Coal mining became a large-scale industry from the 18th century to 1950s. With coal being the main energy source, the low cost and low abundance of this material made coal mining important for the growth of the economic industry. Despite a shift in the use of energy source from coal to electromechanical generators, coal still remains a valuable energy source via modern-day open-pit extraction methods.
An early method of extracting coal involved tunnelling into the earth; however, this procedure resulted in the production of harmful vapours (i.e., carbon monoxide with 0.1 per cent concentration capable of causing death within minutes of exposure; carbon dioxide; and hydrogen sulphide) during the oxidation of coal as a result of spontaneous combustion in mining procedures. The use of sensors designed for gas detection in mines has developed from the well-known miner’s canary to infrared sensors.
A Miner’s Canary
The canary (Serinus canaria) was commonly used by miners to help alert them to the presence of toxic vapours such as the odourless carbon monoxide in hazardous areas of underground mining pits. Use of a canary was a low-tech gas sensor solution to safety for miners in an uncertain working environment.
In the event of an explosion or a fire in a mine, rescuers would descend into the mine carrying a canary in a cage. A canary was known to be sensitive to carbon monoxide poisoning and so any display of distress from the canary would alert the miners to toxic gas exposure and give them enough time to return to safety. Mice were also used as carbon monoxide detectors for miners; however, canaries displayed more obvious signs of distress to the presence of small concentrations of carbon monoxide. For example, in the presence of toxic concentrations of carbon monoxide, a canary would sway from side to side before falling off its perch (figure 1).
Figure 1. Rescue miner using a canary to help detect noxious gases in a mining pit.
By 1987, application of canaries in mining pits to help detect noxious gases was phased out by the government. Use of canaries was not effective or safe enough to detect the presence of pollutants in the atmosphere and maintenance was higher, which wasn’t ideal in a constricted underground working environment. Modern-day technology was now favoured and considered to be a cost-effective long-term method to gas monitoring in mining pits.
The mid-1960s introduced catalytic pellistors as successors to the canary. This sensor is made of two platinum wires positioned within a ceramic mass. Both wires are connected to a Wheatstone-bridge circuit. Part of the ceramic mass contains a bead catalyst responsible for oxidation and an additional ceramic bead that will inhibit the process of oxidation.
A working principle to the pellistor involves transmitting an electrical current through the platinum wire which then generates heat to a temperature ideal for oxidation of a noxious gas (>450°C). During exposure to a toxic vapour, the gas diffuses through a permeable membrane in the sensor, becomes oxidised and initiates a catalytic reaction that starts to create resistance of the metal coils. The pellistor also contains an inactive ceramic bead to control impact from ambient temperature and humidity interfering with the pellistor. Presence of a gas flow results in the generation of heat between the two metal coils altering the resistance of the conductive wire, which creates an imbalance in the bridge circuit directly proportional to the gas concentration (figure 2). The pellistor is designed to combust carbon-based compounds including carbon monoxide, but combustion of this has an impact on neighbouring combustible compounds.
Figure 2. Schematic drawing of a basic pellistor.
Pellistors are useful for sensing a wide range of flammable gases and toxic vapours. Application of the pellistor involves positioning the sensor in a strategic point within an environment at risk of contamination by hazardous gases. Although the pellistor is a more effective detector for poisonous gases, it still carries many limitations, such as:
- A pellistor will malfunction when contaminated by chemicals containing halogen, sulphur, chlorine compounds, or metals containing lead or silicon.
- Contamination by silicone will impact the sensitivity of the pellistor.
- Regular calibration and high maintenance of the pellistor doesn’t make this sensor cost effective.
- These sensors are prone to a calibration drift and so tend to be unreliable and give inaccurate readings on gas contamination in the surrounding area.
Infrared LED-based gas sensor
Infrared (IR) LED gas sensors engineered with a dual beam, have become the preferred method to monitoring gas. Use of such modern-day technology has also helped shape the mining industry to the present day, especially when considering the safety of a mining environment. Figure 2 explains how a modern-day IR gas sensor works to monitor gases in the environment.
Figure 2. Miran SapplRe XL gas analyser
Compared to existing old-age methods for monitoring gas, IR-based gas monitoring carries certain advantages:
- IR gas sensors carry a wide measurement range which allows for the detection of gas concentrations from a few parts per million to 100%
- There is a rapid response rate, approximately 0.1 seconds from the time it takes for the gas to enter the analyser to the time it take to measure the gas concentration.
- Background gases cannot affect the IR LED gas sensor.
- Individual gases can be measures accurately.
The evolution of gas sensors appears to have shaped the mining industry, allowing technology such as the IR gas sensor set to work effectively for a period of five years with increased accuracy in the measurement of noxious gases. Calatytic sensors such as the pellistor are more susceptible to contamination, and with limited inspection principle will struggle to compete with IR technology.
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