Gas Detectors – Gas Analyzers
Common questions – information
What is Accuracy?
The “accuracy” of a measurement refers to how close a measurement is to a “true” (actual) value. The accuracy of most instrumentation, is dependant on the accuracy of the device, or method used for calibration. Therefore, most process instruments cannot be quoted for accuracy as they may only be as “accurate” as their calibration. What is normally meant in this case is “reproducible” or “precise”.
What is Precision?
Precision is a measure of the spread of different readings. Precision and accuracy are unrelated to each other, meaning that you can be very PRECISE but not ACCURATE. In practical terms precision is often more relevant to an online analyzer than accuracy. Precision is also as a synonym for the resolution of the measurement e.g. a measurement
that can distinguish the difference between, 0.01 and 0.02 is more precise (has a greater resolution) than one that can only tell the difference between 0.1 and 0.2 even though they may be equally accurate.
What is Resolution?
Resolution refers to the smallest change that a sensor can detect in the quantity it is measuring. Resolution of an online instrument can be affected by the sensor itself, the manner of digitization and the capability of the display. In the past the resolution was limited by the display (a small analogue gauge), so ‘resolutions’ were often reported as the ability to read a gauge. The development of digital displays, means that the display is no longer the limiting factor, but is often still used to define the resolution. Quoting a resolution which is better than the precision is quite misleading. For example quoting to a resolution of the display of an online O2 meter at 0.001 is misleading if the sensor precision is > 0.01.
What is flammable risk?
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 ‘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).
What is toxic risk?
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.
What is Oxygen risk?
The normal concentration of oxygen in the atmosphere is approximately 20.9% volume. 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.