Factors in the selection of a suitable detection system
There are a wide range of factors which must be considered in order to ensure that a gas detection system is suitable for its intended application. Those presented below represent some of the more important / straightforward to assess factors, however there are many others which must also be considered. While they are intended to direct the user towards suitable technologies, these factors are not sufficient in themselves to confirm the suitability of a particular technology for a specific application.
All of the factors given are intended to be relevant to the selection of instruments for safety applications (since the primary intention of this project is to improve practice in relation to safety devices).
Nature of the gas hazard
There are obviously a vast range of flammable, toxic, and asphyxiant gases and vapours which may be present at potentially dangerous levels in an industrial environment. While many of the available technologies are able to detect a range of different gases, most commercial implementations are specific to either flammable gases, a particular toxic species, or oxygen. In some circumstances merely identifying the possible presence of flammable or asphyxiant species may be sufficient. For example, the hazard associated with any non-flammable non-toxic asphyxiant species can be monitored by using an appropriate oxygen sensor. If a number of flammable gases may be present in the atmosphere then an instrument based on a non-selective technology, eg. a pellistor sensor, is likely to be appropriate. In the case of toxic hazards then it is imperative that the exact species which may be present are identified. The reason for this is that most toxic gas sensors are specific to (or at least highly selective towards) a particular species. While some technologies may exhibit cross sensitivity to other species, this cannot be relied upon to maintain safety.
It should be noted that some gases are flammable, toxic, and asphyxiant, although the relevant concentrations can differ significantly. As an example consider the case of carbon monoxide: a mixture of 0.03 % carbon monoxide in air is toxic, a mixture of 11 % carbon monoxide in air is flammable (and toxic), a mixture of 20 % carbon monoxide in air is asphyxiant by virtue of the amount of oxygen displaced (and is also flammable and toxic). When monitoring such gases (which also include ammonia and hydrogen sulphide) it is important to be aware of which hazard is of concern.
Suitable sensor technology for flammable, toxic or asphyxiant hazards
Anticipated concentration range
In many cases the choice of instrument will depend on the likely concentration of the gas of interest. Although some technologies are in theory capable of measuring concentrations over a very wide range, it would be unusual to find a single commercial instrument which could cover the whole range. In most safety applications only a relatively small dynamic range is required to satisfy the intended purpose. If it is necessary to monitor concentrations over a wide dynamic range then it may be necessary to use two (or more) different types of sensor together. For example a combination of a pellistor based instrument and either a thermal conductivity or IR instrument could be used to measure flammable gas concentrations between around 1 % LEL up to 100 % vol. Such combinations work well to counteract the limitations of individual types of sensor (eg. the pellistor would not measure concentrations above 100 % LEL, nor could it be relied upon to indicate reliably in an oxygen depleted atmosphere).
The most commonly encountered concentration units are likely to be:
ppm - expresses concentration in terms of the number of parts of the component of interest in one million parts of the sample. This unit is used to represent the lower end of the concentration range, as 10000 ppm is equivalent to 1 % Vol. Low gas concentrations can also be found expressed in the SI units of mg/m3.
% Vol - the volume ratio, volume composition, or v/v. This expresses as a percentage the ratio of the volume of the component of interest to the total volume occupied by the sample under standard conditions. These units are generally used to represent the higher end of the concentration range, as 100 % vol constitutes a pure gas sample.
% LEL - the LEL (Lower Explosive Limit) is the lowest volume ratio of a flammable gas or vapour in air which will form an explosive atmosphere. The actual volume ratio depends on the particular flammable species. Flammable gas concentrations below this limit are commonly stated in terms of % LEL. The % LEL expresses as a percentage the ratio of the volume ratio of the flammable component to the volume ratio of the LEL for that species. For example, to express 2.5 % Vol methane as a % LEL value (an LEL value of 5.0 % vol is commonly used for methane, although other values are also used)
%LEL=
volume ration of flammable species x100%
volume ratio of lower explosive limit
%LEL=
2.5% x100%
5.0%
= 50%LEL
Suitability of sensor technologies for ppm, %LEL or %vol
Type of apparatus required
Instruments can normally be classified as being one of four types: personal, portable, transportable, or fixed. These types are defined as:
Fixed - an apparatus which is intended to have all parts permanently installed.
Transportable - an apparatus not intended to be portable, but which can readily be moved from one place to another.
Portable - spot reading or continuously sensing apparatus that has been designed to be readily carried from place to place and to be used whilst being carried. Generally battery powered.
Personal - a device, attached to a person, that monitors the atmosphere in their breathing zone so that their exposure to a particular gas (or gases) can be determined.
Personal instruments provide a reliable means of measuring the exposure of individual personnel to specific gases; often a particular toxic species, but monitoring of exposure to flammable or asphyxiant hazards is also common. While many instruments will only have one channel for monitoring a particular hazard, instruments are available with up to five independent channels if necessary. These multi-channel instruments can be particularly useful for monitoring the exposure of personnel who move around a large site and may be exposed to a range of different hazards. Portable instruments are available in a range of sizes, designs and specifications. They have the potential to be very flexible, being suitable for uses such as leak seeking, spot checking and area monitoring. Transportable instruments are suitable for temporary location in an area where, perhaps due to the nature of work being performed, there is a temporary risk of a hazardous gas being present. Depending on the nature of the apparatus, transportable instruments would generally be moved less than once per day. It should be noted that although some of the available technologies can be obtained in commercial instruments with a range of sizes and portabilities, a number of the technologies are restricted to non-portable equipment by their size and / or robustness.
Environmental considerations
It is important to ensure that any instrument is able to perform satisfactorily under the environmental conditions which it may encounter when in service. These conditions could include extremes of temperature, pressure, humidity, air speed and a number of other factors.
The sensitivity of a particular instrument to variations in the environmental conditions is dependent on both the underlying sensor technology and any compensation / protection included by the manufacturer. Forms of compensation / protection may include temperature compensation, filters, weather protection housings, etc. Some of these may be fitted as standard to all units, others may be optional accessories. This highlights the importance of being aware of any potential environmental extremes and discussing these fully with instrument suppliers prior to purchase.
Although there is no universal definition of what constitutes 'normal' and 'extreme' environmental condition, useful guidance is available from several international standards (particularly BS EN 45544 for toxic gas detectors and BS EN 61779 for flammable gas detectors). These define reference operating conditions and also the ranges of conditions over which an instrument might be expected to perform adequately. If independent testing has been carried out then it will often have been performed to assure compliance with specified performance criteria under the conditions described in the relevant standards. These environmental conditions, which are outlined in the table below, cannot cover all possible eventualities but are intended to include those which may be encountered in the majority of applications. Since some technologies are more sensitive to variations in environmental conditions than others, it is highly advisable to seek advice from the instrument manufacturer prior to purchase.
Condition
BS EN 61779 (flammable)
BS EN 45544 (toxic)
Temperature
-10 to +40ºC (-25 to +55ºC for flammable gas detectors with remote sensors)
-10 to +40ºC
Humidity
20 to 90 % rh at 40ºC (corresponding to a dew point of between 12.8 and 38.0ºC)
20 to 90 % rh at 20ºC (corresponding to a frost / dew point of between -3.2 and 18.3ºC)
Pressure
80 to 110 kPa
90 to 110 kPa
Vibration 10 to 30 Hz, 1.0 mm amplitude.31 to 150 Hz, 2g peak acceleration
10 to 55 Hz, 0.15 mm amplitude
Air speed Up to 6 m/s (up to 13 mph)
0.01 to 4.0 m/s (0.02 to 8.9 mph)
Neither BS EN 45544 nor BS EN 61779 stipulates an ingress protection rating or a performance requirement in the presence of actual dust. It is difficult to specify what constitutes a significant level of dust, however those sensor technologies which rely on monitoring the intensity of a light beam are likely to be affected by relatively low levels of dust.
It must be noted that even instruments which fully meet the requirements of BS EN 45544 or BS EN 61779 for environmental variations need only do so when all other environmental parameters are at their reference values (with the exception of the high humidity high temperature test for flammable gas detectors). Compliance with either standard does not provide assurance that the instrument will be able to meet the required performance criteria when simultaneously exposed to two or more environmental extremes. It is recommended that the manufacturer is consulted and / or independent testing carried out if it is likely that such a condition could arise.
Presence of oxygen in the sample
While many safety applications require the measurement of low concentrations of hazardous species (either flammable or toxic) in atmospheric air with an oxygen level around 21 % vol, this is not always the case. A reduced oxygen level may affect instrument performance, depending on the type of technology employed, as some sensors require a minimum concentration to operate correctly.
When selecting an instrument for use in areas with a potentially low oxygen concentration a distinction can be made according to the purpose of the instrument. If it is solely to protect plant equipment (e.g. from a flammable hazard) then an instrument not dependent on oxygen for its operation may be the best option. If the purpose is to ensure the safety of personnel entering the area then a combination of an oxygen sensor and a suitable flammable and / or toxic sensor (not necessarily one which will operate without oxygen) may be more appropriate.
