Calibration Gas detectors are one of the most
important safety devices that a miner uses. His helmet and his lamp would also rank in this ‘must have’ category. Before entering the mine, the lamp can be tested for functionality with a visual check. But how can we ensure that the gas detectors are working? For them, a functional test is also conducted at the beginning of every working day, or even every shift. To make this functional test, or ‘bump test’ as it is often described, specialty gas mixtures containing low levels of the gases to be detected are passed over the sensors in the gas detector. This simulates the real life danger scenario and should trigger the audible and visual alarms on the device. Only when the miner is satisfied that the detector has reacted appropriately to these simulated conditions may they proceed into the confined space underground.
Gas detector selection
There are two broad types of gas detectors: portable and fixed. According to Barbara Avelar, “portable gas detectors are more commonly used in the mining industry, because as the mine goes deeper the adjustment required for this equipment is minimal, while using a fixed detector would require more maintenance and changes in the equipment at various locations”. Fixed gas detection systems are more common in surface operations, for example in the oil and gas industry and they are often used to detect various gases. For example, the Dräger Polytron 8100 EC can detect a range of toxic gases and oxygen. Regarding the portable gas detectors, there are single, two gas or multi-gas detectors. Avelar continues, “for underground mining operations in Brazil it is most common to use a multigas detector, such as the X-am 5000 from Dräger”. When choosing a gas detector several
criteria must be met. Firstly, the gases to be detected must comply with the local safety legislation and fit the hazards encountered in the particular mine. Beyond that, many factors come into play such as the sensor performance, robustness of the unit, its size and weight, the ease with which the functional test can be performed, data capture for audit purposes and serviceability. Christoph Feyerabend, marketing manager for gas detectors used in the mining segment at Dräger added, “the pace of innovation with gas detectors is stunning. It is highly likely that this has been one of the factors contributing to
improvements in mine safety in recent decades. The X-am 5000 from Dräger can measure up to five gases typically encountered in mines. To go one step beyond that, we have introduced the X-am 5600 which also
detects a sixth component, CO2, by means of a dual infra-red detector, providing a highly accurate measurement performance and long service life.” The inclusion of additional sensors is no
great technological challenge, but with each addition sensor the size, weight and battery consumption of the gas detector can increase and the challenge is to bundle all of that together in a practical and portable design. Feyerabend goes on to say, “the extension of the gas range was something that our customers in the mining industry were particularly keen to see and we are experiencing a general trend across other industries such as petrochemicals processing for a wider range of sensors to be built into our detectors. Our X-am 5600 is, we believe, the most compact six gas detector available on the international market today”.
Gas detector maintenance and sensor calibration
The sensors used in the gas detectors often rely on electrochemistry and many of the sensors require calibration or replacement at prescribed intervals to avoid the problems of ‘drift’ in the measured result. This calibration event is more precise than the daily ‘bump test’ and whilst the bump test generally occurs at the mine pit head, the calibration generally takes place at an off-site service laboratory. Alternatively, the detector can be returned to the manufacturer’s service facility for a general overhaul which may also involve replacement of some of the sensors contained in the gas detector. The frequency of testing the gas detectors
depends on three layers of rules or standards. Feyerabend expands on this point, “in Australia, NSW Resources legislate the frequency of detector testing. In Germany, there are strict codes of practice such as the T021/T023 from the BG RCI (the trade association for raw materials and chemical producers) that are rigorously adhered to. And, in addition to these rules the manufacturers of the gas detection devices will provide usage instructions based on field experience and best practices.” These rules ensure that safety standards are met and mean that mining operators can comply with requirements made by insurance companies which underwrite mining operations. The authorities that write these rules are also engaged in compliance validation to ensure that the gas detectors have indeed been tested appropriately. Feyerabend again, “of course, electronic documentation with a calibration and testing system such as our Dräger X-dock can make life a lot easier to maintain accurate records. This enables periodic inspection and ensures that the required data is available in the case of an
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investigation after an incident.” To assist with the recording and
documentation of gas detector calibration and testing, many companies are now turning to digital technology. According to Feyerabend, “the X-am 8000 is very new in our range, adding even more flexibility with regards to sensors, for example a PID sensor. It also has better user guidance, an internal pump and a wireless Bluetooth interface. This is the new flagship model geared towards specialist applications for confined spaces such as mine rescue. With the Bluetooth interface it will be able to tie into the Dräger CSE Connect cloud-based software solution via a smartphone app. This will pave the way towards paperless documentation and management of clearance measurements.”
Gas detector calibration Gas mixtures
The availability of ‘bump test’ gas mixtures is high. Many specialty gas suppliers around the world are able to produce cylinder gases for this purpose because the certification and accreditation requirements are relatively straight forward. The availability of accredited ISO 17025 or ISO Guide 34 (recently updated to ISO17034:2016) gas mixtures, which are the recommended choice for gas detector calibration is, on the other hand, not such a simple matter. Victor Chim, the specialty gas business
development manager at Coregas Pty in Australia comments on his experience with accredited calibration gas mixtures. “Our specialty gases accreditation journey began in 1997 when we achieved ISO17025 accreditation as a calibration laboratory for calibration gas mixtures. Subsequently, Coregas achieved ISO Guide 34 accreditation in 2002 which made us the first accredited gases reference material producer in Australia. Furthermore, ISO 17034:2016 will be implemented from 2018 and we will have the accreditation assessment soon.” The accreditation authority responsible for Coregas production and testing operations is NATA, the National Association of Testing Authorities Australia which is sole accreditation body in Australia. NATA is one of the founding members of, and currently holds the secretariat for, the
September 2018 Instrumentation Monthly
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