when reducing gases (hydrocarbons of any species, hydrogen and carbon monoxide) are present in the sample gas
At operating temperatures of 650°C, the reducing gases will react with the oxygen, consuming it prior to measurement thus producing a lower than actual oxygen reading. The magnitude of the error is proportional to the concentration of reducing gas.
Conclusion
Selecting the right gas detector and attendant sensor is a crucial safety decision. I hope this and the previous installments help you make the right decision for your workplace and workforce.
posted by Mike B at 12:02 PM
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