Allt om gasdetectors - Hitta gasdetektor utifrån dina behov här.
All about gasdetectors
In many industries, there may be potential hazards such as lack of oxygen, explosion danger and toxic gases. To protect persons there may be used portable gas detectors and stationary gas alarming systems for continuous measurements which gives an alarm or warning as soon as the danger might arise - so the user can manage to leave the area, or for example to an increased ventilation of a room or space.
Considerations when choosing gas detectors
When working in tanks, containers, enclosed spaces and sewers use portable gas detectors (available as single and multi-gas detector) to analyze whether the work environment is safe before entry and during work in the area.
Depending on the number of sensors, the equipment can simultaneously detect and measure:
Lack of oxygen or increased oxygen content
Explosive gases and vapors
|Gas detectors||Gas(ses) measured||Number of gasses|
|Multi Gas / Fixed gas detector||Oxygen O2, carbon monoxide CO, hydrogen sulphide H2S and combustible gases CH4 (LEL)||4|
|Mobile single gas detector||Level of oxygen / oxygen, O2||1|
|Mobile single gas detector||Carbon monoxide, CO||1|
|Mobile single gas detector||Sulphur dioxide, SO2||1|
|Mobile single gas detector||Hydrogen / hydrogen, H2||1|
|Mobile single gas detector||Hydrogen sulphide / hydrogen sulfide, H2S||1|
|Mobile single gas detector||Nitrogen oxides, nitrogen oxides or nitrogen oxide NO2 / NOx||1|
|Mobile single gas detector||Chlorine / chlorine, CL2||1|
|Mobile single gas detector||Ammonia, NH3||1|
What is gas?
The name "gas" comes from the word chaos, which neatly summarises the dangers and confusion when working with gases.
A gas is actually a swarm of particles moving randomly and chaotically, constantly colliding with each other and the walls of any container. The real volume of the particles is minute compared to the total space which they occupy and this is why gases fill any available volume and are readily compressed. The average speeds of gas molecules are of the order of 100s of metres per second and they are colliding with each other billions of times per second. This is why gases mix rapidly and why they exert pressure.
This constant motion is easily demonstrated by releasing a small amount of odorous gas in a room. Within seconds the gas can be smelt in all parts of the room. These properties apply to substances, which are normally gaseous, and to vapours from evaporated liquids.
A volume of any gas at the same temperature and pressure contains the same number of molecules irrespective of what the gas is. This means that measuring gas by volume is very convenient. Gas measurements at higher levels are in % (volume) and at lower levels parts per million, ppm (volume).
Whilst different gases have different densities, they do not totally separate into layers according to their density. Heavy gases tend to sink and light gases tend to rise, but their constant motion means that there is continuous mixing (i.e they do not behave like liquids).
So in a room where there is a natural gas (methane) leak, the gas will tend to rise because it is lighter than air but the constant motion means that there will be a considerable concentration at floor level. This will happen in perfectly still conditions but if there are any air currents, the mixing will be increased.
Air is a mixture of gases, typically:
Nitrogen 77.2 %
Oxygen 20.9 %
Water Vapour 0.9 %
Argon 0.9 %
Carbon Dioxide 0.03 %
Other Gases 0.07 %
Because its composition is reasonably constant, air is usually considered as a single gas, which simplifies the measurement of toxic and flammable gases for safety and health applications.
In order for gas to ignite there must be an ignition source, typically a spark (or flame or hot surface) and oxygen.
For ignition to take place the concentration of gas or vapour in air must be at a level such that the ‘fuel’ and oxygen can react chemically. The power of the explosion depends on the ‘fuel’ and its concentration in the atmosphere. The relationship between fuel/air/ignition is illustrated in the widely known ‘fire triangle’.
The 'fire tetrahedron' concept has been introduced more recently to illustrate the risk of fires being sustained due to chemical reaction.
With most types of fire the original fire triangle model works well – removing one element of the triangle (fuel, oxygen or ignition source) will prevent a fire occurring. However, when the fire involves burning metals like lithium or magnesium, using water to extinguish the fire could result in it getting hotter or even exploding. This is because such metals can react with water in an exothermic reaction to produce flammable hydrogen gas.
Not all concentrations of flammable gas or vapour in air will burn or explode. The Lower Explosive Limit (LEL) is the lowest concentration of ‘fuel’ in air which will burn and for most flammable gases it is less than 5% by volume. So there is a high risk of explosion even when relatively small concentrations of gas or vapour escape into the atmosphere.
LEL levels are defined in following standards: ISO10156, and IEC60079. The ‘original’ ISO standard lists LELs obtained when the gas is in a static state. LELs listed in the EN and IEC standards were obtained with a stirred gas mixture; this resulted in lower LEL’s in some cases (i.e. some gases proved to be more volatile when in motion).
Gases and vapours produced, under many circumstances, have harmful effects on workers exposed to them by inhalation, being absorbed through the skin, or swallowed. Many toxic substances are dangerous to health in concentrations as little as 1ppm (parts per million). Given that 10,000ppm is equivalent to 1% volume of any space, it can be seen that an extremely low concentration of some toxic gases can present a hazard to health.
Gaseous toxic substances are especially dangerous because they are often invisible and/or odourless. Their physical behaviour is not always predictable: ambient temperature, pressure and ventilation patterns significantly influence the behaviour of a gas leak. Hydrogen sulphide for example is particularly hazardous; although it has a very distinctive ‘bad egg’ odour at concentrations above 0.1ppm, exposure to concentrations of 50ppm or higher will lead to paralysis of the olfactory glands rendering the sense of smell inactive. This in turn may result in the assumption that the danger has cleared. Prolonged exposure to concentrations above 50ppm will result in paralysis and death.
Definitions for maximum exposure concentrations of toxic gases vary according to country. Limits are generally time-weighted as exposure effects are cumulative: the limits stipulate the maximum exposure during a normal working day.
The normal concentration of oxygen in the atmosphere is approximately 20.9% volume. Oxygen levels can be dangerous if they are too low (oxygen depletion) or too high (oxygen enrichment). The same oxygen monitor will alert to both enrichment and depletion.
In the absence of adequate ventilation the level of oxygen can be reduced surprisingly quickly by breathing and combustion processes.
Oxygen levels may also be depleted due to dilution by other gases such as carbon dioxide (also a toxic gas), nitrogen or helium, and chemical absorption by corrosion processes and similar reactions. Oxygen sensors should be used in environments where any of these potential risks exist.
When locating oxygen sensors, consideration needs to be given to the density of the diluting gas and the “breathing” zone (nose level). For example helium is lighter than air and will displace the oxygen from the ceiling downwards whereas carbon dioxide, being heavier than air, will predominately displace the oxygen below the breathing zone. Ventilation patterns must also be considered when locating sensors.
The table below shows the effect of a diluting gas on the level of oxygen.
|CONCENTRATION OF DILUTING GAS||RESULTING OXYGEN CONCENTRATION|
Oxygen monitors usually provide a first-level alarm when the oxygen concentration has dropped to 19% volume. Most people will begin to behave abnormally when the level reaches 17%, and hence a second alarm is usually set at this threshold. Exposure to atmospheres containing between 10% and 13% oxygen can bring about unconsciousness very rapidly; death comes very quickly if the oxygen level drops below 6% volume.
The hazard presented by oxygen deficiency is easily under-estimated; especially as risks can exist in non-industrial environments such as cellars or bars where CO2 and nitrogen are used. Oxygen depletion due to corrosion or bacterial activities presents a significant risk in confined spaces such as pipes, vessels, sewers and tunnels. Oxygen sensors are often installed in laboratories where inert gases (eg nitrogen) are stored in enclosed areas.
Increased levels of oxygen may dramatically increase the flammability of any combustible matter. If oxygen levels exceed 24% volume, even materials such as clothing which might normally just smoulder may burst into flame.
The risk from oxygen enrichment exists where pure oxygen is stored; for example in hospitals and industrial gas manufacturing and distribution plants. Oxygen sensors with rising alarms set at 23.5% volume are typically used in such environments.
How to choose gas detector
Choose by gas
When you are to decide on the best gas detection solution for your situation, we suggest considering the following questions:
What is required? A fixed system or portable instrument?
Is there a single defined hazard, or do you need to monitor several gases?
Does it require ATEX/UL or other certification or approvals?
Is the area to be monitored hot or wet?
If a fixed system:
a) Is the sample to be monitored of high pressure or flow?
b) Does it need a controller or will the detectors communicate with an existing system?
c) If it is an existing system, what type of controller is being used?
When you have considered some of the quetions above, it will be easier to decide on a proper gas detection solution.