Abstract
In this work, performance improvements are described for a low-power consumption non-dispersive infrared (NDIR) methane (CH4) gas sensor using customised optical thin film bandpass filters (BPFs) centered at 3300 nm. BPFs shape the spectral characteristics of the combined mid-infrared III–V based light emitting diode (LED)/photodiode (PD) light source/detector optopair, enhancing the NDIR CH4 sensor performance. The BPFs, deposited using a novel microwave plasma-assisted pulsed DC sputter deposition process, provide room temperature deposition directly onto the temperature-sensitive PD heterostructure. BPFs comprise germanium (Ge) and niobium pentoxide (Nb2O5) alternating high and low refractive index layers, respectively. Two different optical filter designs are progressed with BPF bandwidths (BWs) of 160 and 300 nm. A comparison of the modelled and measured NDIR sensor performance is described, highlighting the maximised signal-to-noise ratio (SNR) and the minimised cross-talk performance benefits. The BPF spectral stability for various environmental temperature and humidity conditions is demonstrated.
Highlights
There are increasing requirements for monitoring and controlling the concentration of methane (CH4 ) in such fields as environment, landfill sites, fracking facilities, security, industry and agriculture [1,2,3,4]
A bandpass BW of 300 nm was chosen for this work, and we build upon the previous work by comparing the effect of a narrower BW (160 nm) bandpass filter
The optical coating designs were built into a comprehensive non-dispersive infrared (NDIR) gas sensor Mathcad model, and their effects on the sensor signal accuracy and reduced cross sensitivity were determined, the adjacent water absorption band at 2.8 μm
Summary
There are increasing requirements for monitoring and controlling the concentration of methane (CH4 ) in such fields as environment, landfill sites, fracking facilities, security, industry and agriculture [1,2,3,4]. The methods for monitoring methane concentration are based on various detection methods, including electrochemical [7], gas chromatography [8], thermal conductivity [9], catalytic combustion [10,11], photoionisation [12] and non-dispersive infrared (NDIR) [13,14,15] Among these methods, NDIR gas sensors have advantages such as simple operation and maintenance, a long service life, fast response time, high measurement accuracy and fail-to-safe operation. Optical BPFs are incorporated to improve the NDIR gas selectivity by narrowing the total bandwidth (BW) of the spectral response of the source-detector system, and thereby preventing cross-sensitivity with other gases and water vapour One such solution to the problem of gas selectivity would be to use tunable semiconductor diode lasers; such a solution is not favourable due to high cost. BWs were progressed to assess the effect of the filter BW on the sensor accuracy and the cross-talk reduction capability with the ultimate goal of determining whether a BW is (if any) more suited to sensor commercial production
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