Catalytic Bead vs. NDIR: Comparison of Popular Gas Detection Technologies

Catalytic bead sensors, also known as pellistor sensors, have been around for decades. Introduced in the 1960s, it was the first type of combustible gas sensors that replaced caged canaries that had been used by miners since time immemorial. The technology behind these sensors relies on a refractory bead, typically made of alumina, with a porous structure and a platinum wire coil passing through it. The material of the bead is a catalyst that heats up upon contact with a gas due to its combustion. Heating, in turn, changes the electric resistance of the coil, and the drop (relative to the reference bead) is registered as a detection event. Since their introduction to the market around 60 years ago, the design of these sensors remained mostly intact, with some improvements made primarily to the quality of bead coatings and bead size for energy-saving purposes.

Pellistor-based sensors are fairly versatile thanks to their ability to react differently to different gases, both in terms of reaction time and its intensity. This happens due to the following reasons:

-          Gas-catalyst reactivity. This factor defines the intensity of the chemical reaction between the catalytic coating and the gas that it comes into contact with. Different bead coatings react to different gases with different intensity.

-          Speed of gas diffusion due to Brownian motion. Larger molecules, like long hydrocarbon chains, diffuse through the atmosphere and filters slower than smaller ones, and it takes them more time to reach the necessary concentration that will trigger a chemical reaction with the bead.

-          Combustion temperature. Different gases burn (combust) with a different heat output, resulting in a varying amount of heat being passed to the bead and, subsequently, a different voltage surge in the sensor’s circuit.

-          Lower Explosive Limit volume. Different gases become combustible at different concentrations, so a gas with a higher LEL will produce a more intense reaction with a catalytic bead.

Disadvantages of the older technology

Although simple and relatively inexpensive, this technology has a number of shortcomings. First and foremost, pellistor sensors use a chemical reaction to detect gas, which causes the sensor’s gradual degradation and even complete coil burn-out if the sensor is exposed to the environment with a high concentration of hydrocarbons. As the result, the sensors’ lifespan gets shorter, they require fairly frequent checks and re-calibration, and most manufacturers only guarantee their stable operation for the first 2 or 3 years.

The second serious drawback of the technology is its susceptibility to chemical poisoning by residue formed by compounds that are typical for the gas and oil industry: silicone vapors, various derivatives of hydrocarbons, hydrogen sulfide and many more. These contaminants gradually decrease the sensor’s ability to efficiently detect gas and eventually render it useless, creating a potentially hazardous situation.

The third flaw of the catalytic bead technology is the lack of intrinsic safety. To prevent combustion from the activation of the catalytic bead’s coating, some models use flame arrestors that may have a dramatic impact on the sensor’s sensitivity. Such a sensor may not be able to react to even a fairly high concentration of particular gases.

The fourth downside of pellistor-based sensors is that since the combustion process requires the presence of oxygen, their efficiency is seriously crippled in low-oxygen environments, while inert and airless spaces (that are often found in the oil and gas industry) render them completely unusable. In such conditions, pellistor-based sensors may remain deaf to the growing gas concentration, even if it’s way past LFL/LEL thresholds.

Finally, catalytic bead sensors are known for their high power consumption that drains the gas detector’s battery, substantially undermining its mobility.

NDIR sensors as an alternative to pellistor-based devices

Optical IR sensors emerged as a powerful alternative to the ageing catalytic bead technology. Based on an entirely different approach to gas detection, these new-generation sensors relied on the analysis of the spectrum of light radiated from an IR emitter and reflected onto a receiver inside the measurement chamber of a gas sensor. Different gases absorbed light of a particular wavelength and the difference between the reference value and the actual measurement was interpreted as the level of gas present. The latest generation of IR sensors, such as the MIPEX family of products, take this concept even further by using, among other innovations, a modern nondispersive infrared (NDIR) LED emitter boasting exceptionally low power consumption and longevity. Just like their predecessors, these sensors measure a particular light spectrum to rule out false positives due to dust, water vapors and other substances, and are completely immune to substance poisoning of any sort. Since combustion and chemical reactions are not part of the process, NDIR sensors are shock-proof, intrinsically safe and can be used in fully inert and low-oxygen environments without limitations. Some sensor designs feature a dual-beam/dual-receiver configuration for redundancy and fail-safety – in the unlikely event of one of the components failing, the sensor will remain operational. Finally, the most modern NDIR devices, such as aforementioned MIPEX line of sensors, offer extremely low power consumption: under 3 mWt compared with over 200 mWt typical for catalytic bead sensors.

However, the otherwise perfect optical sensors also have flaws. Due to the limitations of the technology itself, NDIR sensors are not as universal as their pellistor-based counterparts. Configured to monitor a limited spectrum of wavelengths, NDIR sensors are really good at detecting large-molecule gases, such as CH4 and most hydrocarbons, but fail to detect gases like hydrogen and acetylene, which, however, still makes them a perfect choice for multi-gas tools with a focus on hydrocarbons.

Summary

Latest-generation NDIR optical sensors are deprived of the operational limitations of their pellistor-based predecessors and offer the following advantages:

-          Dramatically lower power consumption for longer maintenance-free operation in portable and stationary units

-          Intrinsic safety thanks to the absence of combustive elements

-          Possibility to operate in inert and low-oxygen environments

-          Shock-resistance and immunity to chemical poisoning

-          Substantially longer lifespan and non-serviceable intervals

-          Increased sensitivity to gases with large molecules (mostly hydrocarbons)

-          Minimal signal drift and digital interfaces

-          Applicability for the most compact, portable and autonomous gas detectors (gas clips)

MIPEX offers an entire family of cutting-edge NDIR sensors based on proprietary technologies and technological know-how's that are suitable for the most demanding designs. From the core MIPEX-02 model to the revolutionary MIPEX-04, these sensors can be used in a broad range of devices created for the most complex environments and application types. 

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