Gas Detection 5
powered devices the instrument run time is directly affected. In response, pellistor manufacturers have sought to develop smaller beads using platinum wire as thin as 10µm, and although there is a trade-off between power and poison resistance, most portable fl ammable gas detectors are fi tted with the newer generations of low power, portable pellistor sensors.
The adoption of pellistor technology in portable applications introduced a further challenge; since the instruments are typically clipped to the worker’s belt or lapel, they became subject to signifi cant mechanical shock and impacts. The beads inside the sensor are relatively fragile and extreme shock can cause damage to the internal welds resulting in sensor failure. Pellistor manufacturers responded by including shock absorbing packing materials directly in contact with the sensor beads, thus protecting them against all but the most severe impacts.
Optical Technology – the New Kid on the Block?
Although optical and in particular infra-red technology had been available for many years for the measurement of fl ammable gas in laboratory grade equipment, it was not until the early 90’s that sensors which were of a suffi ciently low price and small size became available for use in industrial safety applications. Rather than combustion, infra-red sensors rely on the absorption of particular wavelengths of light by fl ammable gases such as hydrocarbons and by the use of highly sensitive pyroelectric detectors, the presence of explosive gas can be determined. Since optical measurement is entirely non-chemical, infra-red sensors offered a signifi cantly longer working lifetime than the then-commercially available pellistors, they worked typically at lower power and required less frequent calibration. As a result, there has been a migration from pellistor technology to infra-red technology in gas detectors for industrial safety, although the shift has been relatively slow since optical technology remains more expensive and requires more sophisticated driver electronics than pellistor technology. However, as volume increases, the cost of infra-red sensors will reduce and sensors are now available with integrated driver electronics to facilitate their adoption and reduce the complexity of the required circuitry.
Pellistors – the Future?
It is not however all bad news for catalytic bead sensors. A fundamental essence of the technology is that it senses fl ammability – if the gas can burn, it will burn on a pellistor and there are some gases such as hydrogen which cannot be detected using infra-red sensors. In addition, there are some applications where the potential hazard is not completely known and could be one of a range of different fl ammable gases – for these applications the relative sensitivities of pellistors make it a more appropriate sensing choice. The higher sensitivity of non-laser based infra-red sensors to gases such as ethane and propane may lead to false alarms.
Furthermore, the inherent construction of pellistors has benefi ted from a complete overhaul with the launch of SGX Sensortech’s new MPEL. The new range of sensors are built on a micro-electromechanical (MEMS) system which removes the need for a mounted bead design and instead uses an etched micro-silica heater platform. The new support structure using well proved catalyst technology allows pellistors to be built in high volume at the wafer level using semiconductor manufacturing techniques providing a much greater degree of reproducibility. In addition, the low thermal mass of the sensor provides a signifi cant reduction in the power required and better still, offers the opportunity to drive the sensor in pulsed mode. As an example, a typical portable pellistor might require more than 200 milliwatts to operate; the MPEL offers a 50% reduction in power when voltage is applied constantly and a 90% reduction in pulsed mode. This potentially equates to an instrument run time extending from 12 hours to 5 days.
Since the sensor is manufactured on a solid substrate, the new pellistor is no longer susceptible to mechanical shock and offers robustness equivalent to an optical sensor. Poison resistance has not been compromised either, as a result of careful catalyst selection and deposition combined with novel fi lter technology. It is also the fi rst sensor to be certifi ed intrinsically safe for all Group IIC gases which extends the range of applications for which it can be used and also simplifi es the certifi cation of any instrument using it. It is available in a range of housings and as a result of its intrinsically safe status is ideally suited for adoption in customised packages.
Figure 2: SGX Sensortech MPEL
Despite the competitive threat of optical technology, we at SGX Sensortech believe the future remains bright for pellistor technology. Whilst some fl ammable gas sensing applications are ideally suited to optical gas sensing, with the recent advances in technology and design, the vast majority of industrial safety applications and instruments will continue to benefi t from the tried and tested, low cost, high performance gas detection provided by pellistors.
The Pellistor is Dead. Long live the Pellistor!
Latest Capability Extension to Gas Detector Announced
Crowcon (UK), the gas detection specialist, today announces the release of the latest IRmax capability extension, enabling the detection of 29 hydrocarbon gases or vapours. As a key component of Crowcon’s Fire & Gas Safety portfolio, the infrared gas detector can now be calibrated for more obscure gases or vapours like toluene, cyclohexane, isopropyl alcohol, methyl ethyl ketone (MEK) and paraxylene, along with the more common hydrocarbons such as methane, ethanol and LPG.
The IRmax is an ultra-compact infrared (IR) gas detector which delivers rapid, fail-safe detection of hydrocarbon gases with very low power consumption. Far smaller and lighter than comparable IR detectors, the IRmax is easy to install in even the most inaccessible locations.
Conventional IR gas detectors use power consuming heaters to prevent condensation on windows and mirrors. Not so the IRmax. Instead, its optical components are treated with the highly durable, hydrophobic coating STAY-CLIR that completely prevents signal faults due to condensation. Because the IRmax contains no optical surface heating components, power consumption is dramatically reduced. It requires less than 1 Watt of power, typically 75-90% lower than other IR gas detectors. A gas detection system using IRmax detectors therefore requires smaller, lower cost power supplies and battery back-up systems. Longer cables can also be used and more detectors can be powered on addressable networks.
Featuring dual-wavelength IR sensor technology in a rugged 316 stainless steel package, the ultra-compact IRmax provides rapid, fail-safe detection of potentially explosive hydrocarbons under the most extreme conditions. Capable of operating in temperatures ranging from -40 to +75o
C, Crowcon’s IRmax units are used in some of the most extreme climates on earth, ranging from the Siberian tundra to the Arabian desert and from the North Sea to the Niger Delta.
Other features of IRmax include: Options for HART and RS-485 Modbus communications, IEC61508 SIL 2 Sira, ATEX, IECEx, Inmetro, gas and dust zone certifi cations, Hand-held Intrinsically Safe calibrator, and automatic optical obscuration monitoring.
This latest development to IRmax makes it a key component in the complete Crowcon Fire & Gas offering, ensuring every safety aspect is covered.
For More Info, email: email:
For More Info, email:
31313pr@reply-direct.com The SGX MPEL provides;
Low power High resistance to poisons Mechanical robustness
For More Info, email: email:
For More Info, email: email:
For more information, visit
www.sgxsensortech.com or email
sales.is@
sgxsensortech.com
2010ad@reply-direct.com
www.envirotech-online.com IET September / October 2014
The Next Generation in Flammable Sensing
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