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Gas Detectors

Published: 10th Dec 2010 in OSA Magazine

Confined spaces contain two types of hazards: atmospheric and physical. About 40% of all confined space incidents involve hazardous atmospheres. For this reason regulators emphasise the requirement to analyse the air for hazardous conditions prior to entry and that spaces be monitored periodically, or in certain situations, continuously, while occupied.

In the USA, OSHA’s confined space standard (29CFR1910.146) states that before an employee enters a permit-required confined space the internal atmosphere shall be tested, with a calibrated direct reading instrument, for the following conditions in the order given: 1) oxygen content, 2) flammable gases and vapours and 3) potential toxic air contaminants.

The purpose of this article is to review the instruments commonly available and considerations for finding the one that meets the CSE needs of your worksite.

A July 2010 incident in Pittsburgh, Pennsylvania is representative of many confined space incidents:

An employee with less than two months’ experience had been working in a shaft of the plant’s sewer system. He was observed trying to climb out but fell back into the hole, apparently overcome by toxic fumes, possibly methane gas. Three people, the plant superintendent, an inspector from an engineering firm, and a contract worker, climbed into the hole to help the employee. They were all overcome within seconds. Fortunately, firefighters with breathing apparatuses, ropes and harnesses arrived in time to pull them to safety. The first employee died from the fall. The other three ‘rescuers’ were treated at the hospital and have recovered from their injuries.

It’s very unfortunate and all too common with confined space incidents that good-intentioned but poorly equipped or undertrained rescuers become victims. Having the proper equipment is a first step towards avoiding any confined space incident.

The subsequent steps - which are equally important but are not addressed in this article - involve operator qualification, implementing a thorough calibration plan to assure the ongoing accuracy of the gas detectors and implementing comprehensive control of work practices for all aspects of confined space entry activities.

Portable gas detection equipment

Awareness of the hazards in your workplace, a basic understanding of chemistry, and knowing what conditions and circumstances can disrupt various types of detectors are essential to choosing the best detectors for your work environment. Recall that the three areas required to be addressed by a detector for confined space work are oxygen deficiency/enrichment, combustible gases and toxics. There are two broad categories of gas detection instruments: indirect reading and direct reading. OSHA requires direct-reading instruments, which this article addresses exclusively.

Gas detectors can also be divided into single-gas detectors and multiple-gas detection devices. Due to the requirement to detect oxygen, combustible gases and toxic gases only multi-gas devices should be used for confined space. The typical multi-gas units detect four gases - oxygen, carbon monoxide, hydrogen sulfide, and methane as the combustible gas - however, some manufacturers offer units with plug-in modules for detection of a range of additional toxic gases, including ammonia, chlorine, hydrogen cyanide, phosphine, and sulfur dioxide. Fortunately, over the past 40 years electronics have shrunk in size and cost to the point that there are many competitive choices of direct reading multi-gas detector technology on the market today. The most common include electrochemical sensors, semiconductor sensors, catalytic bead sensors, and the photo-ionisation detector (PID). A buyer should work with the manufacturer to configure the multi-gas unit that meets their worksite needs.

Electrochemical component

Oxygen and about 30 other gases are commonly detected by means of an electrochemical element in handheld meters used for confined space entry.

This technology has been used since the 1930s and is still well established in today’s handheld market thanks to integrated circuits, microprocessors and semi-conductors. Inside this element is typically an electrolyte solution contained inside a porous housing, which has two or more electrodes. These are sometimes referred to as wet chem elements. The housing is covered by a membrane that keeps the electrolyte contained inside the housing yet allows air to pass through. Molecules from the ambient air penetrate the membrane and react with specific substances to change the flow of electrical current.

The current is then analysed and the result displayed as concentration on a read out. The manufacturer determines the detector’s ability to detect specific types of gases based on the choice of membrane, the number of electrodes, the alloy of the electrodes and the type of electrolyte. The potential downsides to electrochemical elements commonly include:

• Variations in humidity, air pressure, and temperature can affect the accuracy of certain elements and shorten the life of the instrument. For this reason it’s necessary to use the device within the recommended operating ranges of these environmental factors. A two to three year life is common for standard
electrochemical sensors
• Certain substances can cause false readings due to adverse reactions in the electrolyte. It’s common that exposure to organic vapours can significantly shorten the life of the sensor. This is referred to as poisoning the element. There are also cross-sensitivities - some gases may result in a response without poisoning the sensor. The user must understand their environment, the potential gases present, and possible sensitivities

Manufacturers have addressed these deficiencies by utilising a variety of electrolytes, installing more sophisticated circuitry, providing monitoring and compensating electronic devices, and improved construction design and materials. It’s critical to check with the manufacturer to be aware of and understand all aspects of a particular model to assure your specific needs can be met.

Galvanic cells for oxygen detection

Galvanic cell technology is similar to the electrochemical technology. The cell works by measuring the current generated when a gas reacts with reagent in the electrolytic cell. Galvanic cells are specifically designed for monitoring low and high concentrations of oxygen, where either situation can be hazardous to personnel and process equipment. The O2 in air diffuses through a selective membrane into the galvanic cell and reacts with a noble metal (Pt, Ag) electrode. This reaction creates an electrical potential between this electrode and a base metal (lead) electrode, causing current to flow through an external circuit. The external circuit uses a resistor to create a voltage drop proportional to O2 concentration. Temperature compensation is required due to the sensitivity of the cell to ambient temperatures.

In a galvanic cell sensor, no external power source is needed to detect the gas. Its output is linear and proportional to O2 concentration up to 40% volume. These sensors are small, lightweight and inexpensive. Maintenance is minimal and recalibration infrequent.

Combustible gases detector component

These detectors sense combustible gases by causing a small but actual combustion of gases within a sensing chamber. These detectors have been used in industrial settings since the 1940s but only became truly portable and suitable for multi-gas use in 1978. They contain a catalytic - or bead - sensor consisting of a flame arresting material to contain the combustion, encasing two chambers each, which contain a coiled wire filament. One chamber is designed to allow air to enter it. Its coil is coated with platinum or palladium to serve as a catalyst for combustion. The wire in the other chamber is sealed to prevent air from contacting it and serves as a baseline for resistivity. Both coils are heated by the battery to temperatures of 500°F or higher. When combustible gases are exposed to the coated coil they ignite and the increased temperature causes an increase in electrical resistance.
The temperature increase and the change of the coil’s electrical resistance are used as inputs to calculate and display combustibility as ‘percent LEL’ (Lower Explosive Limit).

These sensors offer good linearity, however, they are most accurate in concentrations between 1,000 and 50,000 PPM. They do not measure trace amounts of gas (under 200 PPM) and, therefore, are of no use in determining toxic levels.

The disadvantages of catalytic or bead combustibility sensors are:

• They must have a minimum of 16% oxygen content in the air to allow combustion to take place and thus to give an accurate read out
• The sensor can be influenced by exposure to high concentrations of combustible gases and can be ‘poisoned’ or damaged by silicone vapours, sulfur compounds, and corrosive gases including chlorine and chlorinated solvents
• The readings can be affected by humidity and water vapour condensation
• They respond poorly to low energy hydrocarbons such as oil vapours
• They tend to lose their linearity over time
• The heat of combustion varies for different gases so they need to be calibrated for a specific gas. They can, however, be used for a variety of gases using published correction factors
• They are not recommended for use in an acetylene atmosphere as the internal flame arrestor will not prevent propagation of the flame outside
the instrument. This is to say they are not intrinsically safe in an acetylene environment

A second type of combustible detector uses a Metallic Oxide Semiconductor, MOS, or solid state combustible gas sensor. These sensors have been used in industrial settings since the 1970s and consist of a housing containing an electric conductor. This conductor has a heating element (operating between 150 degrees F to 350 degrees F) and a bead of metal oxides. As electric current travels through the bead when exposed to clean air, a baseline resistance is established. When a contaminated gas comes into contact with the sensor surface, a change in sensor resistance occurs. The sensor resistance will vary significantly even with small quantities of gas (less than 200 PPM). This sensor has a long operating life (three to five years), is very rugged and will recover better from high concentrations of a gas that could damage other types of sensors. The downside of MOS sensors is:

• They require oxygen to work accurately, although not as much as the catalytic types
• Some heating elements have a high demand for power, which requires large battery packs
• The readings can be affected by humidity and water vapour condensation

A third type of combustible detector uses an infrared sensor. These sensors utilise the selective absorption of infrared radiation by gases to determine the concentration and combustibility. They work well for measuring combustible gases at a distance (up to 100 metres), they can be used without entering the confined space if a window is available, and they work well in low oxygen levels and in acetylene atmospheres. They are, however, significantly more expensive than catalytic or MOS sensors. They are unable to detect hydrogen, acetylene, ammonia and carbon monoxide.

Photo Ionisation Detector element

Typical Photo Ionisation Detectors (PID) detect volatile organic compounds and limited other gases in concentrations that range from sub-parts per billion to 10,000 parts per million (PPM). They are capable of giving instantaneous readings and continuous monitoring. This detector is an efficient and inexpensive type of gas detector but its limitations need to be understood to be used effectively in confined space entry applications.

The PID uses an ultraviolet lamp that emits photons, which are absorbed by a designated compound in an ionisation chamber. The lamp is rated to a specific ionisation potential measured in terms of electron volts. When a gas molecule passes through the light emitted from the lamp, the molecule is either ionised or it is not. Some gases ionise at lower energy levels than others. If the gas is ionised, positive and negative ions are collected on electrodes and a current is generated. This produces a signal that is directly proportional to the amount of ions present at the electrodes. The current generated provides a measure of the concentration and can be directly related to parts per million. The PID is most commonly used to detect Volatile Organic Compounds (VOC) in fugitive emissions, landfills, soil, sediment, air and water but due to their remote sensing ability and broad-spectrum nature can be useful in certain types of confined space entry work.

Drawbacks to PID relative to Confined Space Entry:

• The detector itself measures the amount of positive and negative ions detected on the electrodes. These ions can come from any compound ionised. The device does not distinguish what the compound actually is. Only if a specific VOC is known by the tester to be the only VOC present in the confined space will the tester know the concentration of that VOC
• Another limitation to be aware of is that many PID respond to humidity.

If a high-humidity sample is taken, the water vapour could cause false positive readings

Given these drawbacks, the most appropriate application of PID instruments is as a secondary toxic gas detector element in a multi-gas unit.

Consideration of electronic features

Once the sensing components of the detector have been selected it’s time to focus on the ‘whistle and bells’ provided with the unit. The basic units will have an alarm that signals a potentially hazardous reading has occurred. However, there are many more features to assure that information provided is reliable and useful. Keep in mind that life and death decisions will be made based on the data provided. Long life and ease of use are important but accuracy and dependability are paramount. The instrument’s response time, accuracy, precision, protection from interference, reading drift and sensitivity are all factors that can differentiate a basic unit from a good investment.

Accuracy and precision: Accuracy refers to the agreement between a measurement and the correct value. If a detector registers 5.0% when the methane concentration is known to be exactly 5.0%, the detector is said to be accurate. Accuracy cannot be discussed meaningfully unless the true value is known. Precision refers to the repeatability of the measurement. If each reading for several identical samples indicates 5.3% this detector is said to be very precise. Whether it is accurate is another matter.

Certifications by an independent product safety organisation or process such as Ex, CSA, UL, FMR, or CE provide assurance that the device conforms to internationally recognised performance standards.

Data logging describes the unit’s ability to internally store a series of readings for later review. This can be valuable when looking for documenting results, analysing trends, comparing activities and incident investigations.

Electromagnetic (or radio) frequency interference protection is the unit’s ability to protect the readings from interference caused by radio waves, pulsed power lines, transformers, and generators. EMI/RFI protection is expressed in terms of immunity to a stated amount of watts of radio transmission at a specific distance.

Explosion proof means the enclosure around the device is sufficiently robust such that it will not allow an internal explosion to propagate outside the enclosure and potentially cause an external explosion. This is a critical feature when exposures might include combustible compounds.

Intrinsically safe means the detector is safe to use in an explosive environment. This is achieved by limiting the electrical and thermal energy used in the device below the threshold that would ignite an explosion in a hazardous atmosphere (within Zone 0, 1 and 2 or Division 1 or 2 Areas). This is a critical feature when exposures might include combustible components.

Modems are provided in some units as a feature that allows real-time transmission of data to a base unit. This feature allows others to monitor the sensor data from a remote location. These can utilise wireless, blue tooth, hard line, or radio technology. Be aware that this feature does not serve to replace local management of confined space entry activities.

Response time is the time period between obtaining data from the sensors and displaying it on a read out. This time period depends on what information is collected, the sensor response, how the unit of measurement is being used (e.g. % LEL or PPM). Response time can range from milliseconds to minutes.

Sensitivity is the unit’s ability to accurately measure changes in concentrations. The hazards presented by the substance being measured help determine the requirement for sensitivity. For instance, the Time Weighted Average (TWA) OSHA Permissible Exposure Limit (PEL) for the construction industry is 10 PPM of H2S and it is Immediately Dangerous to Life and Health (IDLH) at 100 PPM. Sensitivity is critical in the case of an H2S detector. On the other hand, carbon dioxide’s TWA PEL is 5,000 PPM, and is IDLH at 40,000 PPM; therefore, the instrument’s sensitivity is not as critical because a 1000 PPM change in concentration is not as significant to the worker’s health.

Mechanical Aspects

Beyond the sensing technology and the electronic features there are important features of the gas detector that may be deemed critical by the user.

These include:

• Method of taking a sample. To accurately sample a confined space without entering the operator must understand how the air to be tested comes into contact with the sensor. There are two common methods manufacturers use to remotely expose the sensor to the atmosphere:
• Drawing a sample to the sensor through a tube, or
• Using a detachable sensor assembly that can be extended into the atmosphere
• The use of a tube to suck in a sample is the most common for remote acquisition of a sample. This usually involves extending a sampling tube into the confined space while the tester remains at a safe distance. The tube is equipped with a hand pumped bulb or a battery powered aspirator. Be aware of the tube’s length and how many manual strokes or minutes of powered pumping are required to adequately purge the tube volume. It’s essential to assure the integrity of the sampling method. Tubes with leaks, inadequate pumping mechanisms, inadequate purging of the system or contaminants in the tubes can defeat even the smartest and most expensive technology. Some manufacturers overcome tube limitations by designing a removable sensor that can be extended into the area to be tested. This avoids issues with purging
• Robustness of the unit. Gas detectors are used and abused in the worst of circumstances and must be able to withstand this - frequently. Rain, water, heat, ice, mud, sand, humidity, and being dropped are everyday occurrences in some workplaces. The more durable units are water proof and shock resistant
• The unit’s displays and alarm system must meet the requirements of the workplace. If you can’t read or hear the output the instrument has no value. Dual aural and visual alarms are best. Both sounds and lights must exceed the ambient noise and lighting of the confined space, and be heard and be seen by the user. Most instruments are equipped with both a warning signal and an alarm condition. Some units allow the user to set the alarm points while others are pre-set at the factory. The unit should be tamper resistant and default to an alarm mode in the event of battery or sensor failure
• Batteries need to last either for the duration of the work or until replacements are available. Some units are equipped with lithium-ion batteries that can support the unit for 24+ hours of operation. Other units are designed to operate on readily available standard-sized batteries such as AA, AAA or C. It’s common to have one or more sets of batteries charging so there are always fully charged spares to serve as backup. Whatever you select, it’s essential the unit does not run out of power while monitoring a confined space
• Switches, buttons and knobs should be secure enough so that they cannot be knocked away from their set point, but workers should be able to operate them with gloves on. Gauges and/or displays should be large and easily read and understood

It’s to your advantage to consult with manufacturers and suppliers to determine exact features and limitations of specific models.

Summary

There are many types of portable gas detection units on the market today. It’s common for manufacturers to assist in selecting the right instrument for your particular needs. Many manufacturers also offer loaner or rental programmes for users to become familiar with their various units prior to purchase. These instruments represent sophisticated hand-held technology that at certain times may well be the only thing that stands between you and sudden death. Understand the hazards in your work place, shop the market, and then buy the best you can afford.

Disclaimer

Although every care had been taken in providing this information the author accepts no responsibility or liability for any consequences arising from the use of such information.

Author Details

David W Moore is an American health, safety and environmental advisor living on Bainbridge Island, Washington, USA. Mr Moore holds a B.S. and M.Sc. in Chemical Engineering, both from Georgia Tech. He has 38 years of experience in the US domestic and international oil and gas industry. In the past several years Mr Moore has provided significant onsite HSE leadership to the China West to East Pipeline Project (Gansu Province in Western China), the BTC Pipeline Project (Eastern Turkey) and the Tangguh LNG Plant Project (West Papua, Indonesia). Mr Moore is an active member of the American Society of Safety Engineers (ASSE) and can be contacted at: dmoore.che71@gtalumni.org






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Published: 10th Dec 2010 in OSA Magazine

Author


David Moore



David Moore

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dmoore.che71@gtalumni.org

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