Gas detectors can be found in all walks of life, from food processing plants to parking garages, from aeroplanes to casinos. Any place that can have a potential lack of oxygen or presence of toxic gas needs a gas detector present to monitor the safety of people. Some common uses during field projects are: confined space entry, well drilling, soil screening, area monitoring, worker safety, indoor air quality, and leak detection.Of course they have been around for a very long time, starting with the canary, which was sadly a one shot trick that when subjected to methane tended to die rather quickly, with no audio visual alarm capabilities other than a slight chirp and a total lack of motion. Fortunately, technology has advanced significantly and we find ourselves at this point in time with some very sophisticated electronic equipment. Let’s cover some basics:
- One ppm is one part in 1,000,000 parts. Generally, ppm (parts per million) is the lowest unit of measurement 10,000 ppm = 1% by volume
- LEL (Lower Explosive Limit) is the next unit of measurement and is a percentage of the explosive %(vol) level of a compound
- 100% LEL is the lowest concentration at which a flammable substance can produce a fire or explosion when ignited
- UEL (Upper Explosive Limit) is the maximum concentration of gas in air that will burn
- Each compound (gas) has a different LEL, or the point at which the compound will burn or become explosiv
- Most flammable compounds become explosive at less than 5% volume
- Each gas has a different LEL and UEL
- The percentage of gas is the highest unit of measurement, which is the amount of pure gas
How do gas detection sensors work?Modern technology will no doubt make the below explanation outdated as it strides forward, so quickly below are the basics. The oxygen sensor is an electromechanical sensor. Any gas that can be oxidised or reduced electromechanically can be detected by means of a fuel based electrochemical sensor. The consumption of oxygen produces a current that is linearly proportional to the concentration of gas in air. Since the oxygen sensor is constantly exposed to oxygen, the normal life of the sensor is between one and two years. The combustible sensor consists of two coils of fine platinum wire each embedded in a bead of alumina, connected electrically in a bridge circuit. One of the beads is impregnated with a special catalyst that promotes oxidation and the other is treated to inhibit oxidation. Current is passed through the coils so that they reach a temperature at which oxidation of a gas readily occurs at the catalysed bead (about 500°C). This raises the temperature further which increases the resistance of the platinum coil in the catalysed bead, leading to an imbalance of the bridge. This output change is linear, for most gases, up to and beyond 100% LEL and response time is only a few seconds to detect alarm levels (typically 20% LEL). The toxic sensors are also electromechanical and operate by the same basic principles as the oxygen sensor. Electromechanical sensors consume minute amounts of gas, the absorption of gas and electric output being controlled by a “diffusion barrier”.
Confined spaces and gas detectionA confined space is any space large enough for someone to enter and perform assigned work that has limited means of entry or exit, and which is not designed for continuous worker occupancy. This covers just about every industry, including utilities, construction, hydrocarbon exploration and processing, petrochemicals, marine, agriculture, food processing and brewing, as well as the emergency services. Employers must assess the risks these workplaces pose to their employers and endeavour to prevent them. In most cases, both assessment and the safe working system will require testing of the atmosphere with gas testing equipment. Confined space gas risks can be divided into three broad categories: combustible gas, toxic gas and oxygen depletion or enrichment.
Combustible gas risksFor combustion to occur the air must contain a minimum concentration of combustible gas or vapour. This quantity is called the LEL. Different compounds have different LELs so it is vital that detectors are capable of detecting at the correct levels. Typically, storage vessels which have contained hydrocarbon fuels and oils present a danger. Other dangers come from fuel leaks: burst fuel containers; pipelines on and off site, gas cylinders and engine driven plant. For workers in pits, sewers and other sub-surface locations, methane formed by decaying organic matter is an almost universal danger.
Toxic gas and vapoursConfined space workers may be exposed to many toxic compounds, depending on the nature of the work. A risk assessment should be made of which toxic substances a worker may be exposed to in any given work situation. When looking at toxic gases related to specific applications, the water industry for example uses many toxic compounds for cleaning and processing both waste and clean water. Hazards such as chlorine, ozone, sulphur dioxide and chlorine dioxide then pose additional risks both in storage and treatment areas.
Oxygen – too high or too low?The normal concentration of oxygen in fresh air is 20.9%. An atmosphere is hazardous if the concentration drops below 19.5% or goes above 23.5%. Without adequate ventilation, the simple act of breathing will cause oxygen levels to fall surprisingly quickly. Combustion also uses up oxygen, so engine-driven plant and naked flames such as welding torches are potential hazards. Oxygen can also be displaced. Nitrogen, for example, when used to purge hydrocarbon storage vessels prior to re-use, drives oxygen out of the container and leaves it highly dangerous until thoroughly ventilated. High oxygen levels are also dangerous. As with too little, too much will impair the victim’s ability to think clearly and act sensibly. Moreover, oxygen-enriched atmospheres represent a significant fire hazard.
Gas detector typesBoth portable and fixed gas detectors can be used for confined space monitoring. Fixed systems typically comprise of one or more detector “heads” connected to a separate control panel. If a detector reads a dangerous gas level, the panel raises the alarm by triggering external sirens and beacons. This sort of installation is suited to larger spaces like plant rooms, which have sufficient room for the hardware or remote stations that are usually unmanned. However, much confined space work takes place in more restricted areas, making compact portable units more suitable. Ease of use, with one button operation, means minimal training is required while increased safety is ensured. Combining one or more sensors with powerful audible and visual signals to warn when pre-set gas levels are reached, portable detectors can be carried or worn wherever they are needed. In addition, a compact instrument is easily carried in a confined space, ensuring that pockets of high gas concentration are not missed. Certain features should be expected in every portable gas detector. Clearly life-saving tools for demanding environments must be as tough as possible, with reliable electronics housed in impact resistant casings. While the need to leave gas sensors exposed to the atmosphere means that no instrument can be fully sealed, a high degree of protection against dust and water ingress is essential. Toughness notwithstanding, a well-designed detector will also be light and compact enough to wear for an entire shift. Finally, because of the difficulties of working in a cramped space, perhaps under poor lighting, instruments should be easy to use. No matter how advanced a detector’s internal architecture or data management options, personnel in the field should be faced with nothing more daunting than a clear display, simple, one-button operation and loud/bright alarms.
Combustible gas sensorsAs detailed below, combustible gas sensors come in the form of catalytic, metallic oxide semiconductor, and infra-red.
Catalytic combustible gas sensorsSearching out explosive atmospheres, catalytic combustible gas sensors detect combustible gases by causing a combustion of gases within the sensor chamber. Not only do catalytic sensors offer good linearity, they can also react to most combustible gases. As resistance change to %LEL is relatively small, however, they do work better in concentrations between 1,000 and 50,000ppm.
As for their drawbacks:
- They do not measure trace amounts (under 200ppm) of gas and so should not be used to determine toxic levels
- To work accurately they require 14% minimum oxygen in air
- Not recommended for use in acetylene atmospheres
- The sensor can be damaged by lead, silicone or other catalytic poisons
- Readings can be affected by water vapour condensation and humidity
- Poor response to low energy hydrocarbons, e.g. oil vapours, kerosene, diesel fuel and commercial jet fuels
- Oftentimes they lose linearity after approximately one year
Metallic oxide semiconductor (MOS) combustible gas sensorAlso known as solid state sensors, Metallic Oxide Semiconductor (MOS) combustible gas sensors have been around for years. With a long operation life of three to five years this is a very resilient sensor that recovers well from high gas concentrations that could damage other types of sensors. As with catalytic sensors, readings of MOS sensors can also be affected by humidity and water vapour condensation. In addition, while not requiring as much oxygen as their catalytic counterpart, they too require oxygen to work accurately. In addition to the above mentioned disadvantages, further MOS sensor specific disadvantages include:
- Heating elements in some MOS sensors require a great deal of power, meaning larger battery packs are required
- Despite responding to many VOCs, HFCs and solvents, MOS sensors are not specific to any single compound