Gas sensing

TDL takes an average reading across the process, so is unaffected by heterogeneous gas

mixtures. If the O2 concentration is greater in one area than in another, an accurate overall measurement will still be achieved.

None of these technologies is “best” for O2 – each has certain advantages for particular applications. For example, Coulometric provides industry-leading purity measurements for electronics-grade gases, but is not nearly so effective for, say, oxidation control, where Paramagnetic is a much better fit. Even when it comes to the same application, the issue may not be clear-cut. For example,

Zirconia is ideal for most combustion control O2 measurements, and is long-proven in the role, but there may be some benefits to using TDL instead, depending on the process conditions.

SENSOR TECHNOLOGIES FOR MULTIPLE GASES One of the main differentiators between TDL

and the other O2 sensor technologies is that TDL can be configured by the manufacturer to measure a number of different gases. Servomex’s TDL analysers, the SERVOTOUGH

Laser 3 Plus range, can be optimised for O2, ammonia, carbon monoxide (CO), or a combined CO and methane measurement. This sensor technology is not alone in its

diversity. Infrared technology is based on the property exhibited by some gases of absorbing unique light wavelengths. This allows a direct, real-time measurement of a specific gas’s concentration within a gas mixture. Like TDL, it can only measure one gas at a time,

but there are a variety of gases it can be calibrated for. As a non-contact measurement, it is particularly effective for flammable or corrosive gas samples, and is also commonly used for monitoring harmful emissions. When more sensitive measurements are needed, Gas Chromatography provides an effective solution. It is an exceptionally accurate method for separating gas mixtures and measuring concentrations down to parts-per- billion (ppb) levels. Servomex offers three sensors based around Gas Chromatography: the Flame Ionisation Detector (FID, Thermal Conductivity Detector (TCD), and the Plasma Emission Detector (PED).

The SERVOTOUGH Laser 3 Plus is a Tunable Diode Laser analyser

The SERVOPRO Chroma can be fitted with a choice of three sensors supported by Gas Chromatography techniques

EXAMPLE: ONE ANALYSER, MANY APPLICATIONS The variety of sensor options available for the Chroma provides a useful illustration of how sensing technology can work for different applications. Even though the analyser platform is the same, by employing a particular sensor the best results can be achieved for a process. In medical gas applications, it can be configured with a TCD to deliver a nitrogen assay

measurement and carbon dioxide (CO2) impurity analysis in nitrous oxide (N2O). When fitted with a FID and methaniser, it provides impurity analysis of

CO in CO2 and N2O. FID sensors are renowned for their sensitivity,

and so are frequently used for hydrocarbon impurity analysis in ultra-high-purity gases. However, under certain conditions, there may be safety concerns over its requirement for a highly flammable, hydrocarbon-free hydrogen fuel gas. As an alternative, the Chroma can be fitted with a PED sensor, which does not require a fuel gas and offers up to 100 times more sensitivity than a FID measurement. The PED is suitable only for trace gas applications (less than 1,000 parts per million), but is highly accurate and has a low cost of ownership compared to other measurement methods. Its very low detection levels make the PED sensor ideal for demanding Pure Gas (P-Gas) measurements inside semiconductor fabs after the purifier stage, which are often 1ppb or less.

THE IMPORTANCE OF EXPERTISE IN SELECTING THE RIGHT SOLUTION Alongside its long experience in supplying gas analysis solutions to industry, one of the reasons for Servomex’s significant expertise in the field is that it researches, develops and builds its own sensors. These are provided by its Hummingbird Sensing Technology division, which manufactures sensors to the highest build quality at a state-of-the-art ISO 9001-certified facility in the UK.

Hummingbird also supplies gas sensing solutions to global industrial and medical equipment manufacturers, and was awarded the Queen’s Award for Innovation in 2016. Servomex’s own applications team are also responsible for developing their own sensor solutions, including the compact Servomex TDL technology, which was created from scratch for the Laser 3 Plus range. Sensing technology is a key factor in effective

gas analysis. However, it is not the only element to consider. Analyser design is also highly important, especially when considering hazardous environments and sampling conditions. As with selecting the correct sensor, extensive

experience and deep applications knowledge are important in overall analyser design. Knowing which technology to select, how to

apply it to process conditions, and make it easy and cost-effective to use, all requires expertise. Much of this expertise is provided to customers through Servomex’s global Systems team, which works in partnership with customers to deliver customised, scalable solutions to perfectly fit the application.


The sheer amount of sensor technologies available to industry operators can be bewildering, and choosing the wrong one may mean your process is less efficient – and therefore more expensive – than it needs to be. That is why a combination of technical

knowledge and application expertise is essential to decide upon the best solution for each and every process. Selecting the right technology not only provides the best results for efficiency, safety and quality, but very often provides a lower cost of ownership across the lifetime of the system.

Servomex Instrumentation Monthly June 2020 27

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