From the early 1960s, millions of people have used pellistors and catalytic bead sensors to safeguard themselves from flammable gas hazards at their workplace. Pellistor technology has been used in various applications ranging from mining and tunneling through to gas, water treatment, oil, and petrochemical works.
When this technology is incorporated into advanced detectors and instruments, it raises alarms in order to take the right safety actions. The VQ548MP micro-electromechanical (MEMS) pellistor is illustrated in Figure 1.
Figure 1. VQ548MP MEMS pellistor
Pellistor technology has become universal due to its extensive range of quantifiable gases and its inbuilt simplicity which eliminates the need for complex driver electronics. Additionally, it has compact sensors and is inexpensive.
However, the introduction of infrared and other optical sensors, which offer some advantages, has outshined the pellistor technology. In this article, the past development of the pellistor is discussed and its future developmental roadmap is reviewed.
What is a Pellistor?
Since there is a greater safety concern for workers and since safety regulations are made stricter in the British coal industry, considerable effort was made in developing flammable gas technology, which eventually led to the development of pellet resistor or the pellistor. A pellistor is a coil of finely wound wire, typically made of platinum, around which a mixture of catalyst and ceramic material is deposited to produce a small bead (Figure 2).
Figure 2. Structure of a pellistor
When an electrical current is applied, the bead is heated to about 300–400 °C. At this increased temperature, the bead will burn on the sensor’s surface upon encountering flammable gas, and thus, there is a further increase in its temperature.
Provided that the resistance of the platinum coil inside the bead is greatly dependent on its temperature, the presence of gas can be measured by placing the sensor in an electrical bridge circuit. The output is in linear proportion with the concentration of flammable gas.
Pellistors are normally used in pairs: a detector bead contains the active catalyst and is made to respond to gas, and a compensator bead is designed without a catalyst and responds to fluctuations in environmental conditions, thereby providing inherent temperature compensation.
Since a pellistor is a heat source, it is a potential ignition point for flammable gas if the concentration goes beyond the lower explosive limit (LEL), and thus the beads are contained in a stainless steel housing fitted with a metal sinter. The purpose of the sinter is to ensure that any flame formed around the bead cannot spread beyond the sensor causing an explosion.
In the past few years, significant enhancements have been made to the technology in order to improve its usefulness. Yet, one major drawback of pellistors is that the catalyst materials employed in the beads are sensitive to reduced sensitivity when exposed to halides, sulfides, silicones, and other gases. The reduction is permanent in a majority of cases and is reversible in specific cases. To overcome these problems, advanced filter materials have been integrated into the sensor designed to protect the sensitive catalyst from such kinds of gases and thus prolong their operating lifetime.
Earlier, pellistor beads were developed using a hard ceramic core on which a catalyst is doped in a stiff shell, but now, it is also possible to design the pellistor bead as a combination of catalyst and ceramic. The latter method not only provides better access to the surface area of the catalyst but also significantly increases the poison resistance.
Another drawback of the pellistor technology is that high power is required to maintain the bead at the correct temperature. This is not a constant problem in fixed gas detection solutions, but in the case of portable battery-powered instruments, the instrument’s run time is directly impacted.
To resolve this problem, pellistor manufacturers have attempted to create smaller beads by using very thin platinum wires. Although there is a trade-off between power and poison resistance, most of the portable flammable gas instruments are incorporated with the most advanced types of portable pellistor sensors that use only a minimum amount of power.
Nonetheless, the application of pellistor technology in portable instruments posed another difficulty. Since the instruments are usually clipped on the belt or lapel of the worker, they were subjected to impacts and mechanical shocks. Since the beads integrated inside the sensor are fragile, severe shock can destroy the internal welds and eventually lead to sensor failure. Therefore, to overcome this, pellistor manufacturers utilized shock absorbing packing materials that stay in direct contact with the sensor beads, safeguarding them from intense impacts.
For a number of years, optical technology, particularly infrared technology, was used to establish flammable gas in laboratory-grade instruments. It was not until the early 1990s that sensors became available for use in industrial safety applications, due to their compact size and affordability. Infrared sensors rely on the absorption of certain wavelengths of light by other flammable gases, including hydrocarbons, and the presence of explosive gas can be easily determined with the help of highly sensitive pyroelectric detectors.
Considering that optical measurement is non-chemical, the infrared sensors offered a longer working lifetime in comparison with the commercially available pellistors on the market. These sensors not only operated at lower power but also required less frequent calibration.
As a result, these factors were responsible for the significant migration from pellistor technology to infrared technology for industrial safety, albeit the transition was relatively slow because optical technology is more costly and needs more sophisticated driver electronics in comparison to pellistor technology. By contrast, as volume increases, the cost of infrared sensors will reduce significantly and there are sensors that are available with integrated driver electronics to encourage their adoption and lower the complications of the required circuitry.
This, however, is not the end for catalytic bead sensors. The technology’s fundamental essence is that it is capable of detecting flammability — when the gas burns, it will burn on a pellistor. However, it is not possible to use non-laser based infrared sensors for detecting gases like hydrogen.
In addition, there are some applications where the probable danger is not fully known and could be one of a range of diverse types of flammable gases. For applications like these, the pellistors provide an appropriate sensing option because of their relative sensitivities. On the other hand, the higher sensitivity of infrared sensors to ethane and propane gases may lead to false alarms.
With the introduction of SGX Sensortech’s MPEL (Figure 3), the development of pellistors has also been refurbished. The newest range of sensors is developed on a MEMS system, which uses an etched micro-silica heater platform and eliminates the necessity for a mounted bead design.
Figure 3. SGX Sensortech MPEL.
The innovative support structure together with the established catalyst technology allows large-scale development of pellistors at the wafer level using semiconductor manufacturing techniques. Additionally, the low thermal mass of the sensor decreases the amount of power required and, at the same time, enables driving the sensor in pulsed mode. For example, a typical portable pellistor may require more than 200 MW to operate; upon applying the voltage continuously, the MPEL offers a 50% reduction in power and as much as 90% reduction in pulsed mode, respectively, corresponding to an instrument run time that extends from 12 hours to 5 days.
Considering that the sensor is developed on a solid substrate, the newest pellistor is impervious to mechanical shock and offers the same robustness as that of an optical sensor. In addition, poison resistance is unaffected since the catalyst is carefully selected and deposited along with the latest filter technology.
As the first sensor to be certified safe for all Group IIC gases, the pellistor extends the range of uses where it can be applied and simultaneously simplifies the certification of instruments utilizing it. The latest pellistor is available in an array of housings and is appropriate for use in customized packages.
According to SGX Sensortech, pellistor technology can be applied to a variety of applications, despite the competitive threat of optical technology. While optical gas sensing is appropriate for certain flammable gas sensing applications — thanks to the latest advancements in design and technology — a huge number of industrial safety equipment and applications will continue to gain from the low-cost and high-performance gas detection provided by pellistors.
This information has been sourced, reviewed and adapted from materials provided by SGX Sensortech (IS) Ltd.
For more information on this source, please visit SGX Sensortech (IS) Ltd.