Mid Sweden University

Sensing gas concentrations using optical absorption offers valuable advantages over other methods ina wide variety of real-world applications from industrial processes to environmental change. One of the most rapidly developing detection techniques on the global market is the non-dispersive infrared method (NDIR). Sensors developed based on this technique satisfy a growing demand for low-cost, reliable and long-term maintenance-free solutions. The technologies available to support this field for sensor key components such as light sources, photo detectors, optic cavities, and electronic components, have advanced rapidly in recent years. This development has led to an increasing number of application fields, due to significant improvements in accuracy, sensitivity and resolution.

However, this technique has limitations related to basic physical principles and sensor design performance. Variation in sensing environments’ temperature and pressure, the impact of water vapour presence and sensor component ageing are the most important interfering factors for investigation. Errors in measured values could be caused by any of these factors because they influence various sensor parts and the environment’s physical properties. The correct interpretation of error sources is one the most difficult and important tasks involved in designing stable, high-precision sensors.

To facilitate investigations into measurement performance limitations, test equipment was developed along with test approaches capable of creating experimental conditions that exceed the tested sensor’ stolerances. The studied resolution limit for long-path sensors is about 100 ppb. For measurements in fresh air concentrations (approximately 400 ppm of CO2), this is equivalent to a precision of less than0.1%. The methods used to reduce possible inaccuracies due to various error sources’ impacts should possess compensatory capabilities and precisions that exceed this value.

To improve the pressure compensation procedure’s performance, a complete advanced system that includes everything from a lab test bench to the supporting software and comprehensive calculation algorithm was developed. The test bench creates pressure conditions that deviate from the reference value by less than 0.2 mbar (or 0.02% of the standard pressure, 1013 mbar).

One of this study’s major findings is the concentration range-independent pressure compensation method. The advanced conditions achieved with the test station also facilitated the discovery and characterisation of the sources of long-term drift in methane concentration measurement.

Att mäta gaskoncentration med optisk absorption har många fördelar jämfört med andra teknikerin om ett stort antal områden från industriella processer till klimatförändringar. En av teknikerna under snabbast utveckling på den globala marknaden är icke-dispersiv infraröd teknik (NDIR). Sådana sensorer uppfyller ett stort antal krav som låg kostnad, hög tillförlitlighet och underhållsfri funktion under lång tid. Nyckelteknologierna som krävs inom området så som ljuskällor, fotodetektorer, optiska kaviteter och elektronikkomponenter har utvecklats snabbt de senaste åren. Utvecklingen har, tack vare förbättrad noggrannhet, känslighet och upplösning, lett till ett ökat antal tillämpningar i fält.

Tekniken har dock prestandabegränsningar relaterade till den fysikaliska principen och sensorkonstruktionen. Variationer i omgivningens temperatur och tryck, inverkan av vattenånga och åldring av komponenter är de dominerande störfaktorerna som behöver utforskas. Mätfel kan orsakas av alla dessa faktorer då de påverkar olika delar av sensorn och dess fysiska omgivning. Korrekt tolkning av felkällorna är en av de svåraste och viktigaste uppgifterna vid konstruktion av stabila sensorer med hög precision.

För att möjliggöra utvärdering av en sensors prestandabegränsningar måste testutrustning och metodik utvecklas för att uppnå testförhållanden som är mer stabila än sensorn. Den studerade upplösningsbegränsningen för sensorn med lång optisk sträcka ligger på gaskoncentrationer i storleksordningen 100 ppb. För mätning av koldioxid i friskluft, med en halt runt 400 ppm, innebär det ett krav på relativ precision bättre än 0,1%. Metoderna som används måste således stabiliseras och kompenseras för noggrannheter med en precision som är ytterligare bättre än så.

För att förbättra prestanda hos tryckkompenserings-proceduren har ett avancerat system utvecklats. Det inkluderar allt från en testbänk med tillhörande mjukvara till nyutvecklade algoritmer för beräkning. Testbänken kan skapa tryckstabiliserade förhållanden som avviker mindre än 0,2 mbar frånreferensvärdet vilket motsvarar 0,02% av omgivningstrycket på 1013 mbar.

Ett av de större framstegen i den presenterade avhandlingen är den framtagna metoden för koncentrationsberoende tryckkompensering. De stabila miljöerna i testbänken möjliggjorde även upptäckt och karakterisering av källorna till långtidsdrift vid mätning av metankoncentration.

Sensing gas concentrations using optical absorption offers valuable advantages over other methods in a wide variety of real-world applications from industrial processes to environmental monitoring. Among the fastest growing developing detection techniques on the global market is the nondispersive infrared method (NDIR). Sensors developed based on this principle meet the increasing demand for low-cost, reliable and long-term maintenance-free solutions. In recent years, key technological components such as light sources, photodetectors, optical cavities, and electronic elements have seen rapid advancements. These improvements have significantly enhanced accuracy, sensitivity, and resolution, thereby broadening the scope of applications.

Despite these benefits, the NDIR technique has inherent limitations rooted in both fundamental physical principles and sensor design constraints. Among the most important interfering factors to investigate are variations in the sensing environment´s temperature and pressure, the presence of water vapour, and the aging of sensor components. Each of these can introduce measurement errors by affecting the sensor´s internal components and the physical properties of the surrounding environment. The correct interpretation of error sources is one the most difficult and important tasks involved in designing stable, high-precision sensors.

A defining characteristic of NDIR sensing is the relationship between light transmittance and the number of absorbing molecules. Any gas molecules within the optical path of the sensor that exhibit infrared absorption will absorb incident radiation. When equipped with an interference filter that selects a specific spectral band, an NDIR sensor can estimate the concentration of a target gas that absorbs within that band. The total absorption within the selected spectral range is then translated into a concentration value.

However, if absorption lines of multiple gas species overlap within the same spectral band, it becomes impossible to selectively determine the concentration of a specific gas. This lack of selectivity is a major limitation of the NDIR method.

Water vapour, in particular, poses a significant challenge, as its absorption lines appear across nearly the entire infrared spectrum relevant to NDIR sensing. This greatly restricts the method´s applicability in environmental measurements, where water vapour is almost always present.

The overarching research goal is to develop a solution for selectively estimating the concentration of a target gas in mixtures where other gases have overlapping absorption lines. Within this context, the more specific objective is to accurately determine the true concentration on methane (CH₄) under environmental conditions, where it is continuously mixed with water vapour. This serves as a representative but challenging example of gas pairs with overlapping spectra.

To address this, several designs of multichannel NDIR sensors were developed and refined to improve selectivity. In support of this effort, a advance system was also built, incorporating calibration test equipment, supporting software, and a robust selection algorithm.

Key findings from the study include an advanced compensation method for pressure-induced errors and a novel technique for selective concentration measurement of gases with overlapping absorption spectra.