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sensors Article Hydrogen Sensor Based on Tunable Diode Laser Absorption Spectroscopy Viacheslav Avetisov 1,*, Ove Bjoroey 1, Junyang Wang 1,2, Peter Geiser 1 and Ketil Gorm Paulsen 1 1 NEO Monitors AS, Prost Stabels vei 22, N-2019 Skedsmokorset, Norway; ove.bjoroyneomonitors.com O.B.; peter.geiserneomonitors.com P.G.; ketil.paulsenneomonitors.com K.G.P. 2 Yinian Sensors Technology Co., Ltd., Shenzhen 518126, China; junyang.wangecamana-tech.com * Correspondence viacheslav.avetisovneomonitors.com Received 1 November 2019; Accepted 30 November 2019; Published 3 December 2019 gid00030gid00035gid00032gid00030gid00038gid00001gid00033gid00042gid00045gid00001 gid00048gid00043gid00031gid00028gid00047gid00032gid00046 Abstract A laser-based hydrogen H2 sensor using wavelength modulation spectroscopy WMS was developed for the contactless measurement of molecular hydrogen. The sensor uses a distributed feedback DFB laser to target the H2 quadrupole absorption line at 2121.8 nm. The H2 absorption line exhibited weak collisional broadening and strong collisional narrowing e ects. Both e ects were investigated by comparing measurements of the absorption linewidth with detailed models using di erent line profiles including collisional narrowing e ects. The collisional broadening and narrowing parameters were determined for pure hydrogen as well as for hydrogen in nitrogen and air. The performance of the sensor was evaluated and the sensor applicability for H2 measurement in a range of 0–10 v of H2 was demonstrated. A precision of 0.02 v was achieved with 1 m of absorption pathlength 0.02 v m and 1 s of integration time. For the optimum averaging time of 20 s, precision of 0.005 v m was achieved. A good linear relationship between H2 concentration and sensor response was observed. A simple and robust transmitter–receiver configuration of the sensor allows in situ installation in harsh industrial environments. Keywords gas sensor; hydrogen sensor; diode laser; TDLAS; WMS; absorption spectroscopy; laser spectroscopy; hydrogen 1. Introduction An increased demand for hydrogen gas sensors is strongly coupled with the expanded use of hydrogen gas H2 in industry [1,2]. Hydrogen is an important feedstock in many industrial processes and applications, including the oil and gas industry, chemical plants, and the steel industry, among others. Refineries use hydrogen in many operations e.g., hydrotreating of various refinery process streams and hydrocracking of heavy hydrocarbons. Analyzing hydrogen in complex and varying gas mixtures is challenging, and measurements are normally performed using gas chromatographs that entail slow response times and high operational costs. Since hydrogen is highly flammable, strict regulations for using H2 safety sensors apply. Many di erent types of hydrogen safety sensors are commercially available [3], and the common principle is a sensing element that is altered e.g., by resistance when in contact with hydrogen. This mode of operation precludes the use of these point-type hydrogen sensors in reactive, corrosive, and/or dusty gas streams. For this reason, contactless hydrogen sensing is highly desired. Diode lasers are extremely attractive for this purpose due to their inherently low-intensity noise and narrow linewidth. These properties enable the highly selective and sensitive probing of narrow and extremely weak H2 absorption lines. As a diatomic homonuclear molecule, H2 has no dipole moment that could create strong optical absorption in the infrared region. The absorption spectrum of H2 is therefore limited to vibrational bands of very weak Sensors 2019, 19, 5313; doi10.3390/s19235313 www.mdpi.com/journal/sensors Sensors 2019, 19, 5313 2 of 13 electric quadrupole transitions [4], which is why laser-based detection of H2 so far has been limited to extractive cavity-enhanced sensors based on cavity ring-down spectroscopy CRDS [5,6], intra-cavity output spectroscopy ICOS [7,8], and optical-feedback cavity-enhanced absorption spectroscopy OF-CEAS [9]. Common in these techniques is confinement of the laser light in a high-finesse optical cavity by using a set of highly reflective mirrors. Up to several kilometers of e ective absorption pathlength can be obtained, which allows for detection of very weak hydrogen quadrupole transitions. However, for use in industrial applications, where extremely high sample purity can be di cult to achieve, contamination of mirrors is an issue. If not properly handled, this will lead to degradation of the sensor sensitivity and can ultimately damage the coatings of the high-reflectivity mirrors. For this reason, cavity-based H2 sensors often impose rigid requirements on gas sampling and conditioning systems and typically require periodic maintenance. Tunable diode laser absorption spectroscopy TDLAS [10,11] is a sensitive and selective method that directly probes the process in situ without having to extract gas samples. Gas sensors based on TDLAS are frequently used for many industrial process-control, emission-monitoring, and safety applications, and are well accepted throughout many industries [12–16]. The underlying measurement principle is inherently contactless, and the instrumentation is consequently not exposed to potentially corrosive process gases. Concentration readings are made available in real time, which is ideal for fast and e cient process control and safety-related measurements. Furthermore, in situ measurements have low maintenance requirements and thus reduce the operational costs. In general, compared to cavity-enhanced techniques, TDLAS has less complexity and is more robust, which has made TDLAS-based sensors the preferred platform for many industrial process-control and safety applications. In this paper, we present the first laser-based infrared hydrogen absorption sensor for in situ hydrogen measurements. The sensor can be reconfigured for extractive measurements for applications where in situ installation is not feasible due to, for example, high pressure and/or high temperature. Optical windows isolate the process gas from the sensor so that the advantage of contactless measurement is maintained. The presented hydrogen sensor is based on the commercial LaserGas II platform [17] manufactured by NEO Monitors AS. The instrument was used to study a selected hydrogen absorption line in terms of line broadening and narrowing e ects. To validate the measurements, lineshape modelling using di erent spectral line profiles was performed. The sensor was capable of measuring hydrogen with a precision of 0.02 v m for 1 s of integration time, which is better than most intended safety applications require assuming a measurement range of 0–10 v. Using a response time of 1 s, an estimated limit of detection LOD for H2 of 0.1 v for 1 m of absorption pathlength was achieved. 2. Sensor Design The developed sensor follows the classical in situ TDLAS design and consists of transmitter and receiver units. The transmitter unit contains a diode laser, collimating optics, a microprocessor board, and all input–output electronics. The transmitter unit also has a built-in cell for H2 validation. The receiver unit incorporates a photodetector, focusing optics, and signal detection electronics amplifier, mixer, etc.. The sensor is based on the wavelength modulation spectroscopy WMS technique, which is well described in the literature [18–20]. This technique has been proven to be very useful in trace gas sensing due to its ability to perform very sensitive interference-free measurements directly in the process or across stacks without sample extraction and preconditioning. Since WMS provides nominally baseline-free absorption signals, it is especially suited for measuring weak absorbance. Recently published comparisons of WMS and direct absorption spectroscopy DAS techniques revealed that WMS is approximately one order of magnitude more sensitive [21–23]. Figure 1a shows a photograph of the LaserGas II sensor mounted on the demo pipe using DN50 flanges, and Figure 1b depicts a schematic diagram and the basic principle of the sensor operation. Sensors 2019, 19, 5313 3 of 13 Sensors 2019, 19, x 3 of 14 Figure 1. a Tunable diode laser absorption spectroscopy TDLAS H2 sensor mounted on a demo pipe. Transmitter unit is on the left and receiver unit on the right. Gas inlet and outlet of the built-in validation gas cell are indicated. b Schematic overview of the principles of sensor operation. A sinusoidally modulated current ramp is applied to the laser, which is swept in frequency across the transition of interest. After interacting with the sample, the absorption information is encoded in the transmitted intensity, which is measured using a photodetector. The photodetector signal is amplified, filtered, mixed, and digitized. Finally, digital signal processing is used to retrieve the concentration and possibly other relevant parameters. A distributed feedback DFB diode laser from Nanoplus Nanosystems and Technologies GmbH emitting near 2122 nm was used in the sensor. The laser had an output power of about 5 mW at the driving conditions specified herein. The temperature of the diode laser was stabilized at around 30 °C with high accuracy typically in the mK range to set the average emission wavelength of the laser. A DC current of 70 mA was applied to operate the laser above its threshold, and a current ramp of 10 mA with a duration of about 2 ms was used to tune the laser about 0.4 cm –1 across the H2 absorption line of interest. The repetition rate of the current ramps was 150 Hz. After each ramp, the laser current was switched off for signal normalization purposes. A sinusoidal current of 2 mA amplitude and frequency f of 100 kHz was added to the current ramp in order to modulate the laser wavelength for WMS implementation. The collimated laser beam was directed through the target gas, captured by the receiver optics, and focused onto an InGaAs pin photodetector Hamamatsu G12183-020K. The signal detected by the photodetector was bandpass-filtered to select the 2f component, and the filtered signal was detected using an analog mixer, a low-pass filter, and an amplifier. The bandwidth of the detected signal was 10 kHz. Further, the 2f WMS signal was digitized using an AD converter and normalized to the measured direct signal. Before calculating the gas concentration, additional digital signal processing was applied to improve the signal-to-noise ratio SNR, which optionally could be digital filtering, wavelet denoising, or baseline fitting. Figure 1. a Tunable diode laser absorption spectroscopy TDLAS H2 sensor mounted on a demo pipe. Transmitter unit is on the left and receiver unit on the right. Gas inlet and outlet of the built-in validation gas cell are indicated. b Schematic overview of the principles of sensor operation. A sinusoidally modulated current ramp is applied to the laser, which is swept in frequency across the transition of interest. After interacting with the sample, the absorption information is encoded in the transmitted intensity, which is measured using a photodetector. The photodetector signal is amplified, filtered, mixed, and digitized. Finally, digital signal processing is used to retrieve the concentration and possibly other relevant parameters. A distributed feedback DFB diode laser from Nanoplus Nanosystems and Technologies GmbH emitting near 2122 nm was used in the sensor. The laser had an output power of about 5 mW at the driving conditions specified herein. The temperature of the diode laser was stabilized at around 30 C with high accuracy typically in the mK range to set the average emission wavelength of the laser. A DC current of 70 mA was applied to operate the laser above its threshold, and a current ramp of 10 mA with a duration of about 2 ms was used to tune the laser about 0.4 cm 1 across the H2 absorption line of interest. The repetition rate of the current ramps was 150 Hz. After each ramp, the laser current was switched o for signal normalization purposes. A sinusoidal current of 2 mA amplitude and frequency f of 100 kHz was added to the current ramp in order to modulate the laser wavelength for WMS implementation. The collimated laser beam was directed through the target gas, captured by the receiver optics, and focused onto an InGaAs pin photodetector Hamamatsu G12183-020K. The signal detected by the photodetector was bandpass-filtered to select the 2f component, and the filtered signal was detected using an analog mixer, a low-pass filter, and an amplifier. The bandwidth of the detected signal was 10 kHz. Further, the 2f WMS signal was digitized using an AD converter and normalized to the measured direct signal. Before calculating the gas concentration, additional digital signal processing was applied to improve the signal-to-noise ratio SNR, which optionally could be digital filtering, wavelet denoising, or baseline fitting. Sensors 2019, 19, 5313 4 of 13 3. Line Selection The absorption spectrum of H2 in the infrared region was very sparse and extremely weak. Figure 2 shows a simulation of H2 absorption using default air broadening parameters listed in the high-resolution transmission molecular absorption database HITRAN [4]. The fundamental 1–0 vibrational electric–quadrupole transitions of H2 are between 1900 and 3000 nm. There are only a few lines visible due to the large rotational constant that follows from the low mass of the H2 molecule. The first overtone band 2–0 of H2 is located between 1100 and 1500 nm. Compared to the fundamental band, the overtones have been studied much more extensively using cavity-enhanced spectrometers [6,24]. Sensors 2019, 19, x 4 of 14 3. Line Selection The absorption spectrum of H2 in the infrared region was very sparse and extremely weak. Figure 2 shows a simulation of H2 absorption using default air broadening parameters listed in the high-resolution transmission molecular absorption database HITRAN [4]. The fundamental 1–0 vibrational electric–quadrupole transitions of H2 are between 1900 and 3000 nm. There are only a few lines visible due to the large rotational constant that follows from the low mass of the H2 molecule. The first overtone band 2–0 of H2 is located between 1100 and 1500 nm. Compared to the fundamental band, the overtones have been studied much more extensively using cavity-enhanced spectrometers [6,24]. Figure 2. High-resolution transmission molecular absorption database HITRAN simulation of H2 absorption 1 v∙m using default air broadening parameters listed in the HITRAN2016 database. From the HITRAN simulations, three transitions in the fundamental vibrational band were identified as potentially suitable for H2 gas sensing 2407 nm 4155.3 cm –1 , 2223 nm 4497.8 cm –1 , and 2122 nm 4712.9 cm –1 . The best available transition for industrial applications is not ne