Gases of environmental concern fall into two basic categories: toxic or combustible. Because of their respective characteristics, the need to detect any or all of them is of paramount importance for plant and personnel safety.
Combustible gases normally have defined flammable (explosive) limits, the lower explosive level (LEL) and the upper explosive level (UEL). LEL and UEL are usually expressed as percent by volume of the material in air (or other oxidant). Substances only become combustible between these two limits. Below the LEL the substance is too 'Lean' to burn whilst above the UEL it would be too 'Rich' to burn. A typical range of measurement for a combustible gas sensor would be 0-100% LEL where the LEL value would have been established, for example 1,1% for hexane or 4% for hydrogen.
Some gases are both toxic and combustible; in these instances toxicity becomes the prime consideration. Carbon monoxide, for example, has a threshold limit value (TLV) of 25 ppm, but also has an LEL of 12,5% (125 000 ppm). The category of the gas is decided by the concentration range of the gas to be measured, so carbon monoxide is considered to be a toxic gas.
The measuring range for toxic gases is established for the various categories of TLVs as follows:
(a) Threshold limit values - time weighted average (TLV - TWA) These values represent the time weighted average concentrations of substances to which nearly all workers may be repeatedly exposed for a normal 8-hour working day and a 40 hour workweek day after day without adverse effect.
(b) Threshold limit value - short term exposure limit (TLV - STEL) STEL is defined as a 15 minute TWA exposure which should not be exceeded at any time during a working day even if the 8-hour TWA is within the TLV-TWA. Exposures above the TLV-TWA up to the STEL should not be longer than 15 minutes, and should not occur more than four times per day.
(c) Immediately dangerous to life and health (IDLH) IDLH concentrations represent the maximum concentrations from which one could escape within 30 minutes without a respirator, in the event of respirator failure, without experiencing any escape - impairment (eg, severe eye irritation) or irreversible health effects.
From the above it can be seen that the correct selection of operating ranges is critical to ensure safety of personnel.
Sensors for general applications
It should be noted that the instruments utilising these sensors are intended to detect the presence of a particular gas in air, and not as a gas analyser. The most suitable and widely used sensor types for ambient air monitoring are as follows:
Solid-state sensors
These were introduced in the 1970s and represented a major breakthrough in the field of ambient air monitoring. They are unique in the fact that they can detect both toxic and combustible gases in concentrations in the low ppm range to as high as 100% LEL and above. Modern sensors are capable of detecting in excess of 150 different gases including some that could otherwise only be detected by sophisticated (expensive) analytical instruments. A major advantage of solid-state sensors is their typical life expectancy of more than 10 years.
Catalytic bead sensors
These sensors can only be used in applications where the detection of combustible gases is required. They only respond well to concentrations from 1000 ppm to % LEL levels. Catalytic Bead sensors are non-specific and will respond to a wide variety of combustibles. Historically they have been known to have a susceptibility to poisoning from hydrogen sulphide, silicones and other substances. Later advances have led to the introduction of catalytic bead sensors that are poison resistant, with good stability and long life expectancy.
Electrochemical sensors
These were used as long ago as the 1950s for the monitoring of oxygen, but today are available for more than a dozen toxic gases in ppm ranges, including carbon monoxide, hydrogen sulphide, and ammonia. For some gases electrochemical sensors exhibit good sensitivity and selectivity with a further advantage of low power requirements. Exposure to high gas concentrations for long periods of time will automatically reduce their useful life.
Sensors for special applications
Infrared sensors are typically used for the monitoring of carbon dioxide in the range of 2000 ppm full scale to 10% by volume, carbon monoxide for 1% full scale and up, and hydrocarbons such as methane 1% to 100% by volume, as well as other hydrocarbons 1000 ppm and up.
Photo-ionisation detectors are used in analytical chemistry due to their fast response and good sensitivity to the family of volatile organic compounds (VOCs). Due to the need for relatively frequent calibration their use is normally restricted to portable instruments.
Selection of sensors and gas detection instrumentation is not a 'black art' - but is for the most part strictly applications orientated.
For more information contact Norman Wolf, PREI Instrumentation, 011 448 2172, [email protected]
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