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Gas Detection Principles

1. Honeywell Analytics

Honeywell Analytics is the World’s leading manufacturer of flammable, toxic and oxygen detection systems, bringing together nearly 200 years of collective experience in the design and creation of some of the most innovative products in gas detection. Suitable for industrial, commercial and domestic applications where people can be exposed to flammable, toxic or oxygen deficiency gas hazards, Honeywell Analytics’ gas detection solutions are available in a variety of fixed and portable ranges.

The company’s core competencies lie in the design, development and manufacture of electrochemical, catalytic, paper tape and infrared sensors. With a high commitment to research and engineering, the company offers solutions to increasingly difficult gas detection and monitoring applications.

Lumidor and Neotronics are the company’s portable ranges that include instruments with disposable multi-gas cartridges and automated calibration facilities; Sieger fixed, robust detectors are crucial for gas monitoring in extreme conditions such as the marine and offshore industry; MDA Scientific is the leading fixed solution for the semiconductor industry, monitoring low levels of highly toxic gases and Zareba, Honeywell Analytics’ newest family of detectors, makes compliance with health and safety regulations in commercial and industrial sites, simple, convenient and cost-effective.

The success and influence of these brands is evident in them having become as much associated with industry-standard processes as with the products they represent - for example, ‘MDA rooms’ are commonly found in the semiconductor manufacturing facilities, while Sieger is used so commonly on offshore oil platforms it has become effectively a shorthand term to describe gas detection technology.

Honeywell Analytics’ success is builton a legacy of innovation in and the development of gas detection technology. Responsible for a number of industry ‘firsts’ that has defined and transformed the theory, practise and language of the gas detection industry, notable innovations include the original Siegistor catalytic bead, development of optical point and open path infrared gas detection technology, the Chemcasette® paper tape cartridge and the world’s first automated electrochemical cell manufacturing facility.

Honeywell Analytics’ commitment to excellence is reflected in our dedication to best practise in customer relations. By adopting a cohesive, unified approach to all aspects of customer relations and service, all enquiries, sales, service and technical support are handled by two ‘customer business centers’ located in Uster, Switzerland and Sunrise, Florida, ensuring our customers receive the high level of advice and support they deserve.

We are a responsible company and take pride in building positive, sustained relationships with all our stakeholders. By the very nature of our business, we are an environmentally-aware company and our working and manufacturing methods reflect this commitment to good environmental practise.

2. Introduction

Industrial processes increasingly involve the use and manufacture of highly dangerous substances, particularly flammable, toxic and oxygen gases. Inevitably, occasional escapes of gas occur, which create a potential hazard to the industrial plant, its employees and people living nearby. Worldwide incidents, involving asphyxiation, explosions and loss of life, are a constant reminder of this problem.

In most industries, one of the key parts of any safety plan for reducing risks to personnel and plant is the use of early-warning devices such as gas detectors. These can help to provide more time in which to take remedial or protective action. They can also be used as part of a total, integrated monitoring and safety system for an industrial plant.

This handbook is intended to offer a simple guide to anyone considering the use of such gas detection equipment. It provides an explanation of both the principles involved and the instrumentation needed for satisfactory protection of personnel, plant and environment. The aim has been to answer as many as possible of the most commonly asked questions about the selection and use of industrial gas detection equipment.

3. What is gas?

The name gas comes from the word chaos. Gas is a swarm of molecules moving randomly and chaotically, constantly colliding with each other and anything else around it. Gases fill any available volume and due to the very high speed at which they move will mix rapidly into any atmosphere in which they are released.

4. Gas Hazards

There are three main types of gas hazard: Flammable, Toxic and Asphyxiant

5. Flammable Gas Hazards

Combustion is a fairly simple chemical reaction in which oxygen is combined rapidly with another substance resulting in the release of energy. This energy appears mainly as heat – sometimes in the form of flames. The igniting substance is normally, but not always, a Hydrocarbon compound and can be solid, liquid, vapor or gas. However, only gases and vapors are considered in this publication.

Flammable Limit

There is only a limited band of gas/air concentration which will produce a combustible mixture. This band is specific for each gas and vapor and is bounded by an upper level, known as the Upper Explosive Limit (or the UEL) and a lower level, called the Lower Explosive Limit (LEL).

At levels below the LEL, there is insufficient gas to produce an explosion (i.e. the mixture is too ‘lean’), whilst above the UEL, the mixture has insufficient oxygen (i.e. the mixture is too ‘rich’). The flammable range therefore falls between the limits of the LEL and UEL for each individual gas or mixture of gases. Outside these limits, the mixture is not capable of combustion. The Flammable Gases Data in section 2.4 indicates the limiting values for some of the better-known combustible gases and compounds. The data is given for gases and vapors at normal conditions of pressure and temperature. An increase in pressure, temperature or oxygen content will generally broaden the flammability range.

In the average industrial plant, there would normally be no gases leaking into the surrounding area or, at worst, only a low background level of gas present. Therefore 5 Flammable Gas Hazards Flammable Limit There is only a limited band of gas/air concentration which will produce a combustible mixture. This band is specific for each gas and vapor and is bounded by an upper level, known as the Upper Explosive Limit (or the UEL) and a lower level, called the Lower Explosive Limit (LEL). the detecting and early warning system will only be required to detect levels from zero percent of gas up to the lower explosive limit. By the time this concentration is reached, shut-down procedures or site clearance should have been put into operation. In fact this will typically take place at a concentration of less than 50 percent of the LEL value, so that an adequate safety margin is provided.

However, it should always be remembered that in enclosed or unventilated areas, a concentration in excess of the UEL can sometimes occur. At times of inspection, therefore, special care needs to be taken when operating hatches or doors, since the ingress of air from outside can dilute the gases to a hazardous, combustible mixture.

Flammable Gas Properties

Ignition Temperature:

Flammable gases also have a temperature where ignition will take place, even without an external ignition source such as a spark or flame. This temperature is called the Ignition Temperature. Apparatus for use in a hazardous area must not have a surface temperature that exceeds the ignition temperature. Apparatus is therefore marked with a maximum surface temperature or T rating.

6. Toxic Gas Hazards

Some gases are poisonous and can be dangerous to life at very low concentrations. Some toxic gases have strong smells like the distinctive ‘rotten eggs’ smell of H2S. The measurements most often used for the concentration of toxic gases are parts per million (ppm) and parts per billion (ppb). For example 1ppm would be equivalent to a room filled with a total of 1 million balls and 1 of those balls being red. The red ball would represent 1ppm.

More people die from toxic gas exposure than from explosions caused by the ignition of flammable gas. (It should be noted that there is a large group of gases which are both combustible and toxic, so that even detectors of toxic gases sometimes have to carry hazardous area approval). The main reason for treating flammable and toxic gases separately is that the hazards and regulations involved and the types of sensor required are different.

With toxic substances, (apart from the obvious environmental problems) the main concern is the effect on workers of exposure to even very low concentrations, which could be inhaled, ingested, or absorbed through the kin. Since adverse effects can often result from additive, long-term exposure, it is important not only to measure the concentration of gas, but also the total time of exposure. There are even some known cases of synergism, where substances can interact and produce a far worse effect when together than the separate effect of each on its own.

Concern about concentrations of toxic substances in the workplace focus on both organic and inorganic compounds, including the effects they could have on the health and safety of employees, the possible contamination of a manufactured end-product (or equipment used in its manufacture) and also the subsequent disruption of normal working activities.

Hygiene Monitoring

The term ‘hygiene monitoring’ is generally used to cover the area of industrial health monitoring associated with the exposure of employees to hazardous conditions of gases, dust, noise etc. In other words, the aim is to ensure that levels in the workplace are below the statutory limits.

This subject covers both area surveys (profiling of potential exposures) and personal monitoring, where instruments are worn by a worker and sampling is carried out as near to the breathing zone as possible. This ensures that the measured level of contamination is truly representative of that inhaled by the worker.

It should be emphasised that both personal monitoring and monitoring of the workplace should be considered as important parts of an overall, integrated safety plan. They are only intended to provide the necessary information about conditions as they exist in the atmosphere. This then allows the necessary action to be taken to comply with the relevant industrial regulations and safety requirements.

Whatever method is decided upon, it is important to take into account the nature of the toxicity of any of the gases involved. For instance, any instrument which measures only a time-weighted average, or an instrument which draws a sample for subsequent laboratory analysis, would not protect a worker against a short exposure to a lethal dose of a highly toxic substance. On the other hand, it may be quite normal to briefly exceed the average, long-term (LTEL) levels in some areas of a plant, and it need not be indicated as an alarm situation. Therefore, the optimum instrument system should be capable of monitoring both short and long term exposure levels as well as instantaneous alarm levels.

Toxic Exposure Limits

European Occupational Exposure Limits:

Occupational Exposure Limit values (OELs) are set by competent national authorities or other relevant national institutions as limits for concentrations of hazardous compounds in workplace air. OELs for hazardous substances represent an important tool for risk assessment and management and valuable information for occupational safety and health activities concerning hazardous substances.

Occupational Exposure Limits can apply both to marketed products and to waste and by products from production processes. The limits protect workers against health effects, but do not address safety issues such as explosive risk. As limits frequently change and can very by country, you should consult your relevant national authorities to ensure that you have the latest information.

Occupational exposure limits in the UK function under the Control of Substances Hazardous to Health Regulations (COSHH). The COSHH regulations require the employer to ensure that the employee’s exposure to substances hazardous to health is either prevented or if not practically possible, adequately controlled. As of 6 April 2005, the regulations introduced a new, simpler Occupational Exposure Limit system. The existing requirements to follow good practice were brought together by the introduction of eight principles in the Control of Substances Hazardous to Health (Amendment) Regulations 2004.

Maximum Exposure Limits (MELs) and Occupational Exposure Standards (OESs) were replaced with a single type of limit - the Workplace Exposure Limit (WEL). All the MELs, and most of the OESs, are being transferred into the new system as WELs and will retain their previous numerical values. The OESs for approximately 100 substances were deleted as the substances are now banned, scarcely used or there is evidence to suggest adverse health effects close to the old limit value. The list of exposure limits is known as EH40 and is available from the UK Health and Safety Executive. All legally enforceable WELs in UK are air limit values. The maximum admissible or accepted concentration varies from substance to substance according to its toxicity. The exposure times are averaged for eight hours (8-hour TWA) and 15 minutes (short-term exposure limit STEL). For some substances, a brief exposure is considered so critical that they are set only a STEL, which should not be exceeded even for a shorter time. The potency to penetrate through skin is annotated in the WEL list by remark “Skin”. Carcinogenicity, reproduction toxicity, irritation and sensitation potential are considered when preparing a proposal for an OEL according to the present scientific knowledge.

US Occupational Exposure Limits:

The Occupational Safety systems in the United States vary from state to state. Here, Information is given on 3 major providers of the Occupational Exposure Limits in the USA - ACGIH, OSHA, and NIOSH.

The American Conference of Governmental Industrial Hygienists (ACGIH) publishes Maximum Allowable Concentrations (MAC), which were later renamed to “Threshold Limit Values” (TLVs).

Threshold Limit Values are defined as an exposure limit “to which it is believed nearly all workers can be exposed day after day for a working lifetime without ill effect”. The ACGIH is a professional organisation of occupational hygienists from universities or governmental institutions. Occupational hygienists from private industry can join as associate members. Once a year, the different committees propose new threshold limits or best working practice guides. The list of TLVs includes more than 700 chemical substances and physical agents, as well as dozens of Biological Exposure Indices for selected chemicals.

The ACGIH defines different TLV-Types as:

Threshold Limit Value – Time-Weighted Average (TLV-TWA): the time-weighted average concentration for a conventional 8-hour workday and a 40-hour workweek, to which it is believed that nearly all workers may be repeatedly exposed, day after day, without adverse effect.

Threshold Limit Value – Short-Term Exposure Limit (TLV-STEL): the concentration to which it is believed that workers can be exposed continuously for a short period of time without suffering from irritation, chronic or irreversible tissue damage, or narcosis. STEL is defined as a 15-minute TWA exposure, which should not be exceeded at any time during a workday.

Threshold Limit Value - Ceiling (TLV-C): the concentration that should not be exceeded during any part of the working exposure.

There is a general excursion limit recommendation that applies to those TLV-TWAs that do not have STELs. Excursions in worker exposure levels may exceed 3 times the TLV-TWA for no more than a total of 30 minutes during a workday, and under no circumstances should they exceed 5 times the TLV-TWA, provided that the TLV-TWA is not exceeded.

ACGIH-TLVs do not have a legal force in the USA, they are only recommendations. OSHA defines regulatory limits. However, ACGIH-TLVs and the criteria documents are a very common base for setting TLVs in the USA and in many other countries. ACGIH exposure limits are in many cases more protective than OSHA’s. Many US companies use the current ACGIH levels or other internal and more protective limits.

The Occupational Safety and Health Administration (OSHA) of the U.S. Department of Labor publishes Permissible Exposure Limits (PEL). PELs are regulatory limits on the amount or concentration of a substance in the air, and they are enforceable. The initial set of limits from 1971 was based on the ACGIH TLVs. OSHA currently has around 500 PELs for various forms of approximately 300 chemical substances, many of which are widely used in industrial settings. Existing PELs are contained in a document called “29 CFR 1910.1000”, the air contaminants standard. OSHA uses in a similar way as the ACGIH the following types of OELs: TWAs, Action Levels, Ceiling Limits, STELs, Excursion Limits and in some cases Biological Exposure Indices (BEIs).

The National Institute for Occupational Safety and Health (NIOSH) has the statutory responsibility for recommending exposure levels that are protective to workers. NIOSH has identified Recommended Exposure Levels (RELs) for around 700 hazardous substances. These limits have no legal force. NIOSH recommends their limits via criteria documents to OSHA and other OEL setting institutions. Types of RELs are TWA, STEL, Ceiling and BEIs. The recommendations and the criteria are published in several different document types, such as Current Intelligent Bulletins (CIB), Alerts, Special Hazard Reviews, Occupational Hazard Assessments and Technical Guidelines.

7. Asphyxiant (Oxygen Deficiency) Hazard

We all need to breathe the oxygen (O2) in air to live. Air is made up of several different gases including oxygen. Normal ambient air contains an oxygen concentration of 20.9% v/v. When the oxygen level dips below 19.5% v/v, the air is considered oxygendeficient. Oxygen concentrations below 16% v/v are considered unsafe for humans.

8. Oxygen Enrichment

It is often forgotten that oxygen enrichment can also cause a risk. At increased O2 levels the flammability of materials and gases increases. At levels of 24% items such as clothing can spontaneously combust.

Oxyacetylene welding equipment combines oxygen and acetylene gas to produce an extremely high temperature. Other areas where hazards may arise from oxygen enriched atmospheres include areas manufacturing or storing rocket propulsion systems, products used for bleaching in the pulp and paper industry and clean water treatment facilities.

Sensors have to be specially certified for use in O2 enriched atmospheres.

9. Typical Areas that Require Gas Detection

There are many different applications for flammable, toxic and oxygen gas detection. Industrial processes increasingly involve the use and manufacture of highly dangerous substances, particularly toxic and combustible gases. Inevitably, occasional escapes of gas occur, which create a potential hazard to the industrial plant, its employees and people living nearby. Worldwide incidents involving asphyxiation, explosions and loss of life, are a constant reminder of this problem.

In most industries, one of the key parts of the safety plan for reducing the risks to personnel and plant is the use of early warning devices such as gas detectors. These can help to provide more time in which to take remedial or protective action. They can also be used as part of a total integrated monitoring and safety system for an industrial plant.

Oil & Gas

The oil and gas industry covers a large number of upstream activities from the on and offshore exploration and production of oil and gas to its transportation, storage and refining. The large amount of highly flammable Hydrocarbon gases involved are a serious explosive risk and additionally toxic gases such as Hydrogen Sulfide are often present.

Typical Applications:

  • Exploration Drilling Rigs
  • Production Platforms
  • Onshore oil and gas terminals
  • Refineries

Typical Gases:

Flammable: Hydrocarbon gases
Toxic: Hydrogen Sulfide, Carbon Monoxide

Semiconductor Manufacturing

Manufacturing semiconductor materials involves the use of highly toxic substances and flammable gas. Phosphorus, arsenic, boron and gallium are commonly used as doping agents. Hydrogen is used both as a reactant and a reducing atmosphere carrier gas. Etching and cleaning gases include NF3 and other perfluorocompounds.

Typical Applications:

  • Wafer reactor
  • Wafer dryers
  • Gas Cabinets
  • Chemical Vapor Deposition

Typical Gases:

Flammable: Flammable: Hydrogen, Isopropyl Alcohol, Methane
Toxic: HCl, AsH3, BCl3, PH3, CO, HF, O3, H2Cl2Si, TEOS, C4F6, C5F8, GeH4, NH3, NO2 and O2 Deficiency.
Pyrophoric: Silane

Chemical Plants

Probably one of the largest users of gas detection equipment are Chemical Plants. They often use a wide range of both flammable and toxic gases in their manufacturing processes or create them as by-products of the processes.

Typical Applications:

  • Raw material storage
  • Process areas
  • Laboratories
  • Pump rows
  • Compressor stations
  • Loading/unloading areas

Typical Gases:

Flammable: General Hydrocarbons
Toxic: Various including Hydrogen Sulfide, Hydrogen Fluoride and Ammonia

Power Stations

Traditionally coal and oil have been used as the main fuel for Power Stations.

In Europe and the US most are being converted to natural gas.

Typical Applications:

  • Around the boiler pipe work and burners
  • In and around turbine packages
  • In coal silos and conveyor belts in older coal/oil fired stations

Typical Gases:

Flammable: Natural Gas, Hydrogen
Toxic: Carbon Monoxide, SOx, NOx and Oxygen deficincy.

Waste Water Treatment Plants

Waste Water Treatment Plants are a familiar site around many cities and towns.

Sewage naturally gives off both Methane and H2S. The ‘rotten eggs’ smell of H2S can often be noticed as the nose can detect it at less than 0.1ppm.

Typical Applications:

  • Digesters
  • Plant sumps
  • H2S Scrubbers
  • Pumps

Typical Gases:

Flammable: Methane, Solvent vapors
Toxic: Hydrogen Sulfide, Carbon Dioxide, Chlorine, Sulfur Dioxide, Ozone.

Boiler Rooms

Boiler Rooms come in all shapes and sizes. Small buildings may have a single boiler whereas larger buildings often have large boiler rooms housing several large boilers.

Typical Applications:

  • Flammable gas leaks from the incoming gas main
  • Leaks from the boiler and surrounding gas piping
  • Carbon Monoxide given off badly maintained boiler

Typical Gases:

Flammable: Methane
Toxic: Carbon Monoxide


Hospitals may use many different flammable and toxic substances, particularly in their laboratories. Additionally, many are very large and have onsite utility supplies and back up power stations.

Typical Applications:

  • Laboratories
  • Refrigeration plants
  • Boiler rooms

Typical Gases:

Flammable: Methane, Hydrogen
Toxic: Carbon Monoxide, Chlorine, Ammonia, Ethylene oxide and Oxygen deficiency

Tunnels/Car Parks

Car Tunnels and enclosed Car Parks need to be monitored for the toxic gases from exhaust fumes. Modern tunnels and car parksuse this monitoring to control the ventilation fans. Tunnels may also need to be monitored for the build up of natural gas.

Typical Applications:

  • Car tunnels
  • Underground and enclosed car parks
  • Access tunnels
  • Ventilation control

Typical Gases:

Flammable: Methane (natural gas), LPG, LNG, Petrol Vapor.
Toxic: Carbon Monoxide, Nitrogen Dioxide

10. Principles of Detection

Many people have probably seen a flame safety lamp at some time and know something about its use as an early form of ‘firedamp’ gas detector in underground coal mines and sewers. Although originally intended as a source of light, the device could also be used to estimate the level of combustible gases- to an accuracy of about 25-50%, depending on the user’s experience, training, age, colour perception etc. Modern combustible gas detectors have to be much more accurate, reliable and repeatable than this and although various attempts were made to overcome the safety lamp’s subjectiveness of measurement (by using a flame temperature sensor for instance), it has now been almost entirely superseded by more modern, electronic devices.

Nevertheless, today’s most commonly used device, the catalytic detector, is in some respects a modern development of the early flame safety lamp, since it also relies for its operation on the combustion of a gas and its conversion to carbon dioxide and water.

Nearly all modern, low-cost, combustible gas detection sensors are of the electro-catalytic type. They consist of a very small sensing element sometimes called a ‘bead’, a ‘Pellistor’, or a ‘Siegistor’- the last two being registered tradenames for commercial devices. They are made of an electrically heated platinum wire coil, covered first with a ceramic base such as alumina and then with a final outer coating of palladium or rhodium catalyst dispersed in a substrate of thoria.

This type of sensor operates on the principle that when a combustible gas/air mixture passes over the hot catalyst surface, combustion occurs and the heat evolved increases the temperature of the ‘bead’. This in turn alters the resistance of the platinum coil and can be measured by using the coil as a temperature thermometer in a standard electrical bridge circuit. The resistance change is then directly related to the gas concentration in the surrounding atmosphere and can be displayed on a meter or some similar indicating device.

To ensure temperature stability under varying ambient conditions, the best catalytic sensors use thermally matched beads. They are located in opposing arms of a Wheatstone bridge electrical circuit, where the ‘sensitive’ sensor (usually known as the ‘s’ sensor) will react to any combustible gases present, whilst a balancing, ‘inactive’ or ‘non-sensitive’ (n-s) sensor will not. Inactive operation is achieved by either coating the bead with a film of glass or de-activating the catalyst so that it will act only as a compensator for any external temperature or humidity changes.

A further improvement in stable operation can be achieved by the use of poison resistant sensors. These have better resistance to degradation by substances such as silicones, sulfur and lead compounds which can rapidly de-activate (or ‘poison’) other types of catalytic sensor.

To achieve the necessary requirements of design safety, the catalytic type of sensor has to be mounted in a strong metal housing behind a flame arrestor. This allows the gas/ air mixture to diffuse into the housing and on to the hot sensor element, but will prevent the propagation of any flame to the outside atmosphere. The flame arrestor slightly reduces the speed of response of the sensor but, in most cases the electrical output will give a reading in a matter of seconds after gas has been detected. However, because the response curve is considerably flattened as it approaches the final reading, the response time is often specified in terms of the time to reach 90 percent of its final reading and is therefore known as the T90 value. T90 values for catalytic sensors are typically between 20 and 30 seconds.

(N.B. In the USA and some other countries, this value is often quoted as the lower T60 reading and care should therefore be taken when comparing the performance of different sensors).


The most common failure in catalytic sensors is performance degradation caused by exposure to certain poisons’. It is therefore essential that any gas monitoring system should not only be calibrated at the time of installation, but also checked regularly and re-calibrated as necessary. Checks must be made using an accurately calibrated standard gas mixture so that the zero and ‘span’ levels can be set correctly on the controller.

Codes of practice such as EN50073:1999 can provide some guidance about the calibration checking frequency and the alarm level settings. Typically, checks should initially be made at weekly intervals but the periods can be extended as operational experience is gained. Where two alarm levels are required, these are normally set at 20-25% LEL for the lower level and 50-55% LEL for the upper level.

Older (and lower cost) systems require two people to check and calibrate, one to expose the sensor to a flow of gas and the other to check the reading shown on the scale of its control unit. Adjustments are then made at the controller to the zero and span potentiometers until the reading exactly matches that of the gas mixture concentration. Remember that where adjustments have to be made within a flameproof enclosure, the power must first be disconnected and a permit obtained to open the enclosure.

Today, there are a number of ‘one-man’ calibration systems available which allow the calibration procedures to be carried out at the sensor itself. This considerably reduces the time and cost of maintenance, particularly where the sensors are in difficult to get to locations, such as an off-shore oil or gas platform. Alternatively, there are now some sensors available which are designed to intrinsically safe standards, and with these it is possible to calibrate the sensors at a convenient place away from the site (in a maintenance depot for instance). Because they are intrinsically safe, it is allowed to freely exchange them with the sensors needing replacement on site, without first shutting down the system for safety.

Maintenance can therefore be carried out on a ‘hot’ system and is very much faster and cheaper than early, conventional systems.

Semiconductor sensor

Sensors made from semiconducting materials gained considerably in popularity during the late 1980’s and at one time appeared to offer the possibility of a universal, low cost gas detector. In the same way as catalytic sensors, they operate by virtue of gas absorption at the surface of a heated oxide. In fact, this is a thin metal-oxide film (usually oxides of the transition metals or heavy metals, such as tin) deposited on a silicon slice by much the same process as is used in the manufacture of computer ‘chips’. Absorption of the sample gas on the oxide surface, followed by catalytic oxidation, results in a change of electrical resistance of the oxide material and can be related to the sample gas concentration. The surface of the sensor is heated to a constant temperature of about 200-250°C, to speed up the rate of reaction and to reduce the effects of ambient temperature changes.

Semiconductor sensors are simple, fairly robust and can be highly sensitive. They have been used with some success in the detection of hydrogen sulfide gas, and they are also widely used in the manufacture of inexpensive domestic gas detectors. However, they have been found to be rather unreliable for industrial applications, since they are not very specific to a particular gas and they can be affected by atmospheric temperature and humidity variations. They probably need to be checked more often than other types of sensor, because they have been known to ‘go to sleep’ (i.e. lose sensitivity) unless regularly checked with a gas mixture and they are slow to respond and recover after exposure to an outburst of gas.

Thermal Conductivity

This technique for detecting gas is suitable for the measurement of high (%V/V) concentrations of binary gas mixes. It is mainly used for detecting gases with a thermal conductivity much greater than air e.g. Methane and Hydrogen. Gases with thermal conductivities close to air cannot be detected E.g. Ammonia and Carbon Monoxide. Gases with thermal conductivities less than air are more difficult to detect as water vapor can cause interference E.g. Carbon Dioxide and Butane. Mixtures of two gases in the absence of air can also be measured using this technique.

The heated sensing element is exposed to the sample and the reference element is enclosed in a sealed compartment. If the thermal conductivity of the sample gas is higher than that of the reference, then the temperature of the sensing element decreases. If the thermal conductivity of the sample gas is less than that of the reference then the temperature of the sample element increases. These temperature changes are proportional to the concentration of gas present at the sample element.

Infrared Gas Detector

Many combustible gases have absorption bands in the infrared region of the electromagnetic spectrum of light and the principle of infrared absorption has been used as a laboratory analytical tool for many years. Since the 1980’s, however, electronic and optical advances have made it possible to design equipment of sufficiently low power and smaller size to make this technique available for industrial gas detection products as well.

These sensors have a number of important advantages over the catalytic type. They include a very fast speed of response (typically less than 10 seconds), low maintenance and greatly simplified checking, using the self-checking facility of modern micro-processor controlled equipment. They can also be designed to be unaffected by any known ‘poisons’, they are failsafe and they will operate successfully in inert atmospheres, and under a wide range of ambient temperature, pressure and humidity conditions.

The technique operates on the principle of dual wavelength IR absorption, whereby light passes through the sample mixture at two wavelengths, one of which is set at the absorption peak of the gas to be detected, whilst the other is not. The two light sources are pulsed alternatively and guided along a common optical path to emerge via a flameproof ‘window’ and then through the sample gas. The beams are subsequently reflected back again by a retro-reflector, returning once more through the sample and into the unit. Here a detector compares the signal strengths of sample and reference beams and, by subtraction, can give a measure of the gas concentration.

This type of detector can only detect diatomic gas molecules and is therefore unsuitable for the detection of Hydrogen.

Open Path Flammable Infrared Gas Detector

Traditionally, the conventional method of detecting gas leaks was by point detection, using a number of individual sensors to cover an area or perimeter. More recently, however, instruments have become available which make use of infrared and laser technology in the form of a broad beam (or open path) which can cover a distance of several hundred metres. Early open path designs were typically used to complement point detection, however the latest 3rd generation instruments are now often being used as the primary method of detection. Typical applications where they have had considerable success include FPSOs, add ‘jettys, loading/unloading terminals, pipelines, perimeter monitoring, off-shore platforms and LNG (Liquid Natural Gas) storage areas.

Early designs use dual wavelength beams, the first coinciding with the absorption band peak of the target gas and a second reference beam which lies nearby in an unabsorbed area. The instrument continually compares the two signals that are transmitted through the atmosphere, using either the back-scattered radiation from a retroreflector or more commonly in newer designs by means of a separate transmitter and receiver. Any changes in the ratio of the two signals is measured as gas. However, this design is susceptible to interference from fog as different types of fog can positively or negatively affect the ratio of the signals and thereby falsely indicate an upscale gas reading/alarm or downscale gas reading/fault. The latest 3rd generation design uses a double band pass filter that has two reference wavelengths (one either side of the sample) that fully compensates for interference from all types of fog and rain. Other problems associated with older designs have been overcome by the use of coaxial optical design to eliminate false alarms caused by partial obscuration of the beam and the use of xenon flash lamps and solid state detectors making the instruments totally immune to interference from sunlight or other sources of radiation such as flare stacks, arc welding or lightning.

Open path detectors actually measure the total number of gas molecules (i.e. the quantity of gas) within the beam. This value is different to the usual concentration of gas given at a single point and is therefore expressed in terms of LEL meters.

Open Path Toxic Infrared Gas Detector

With the availability of reliable solid state laser diode sources in the near infrared region and also the increase in processing power afforded by the latest generation of digital signal processors, it is now feasible to consider the production of a new generation of gas detector for the reliable detection of toxic gases by optical means.

Optical open path and point detection of flammable gas is now well established and has been widely accepted in the Petrochemical industry where they have proved to be a viable and reliable measurement technology. The main challenge in adapting this technology to measure toxic gases is that of the very low levels of gas that must be reliably measured. Typically flammable gases need to be measured at percent levels of concentration. However typical toxic gases are dangerous at part per million (ppm) levels, i.e. a factor of 1000 times lower than for flammable gas detection.

To achieve these very low sensitivities it is not possible to simply adapt the technology used in open path flammable infrared gas detectors. Open path toxic infrared detectors need to utilise a different measurement principle where the instrument probes individual gas lines as opposed to a broad spectral range. This is facilitated by the use of a laser diode light source. The output of the laser is effectively all at a single wavelength and so no light is ‘wasted’ and all of the light emitted is subjected to absorption by the target toxic gas. This provides a significant enhancement of sensitivity compared to open path flammable gas detection techniques and further enhancements are achieved by the use of sophisticated modulation techniques.

Electrochemical Sensor

Gas specific electrochemical sensors can be used to detect the majority of common toxic gases, including CO, H2S, Cl2, SO2 etc. in a wide variety of safety applications. Electrochemical sensors are compact, require very little power, exhibit excellent linearity and repeatability and generally have a long life span, typically one to three years. Response times, denoted as T90, i.e. time to reach 90% of the final response, are typically 30-60 seconds and minimum detection limits range from 0.02 to 50ppm depending upon target gas type.

Commercial designs of electrochemical cell are numerous but share many of the common features described below:

Three active gas diffusion electrodes are immersed in a common electrolyte, frequently a concentrated aqueous acid or salt solution, for efficient conduction of ions between the working and counter electrodes.

Depending on the specific cell the target gas is either oxidized or reduced at the surface of the working electrode. This reaction alters the potential of the working electrode relative to the reference electrode. The primary function of the associated electronic driver circuit connected to the cell is to minimize this potential difference by passing current between the working and counter electrodes, the measured current being proportional to the target gas concentration. Gas enters the cell through an external diffusion barrier that is porous to gas but impermeable to liquid.

Many designs incorporate a capillary diffusion barrier to limit the amount of gas contacting the working electrode and thereby maintaining “amperometric” cell operation.

A minimum concentration of oxygen is required for correct operation of all electrochemical cells, making them unsuitable for certain process monitoring applications. Although the electrolyte contains a certain amount of dissolved oxygen, enabling short-term detection (minutes) of the target gas in an oxygen free environment, it is strongly advised that all calibration gas streams incorporate air as the major component or diluent.

Specificity to the target gas is achieved either by optimization of the electrochemistry, i.e. choice of catalyst and electrolyte, or else by incorporating filters within the cell which physically absorb or chemically react with certain interferent gas molecules in order to increase target gas specificity. It is important that the appropriate product manual be consulted to understand the effects of potential interferent gases on the cell response.

The necessary inclusion of aqueous electrolytes within electrochemical cells results in a product that is sensitive to environmental conditions of both temperature and humidity. To address this, the patented Surecell™ design incorporates two electrolyte reservoirs that allows for the ‘take up’ and ‘loss’ of electrolyte that occurs in high temperature/high humidity and low temperature/low humidity environments.

Electrochemical sensor life is typically warranted for 2 years, but the actual lifetime frequently exceeds the quoted values. The exceptions to this are oxygen, ammonia and hydrogen cyanide sensors where components of the cell are necessarily consumed as part of the sensing reaction mechanism.


Chemcassette® is based on the use of an absorbent strip of filter paper acting as a dry reaction substrate. This performs both as a gas collecting and gas analyzing media and it can be used in a continuously operating mode. The system is based on classic colorimetry techniques and is capable of extremely low detection limits for a specific gas. It can be used very successfully for a wide variety of highly toxic substances, including di-isocyanates, phosgene, chlorine, fluorine and a number of the hydride gases employed in the manufacture of semiconductors.

Detection specificity and sensitivity are achieved through the use of specially formulated chemical reagents, which react only with the sample gas or gases. As sample gas molecules are drawn through the Chemcassette® with a vacuum pump, they react with the dry chemical reagents and form a coloured stain specific to that gas only. The intensity of this stain is proportionate to the concentration of the reactant gas, ie, the higher the gas concentration, the darker is the stain. By carefully regulating both the sampling interval and the flow rate at which the sample is presented to the Chemcassette®, detection levels as low as partsper-billion (ie, 10 -9) can be readily achieved.

Stain intensity is measured with an electro-optical system which reflects light from the surface of the substrate to a photo cell located at an angle to the light source. Then, as a stain develops, this reflected light is attenuated and the reduction of intensity is sensed by the photo detector in the form of an analog signal. This signal is, in turn, converted to a digital format and then presented as a gas concentration, using an internally-generated calibration curve and an appropriate software library. Chemcassette® formulations provide a unique detection medium that is not only fast, sensitive and specific, but it is also the only available system which leaves physical evidence (i.e. the stain on the cassette tape) that a gas leak or release has occurred.

11. Portable Gas Detectors

Flammable and toxic gas detection instruments are generally available in two different formats: portable, i.e. ‘spot reading’ detectors and ‘fixed’, permanently sited monitors. Which of these types is most appropriate for a particular application will depend on several factors, including how often the area is accessed by personnel, site conditions, whether the hazard is permanent or transitory, how often testing is needed, and last but not least, the availability of finances.

Portable instruments probably account for nearly half of the total of all modern, electronic gas detectors in use today. In most countries, legislation also requires their use by anyone working in confined spaces such as sewers and underground telephone and electricity ducts. Generally, portable gas detectors are compact, robust, waterproof and lightweight and can be easily carried or attached to clothing. They are also useful for locating the exact point of a leak which was first detected with a fixed detection system.

Portable gas detectors are available as single or multi gas units. The single gas units contain one sensor for the detection of a specific gas while multi gas units usually contain up to four different gas sensors (typically oxygen, flammable, carbon monoxide and hydrogen sulfide). Products range from simple alarm only disposable units to advanced fully configurable and serviceable instruments with features such as datalogging, internal pump sampling, auto calibration routines and connectivity to other units.

Recent portable gas detector design advances include the use of more robust and lightweight materials for their construction. The use of high power microprocessors enables data processing for instrument self checking, running operating software, data storage, and auto calibration routines. Modular designs allow simple routine servicing and maintenance. New battery technology has provided extended operating time between charges in a smaller and more lightweight package.

Future designs are likely to see the integration of other technologies such as GPS, bluetooth and voice communication as well as the incorporation of gas detection into other safety equipment.

12. North American Hazardous Area Standards and Approvals

The North American system for the certification, installation, and inspection of hazardous locations equipment includes the following elements:

  • Installation Codes – E.g. NEC, CEC
  • Standard Developing Organisations (SDOs) – E.g. UL, CSA, FM
  • Nationally Recognized Testing Laboratories (NRTLs) – Third Party Certifiers e.g. ARL, CSA, ETI, FM, ITSNA, MET, UL
  • Inspection Authorities – E.g. OSHA, IAEI, USCG

The installation codes in North America are the NEC (National Electric Code) for the USA, and the CEC (Canadian Electric Code) for Canada. In both countries these guides are accepted and used by most authorities as the final standard on installation and use of electrical products. Details include equipment construction, performance and installation requirements, and area classification requirements. With the issuance of the new NEC these are now almost identical.

The Standards Developing Organizations (SDOs) work with industry to develop the appropriate overall equipment requirements. Certain SDOs also serve as members of the technical committees charged with the development and maintenance of the North American installation codes for hazardous locations.

The Nationally Recognized Testing Laboratories (NRTLs) are independent third-party certifiers who assess the conformity of equipment with these requirements. The equipment tested and approved by these agencies is then suitable for use under the NEC or CEC installation standards.

In the United States of America the inspection authority responsible is OSHA (Occupational Health and Safety Administration). In Canada the inspection authority is the Standards Council of Canada. To confirm compliance to all national standards both countries require an additional indication on products tested and approved. As an example CSA approved product to USA standards must add NRTL/C to the CSA symbol. In Canada UL must add a small c to its label to indicate compliance to all Canadian standards.

13. European and Rest of World Hazardous Area Standards and Approvals

The standards used in most countries outside of North America are IEC / CENELEC. The IEC (International Electrotechnical Commission) has set broad standards for equipment and classification of areas. CENELEC (European Committee for Electrotechnical Standardization) is a rationalizing group that uses IEC standards as a base and harmonizes them with all member countries standards. The CENELEC mark is accepted in all European Community (EC) countries.

14. ATEX

ATEX = ATmospheres EXplosibles

There are two European Directives that have been law since July 2003 that detail the manufacturers and users obligations regarding the design and use of apparatus in hazardous atmospheres.

The ATEX directives set the MINIMUM standards for both the Employer and Manufacturer regarding explosive atmospheres. It is the responsibility of the Employer to conduct an assessment of explosive risk and to take necessary measures to eliminate or reduce the risk.

ATEX Directive 94/9/EC article 100a

Article 100a describes the manufacturers responsibilities:

  • The requirements of equipment and protective systems intended for use in potentially explosive atmospheres (e.g. Gas Detectors).
  • The requirements of safety and controlling devices intended for use outside of potentially explosive atmospheres but required for the safe functioning of equipment and protective systems (e.g. Controllers).
  • The Classification of Equipment Groups into Categories
  • The Essential Health and Safety Requirements (EHSRs). Relating to the design and construction of the equipment / systems

In order to comply with the ATEX directive the equipment must:

  • display a CE mark.
  • have the necessary hazardous area certification.
  • meet a recognized performance standard e.g. EN 61779-1:2000 for flammable gas detectors.

15. Area Classification

Not all areas of an industrial plant or site are considered to be equally hazardous. For instance, an underground coal mine is considered at all times to be an area of maximum risk, because some methane gas can always be present. On the other hand, a factory where methane is occasionally kept on site in storage tanks, would only be considered potentially hazardous in the area surrounding the tanks or any connecting pipework. In this case, it is only necessary to take precautions in those areas where a gas leakage could reasonably be expected to occur.

In order to bring some regulatory control into the industry, therefore, certain areas (or ‘zones’) have been classified according to their perceived likelihood of hazard. The three zones are classified as:


In which an explosive gas/ air mixture is continuously present, or present for long periods


In which an explosive gas/ air mixture is likely to occur in the normal operation of the plant


In which an explosive gas/ air mixture is not likely to occur in normal operation

16. Apparatus Design

To ensure the safe operation of electrical equipment in flammable atmospheres, several design standards have now been introduced. These design standards have to be followed by the manufacturer of apparatus sold for use in a hazardous area and must be certified as meeting the standard appropriate to its use. Equally, the user is responsible for ensuring that only correctly designed equipment is used in the hazardous area.

For gas detection equipment, the two most widely used classes of electrical safety design are ‘flameproof’ (sometimes known as ‘explosionproof’ and with an identification symbol Ex d) and ‘intrinsically safe’ with the symbol Ex i.

Flameproof apparatus is designed so that its enclosure is sufficiently rugged to withstand an internal explosion of flammable gas without suffering damage. This could possibly result from the accidental ignition of an explosive fuel/ air mixture inside the equipment. The dimensions of any gaps in the flameproof case or box (e.g. a flange joint) must therefore be calculated so that a flame can not propagate through to the outside atmosphere.

Intrinsically safe apparatus is designed so that the maximum internal energy of the apparatus and interconnecting wiring is kept below that which would be required to cause ignition by sparking or heating effects if there was an internal fault or a fault in any connected equipment. There are two types of intrinsic safety protection. The highest is Ex ia which is suitable for use in zone 0, 1 and 2 areas, and Ex ib which is suitable for use in zone 1 and 2 areas. Flameproof apparatus can only be used in Zone 1 or 2 areas.

Increased safety (Ex e) is a method of protection in which additional procedures are applied to give extra security to electrical apparatus. It is suitable for equipment in which no parts can produce sparking or arcs or exceed the limiting temperature in normal service.

A further standard, Encapsulation (Ex m) is a means of achieving safety by the encapsulation of various components or complete circuits. Some products now available, achieve safety certification by virtue of using a combination of safety designs for discrete parts. Eg. Ex e for terminal chambers, Ex i for circuit housings, Ex m for encapsulated electronic components and Ex d for chambers that could contain a hazardous gas.

17. Apparatus Classification

As an aid to the selection of apparatus for safe use in different environmental conditions, two designations, apparatus group and temperature classification, are now widely used to define their limitations.

As defined by standard No EN50014 of the European Committee for Electrical Standards (i.e. Comite Europeen de Normalisation Electrotechnique or CENELEC), equipment for use in potentially explosive atmospheres is divided into two apparatus groups:

Group I

for mines which are susceptible to firedamp (methane)

Group II

for places with a potentially explosive atmosphere, other than Group I mines

Group II clearly covers a wide range of potentially explosive atmospheres and includes many gases or vapors that constitute widely different degrees of hazard. Therefore, in order to separate more clearly the differing design features required when used in a particular gas or vapor, Group II gases are sub-divided as indicated in the table. Acetylene is often considered to be so unstable that it is listed separately, although still included in Group II gases. A more comprehensive listing of gases can be found in European Standard EN50014.

The Temperature Class rating for safety equipment is also very important in the selection of devices to detect gas or mixture of gases. (In a mixture of gases, it is always advisable to take the ‘worst case’ of any of the gases in the mixture). Temperature classification relates to the maximum surface temperature which can be allowed for a piece of apparatus. This is to ensure that it does not exceed the ignition temperature of the gases or vapors with which it comes into contact.

The range varies from T1 (450°C) down to T6 (85°C). Certified apparatus is tested in accordance with the specified gases or vapors in which it can be used. Both the apparatus group and the temperature classification are then indicated on the safety certificate and on the apparatus itself.

North America and the IEC are consistent in their temperature or T-Codes. However unlike the IEC, North America includes incremental values as shown opposite.

18. Ingress Protection of Enclosures

Coded classifications are now widely used to indicate the degree of protection given by an enclosure against entry of liquids and solid materials. This classification also covers the protection of persons against contact with any live or moving parts inside the enclosure. It should be remembered that this is supplementary to and not an alternative to the protection classifications for electrical equipment used in hazardous areas.

In Europe the designation used to indicate the Ingress Protection consists of the letters IP followed by two ‘Characteristic Numbers’ which indicate the degree of protection. The first number indicates the degree of protection for persons against contact with live or moving parts inside, and the second number shows the enclosure’s protection against entry of water. For example, an enclosure with a rating of IP65 would give complete protection against touching live or moving parts, no ingress of dust, and would be protected against entry from water spray or jet. This would be suitable for use with gas detection equipment such as controllers, but care should be taken to ensure adequate cooling of the electronics. The two digit IP rating is a short form more commonly used in Britain. The full international version has three digits after the IP rather than two, e.g. “IP653”. The third digit is impact resistance. The meanings of the numbers are given in the following table.

19. Safety Integrity Levels (SIL)

Certification has essentially been concerned with the safety of a product in its working environment i.e. that it won’t create a hazard in its own right. The certification process (particularly in Europe with the introduction of the ATEX standard pertaining to Safety Related Devices) has now moved on to also include the measurement/physical performance of the product. SIL adds a further dimension by being concerned with the safety of the product in terms of being able to carry out its safety function when called to do so (ref: IEC 61508 manufacturers requirement). This is increasingly being demanded as installation designers and operators are required to design and document their Safety Instrumented Systems (ref: IEC 61511 user’s requirement).

Individual standards applicable to specific types of equipment are being developed from IEC61508. For gas detection equipment the relevant standard is EN50402:2005 Electrical apparatus for the detection and measurement of combustible or toxic gases or vapors or of oxygen. Requirements on the functional Safety of fixed gas detection systems.

Managing safety is about risk reduction. All processes have a risk factor. The aim is to reduce the risk to 0%. Realistically, this is not possible so an acceptable risk level that is ‘As Low As Reasonably Practical’ (ALARP) is set. Safe plant design and specification is the major risk reduction factor. Safe operational procedures further reduce the risk as does a comprehensive maintenance regime. The E/E/PES (Electrical/Electronic/Programmable Electronic System) is the last line of defence in the prevention of accidents. SIL is a quantifiable measure of safety capability of the E/E/PES. In typical applications, this relates to the F&G systems- detectors, logic resolvers and safety actuation/annunciation.

It is recognized that all equipment has failure modes. The key aspect is to be able to detect when the failures have occurred and take appropriate action. In some systems, redundancy can be applied to retain a function. In others, self checking can be employed to the same effect. The major design aim is to avoid a situation where a fault which prevents the system carrying out its safety function goes undetected. There is a critical distinction between reliability and safety. A product which appears to be reliable may have unrevealed failure modes whereas a piece of equipment which appears to declare a large number of faults may be safer as it is never/rarely in a condition where it is unable to do its function or has failed to annunciate its inability to do so.

There are 4 levels of SIL defined. In general, the higher the SIL, the greater the number of failure modes that are accommodated. For Fire and Gas systems the levels are defined in terms of “average probability of failure to perform the intended function on demand”.

Many current fire and gas detection products were designed long before the introduction of SIL and therefore on individual assessment may only achieve a low or no SIL rating. This problem can be overcome by techniques such as decreasing the proof test intervals or combining systems with different technologies (and hence eliminating common mode failures) to increase the effective SIL rating.

20. Gas Detection Systems

The most common method employed to continuously monitor for leakage of hazardous gases is to place a number of sensors at the places where any leaks are most likely to occur. These are often then connected electrically to a multi-channel controller located some distance away in a safe, gas free area with display and alarm facilities, event recording devices etc. This is often referred to as a fixed point system. As its name implies, it is permanently located in the area (e.g. an off-shore platform, oil refinery, laboratory cold storage etc).

The complexity of any gas detection system depends on the use to which the data will be put. Data recording allows the information to be used to identify problem areas and assist in the implementation of safety measures. If the system is to be used for warnings only, then the outputs from the system can be simple and no data storage is necessary. In choosing a system, therefore, it is important to know how the information will be used so that the proper system components can be chosen. In toxic gas monitoring, the use of multi-point systems has rapidly demonstrated their potential for solving a wide variety of workplace exposure problems and is invaluable for both identifying problems and for keeping workers and management aware of pollutant concentrations in the workplace.

In the design of multi-point systems, considerable thought should be given to the various components and to their interconnection. When using catalytic detection sensors, for instance, the electrical cable connections to the sensors would have three cores, each of 1mm squared, carrying not only the output signal, but also power to the electrical bridge circuit, which is located at the sensor to reduce signal voltage drop along the cables.

In the case of toxic (and some flammable) gas monitoring systems, the atmosphere is often sampled at locations remote from the unit and the gases are drawn by pumps to the sensors through a number of synthetic material, narrowbore tubes. Care in design of such systems will include selection of suitable sized pumps and tubes, a sequential sampling unit for sampling each tube in turn and filters to stop particulates or water cutting off the flow of gas.

The bore size of tubing can be critical, since it needs to be both large enough to allow rapid response times with standard size pumps, but at the same time should not be so large as to allow excessive dilution of the sample by air. Each sampling point must be connected to a separate tube and if a number of points are connected to a single, central sensor, it will be necessary to purge the sensor with clean air between samples.

The controllers used in fixed systems can be centrally located or distributed at various locations in a facility according to the application requirements. They come in a control panel and come in either single channel (i.e. one control card per sensor) or multi-channel configurations, the latter being useful where power, space or cost limitations are important.

The control units include a front panel meter or LCD to indicate the gas concentration at each sensor and will also normally have internal relays to control functions such as alarm, fault and shutdown. The number of alarm levels available varies between controllers but typically up to three levels can be set, depending on statutory requirements or working practices within the industry. Other useful features would include alarm inhibit and reset, over-range indication and analog 4-20mA outputs. Often digital outputs are also available for interfacing the controller to a DCS/BMS. It is important to remember that the main purpose of a gas detection system is to detect the build up of a gas concentration before it reaches a hazardous level and to initiate a mitigation process to prevent a hazard occurring. If the gas concentration continues towards a hazardous level then executive shut down and hazard warning alarms are initiated. It is not enough to just log the event or measure the gas levels to which personnel have been exposed.

Cables and junction boxes

In a typical industrial gas detection system such as that just described, sensors are located at a number of strategic points around the plant and at varying distances from the controller. When installing electrical connections to the controller, it is important to remember that each sensor cable will have a different electrical loop resistance depending upon its length. With constant voltage type detectors the calibration process will require a person at both the sensor in the field and at the controller. With constant current detectors or those with a local transmitter, calibration of the field device can be carried out separately to that of the controller.

The sensor cables are protected from external damage either by passing them through metal ducting, or by using a suitable mechanically protected cable. Protective glands have to be fitted at each end of the cable and the sensor is mounted on a junction box to help in making simple, low-resistance, ‘clean’ terminations. It is also very important to ensure that all the gland sizes and screw threads are compatible with the junction box and the external diameter of the cables being used. The correct sealing washer should be used to ensure a weatherproof between the detector and junction box. A further point to remember is that sensor manufacturers normally indicate the maximum loop resistance (not line resistance) of their sensor connections when providing the information to calculate cable core diameters for installation.

Location of Sensors

‘How many detectors do I need?’ and ‘where should I locate them?’ are two of the most often asked questions about gas detection systems, and probably two of the most difficult to answer. Unlike other types of safety related detectors, such as smoke detectors, the location and quantity of detectors required in different applications is not clearly defined.

Considerable guidance is available from standards such as EN50073 Guide for selection, installation, use and maintenance of apparatus for the detection and measurement of combustible gases or oxygen. Similar international codes of practice e.g. National Electrical Code (NEC) or Canadian Electrical Code (CEC) may be used where applicable. In addition certain regulatory bodies publish specifications giving minimum gas detection requirements for specific applications. These references are useful, but tend to be either very generic and therefore too general in detail, or application specific and therefore irrelevant in most applications.

The placement of detectors should be determined following the advice of experts having specialist knowledge of gas dispersion, experts having knowledge of the process plant system and equipment involved, safety and engineering personnel. The agreement reached on the location of detectors should also be recorded.

Detectors should be mounted where the gas is most likely to be present. Locations requiring the most protection in an industrial plant would be around gas boilers, compressors, pressurized storage tanks, cylinders or pipelines. Areas where leaks are most likely to occur are valves, gauges, flanges, T-joints, filling or draining connections etc.

There are a number of simple and quite often obvious considerations that help to determine detector location:

  • To detect gases that are lighter than air (e.g. Methane and Ammonia), detectors should be mounted at high level and preferably use a collecting cone.
  • To detect heavier than air gases (e.g. Butane and Sulfur Dioxide), detectors should be mounted at a low level.
  • Consider how escaping gas may behave due to natural or forced air currents. Mount detectors in ventilation ducts if appropriate.
  • When locating detectors consider the possible damage caused by natural events e.g. rain or flooding. For detectors mounted outdoors it is preferable to use the weather protection assembly.
  • Use a detector sunshade if locating a detector in a hot climate and in direct sun.
  • Consider the process conditions. Butane and Ammonia, for instance are normally heavier than air, but if released from a process line that is at an elevated temperature and/or under pressure, the gas may rise rather than fall.
  • Detectors should be positioned a little way back form high pressure parts to allow gas clouds to form. Otherwise any leak of gas is likely to pass by in a high speed jet and not be detected.
  • Consider ease of access for functional testing and servicing.
  • Detectors should be installed at the designated location with the detector pointing downwards. This ensures that dust or water will not collect on the front of the sensor and stop the gas entering the detector.
  • When siting open path infrared devices it is important to ensure that there is no permanent obscuration or blocking of the IR beam. Shortterm blockage from vehicles, site personnel, birds etc can be accommodated.
  • Ensure the structures that open path devices are mounted to are sturdy and not susceptible to vibration.

Installation Methods

Essentially three installation methods are used Worldwide for electrical equipment in hazardous locations:

1. Cable with indirect entry
2. Cable with direct entry
3. Conduit

Cable Systems

These are mainly used in Europe (although the US and Canadian Electrical Codes list Metal Clad and Mineral Insulated cables for use in Class 1 Div 1 or Zone 1). Explosion proof standards state that cable systems with suitable mechanical protection must be used. The cable is often Steel Wire Armoured (SWA) if used in areas where mechanical damage may occur, or it may be laid in protective conduit which is open at both ends. Certified cable glands are used to safely connect the cable to the enclosure.

Indirect Cable Entry

Indirect entry is into an increased safety ‘Ex e’ terminal area. Line barriers are used on the wires between the terminal chamber and the main enclosure. The installer need only open access the terminal area, not the flameproof enclosure.

Direct Cable Entry

Direct Entry is made into the flameproof enclosure. Only specially certified glands may be used. The type and structure of the cable must be carefully matched to the correct type of gland. The integrity of the protection is reliant on the correct installation by the installer.


Conduit is the main method of installation in hazardous areas throughout the USA. The electrical wires are run as individual wires inside enclosed metal tubes. The tubes are connected to the housings by means of unions and must have a seal within 18 inches of each entrance point. The entire conduit system is flameproof.

21. Global Service and Support Network

A vital part of ensuring that fixed and portable gas detection equipment operates correctly is periodic servicing, maintenance and calibration. Unlike some other types of safety related equipment (e.g. fire detection), gas detection does not have specific legislation or clear guidelines that specify how often it should be serviced. Relevant documents simply state that inspection and maintenance should be carried out frequently by competent, trained personnel and in line with the manufacturers recommendations.

Gas detection applications vary widely and therefore so do the factors that affect the frequency of servicing required to ensure proper operation. It is important that a suitable service period is established for the equipment that takes account of each individual application’s unique set of factors.

Traditionally, gas detection users had their own in-house service departments that were responsible for servicing, maintenance and calibration of their gas detection equipment as well as other safety related equipment. Increasingly, many users now choose to outsource part or all of this function in order to reduce fixed costs and at the same time ensure that people with specialist knowledge of the equipment are responsible for it.

It is also becoming more common for leading gas detection companies to also offer service of third party gas detection equipment as well as their own. As users continue to demand better efficiencies from outsourced suppliers, the trend in the future is likely to require gas detection companies to offer a ‘one stop shop’ for the service and maintenance of complete safety systems.

Gas detection company service departments should also offer other services including site surveys, installation, commissioning and training. Advice from experts in gas detection when conducting a site survey helps ensure the selection of the most suitable detection technologies and most appropriate detector locations. Installation must be carried out in accordance with any site, local or national codes of practice and legislation.

Properly commissioning a system ensures that it is fully functioning as designed and accurately detecting gas hazards. Many companies require that employees who use personal gas detection equipment, or work in areas that have fixed systems installed, are formally trained on the use and routine maintenance of the equipment. Service training departments should be able to offer certified training courses designed to suit all levels of ability from basic gas detection principles through to advanced custom designed technical courses.

Some gas detectors now offer ‘smart’ sensors that are pre-calibrated and can be simply fitted and used without the need for additional calibration and set up in the field. The additional use of Intrinsically Safe (I.S) design can also allow the ‘hot swap’ of these sensors without the need for removing power from the detector. Other recent innovations include the use of ‘auto-cal’ routines where the user is taken through a sequence of ‘on-screen’ calibration steps thereby ensuring correct set up. All of these innovations help keep service times to a minimum while ensuring accurate calibration of the detector.

The modular design of modern gas detectors enables more efficient servicing. The replacement of modules rather than component level service/repair greatly reduces turnaround time and therefore system down time. The economies of scale achieved by volume module level servicing also enables overall servicing costs to be reduced.