Scientists from Japan have developed a novel method for measuring hydrogen gas that employs tunable diode laser absorption spectroscopy (TDLAS) at different pressures and a high-pressure gas cell.
TDLAS is widely utilized for gas analysis, but until now it has had difficulties quantifying low concentrations of hydrogen due to its lower absorption compared to other gases in the near-infrared (NIR) region.
“Our system can significantly improve hydrogen detection systems for safety and quality control, facilitating wider adoption of hydrogen fuel,” Tatsuo Shiina, who led the research, explained. “For example, this system can be reliably used for the detection of leakages in hydrogen fuel cell cars.”
In TDLAS methods, a laser is passed through a Herriott multipass cell (HMPC) containing the target gas. The laser's wavelength is modulated around the gas's target absorption line to remove any environmental noise, while the cell pressure can be tuned to influence the absorption line.
The researchers analyzed the width of hydrogen's strongest absorption line at different pressures and, through a series of simulations, they identified the optimal pressure for a broader absorption line width and the most effective modulation parameters within this line width.
Image: Optics & Laser Technology, Chiba University, License CC BY 4.0
Then, they applied those results to a calibration-free technique. Essentially, instead of processing only the second harmonic signal, as done in traditional TDLAS, they used the ratio of the first and second harmonic. By doing so, the academics were able to normalize the research and realize the optimal measurement conditions for increasing the gas detection limit and stabilizing the wavelength lock for the laser diode.
The researchers also achieved accurate measurements of hydrogen concentrations in a wide detection range from 0.01% to 100%. “Moreover, the results improved with longer integration times, the time period during which light is allowed to be absorbed. At 0.1 second integration time, the minimum detection limit was 0.3% or 30,000 ppm, which improved to 0.0055% or 55 ppm at 30 seconds integration time,” they explained. “However, beyond 30 seconds, the minimum detection limit increased.”
Concluding the research, the group added that despite identifying limitations related to optical elements and laser properties, “the experimental method exhibits excellent overall performance in measuring hydrogen concentrations. This innovative method for precise hydrogen measurement holds potential applications in trace gas sensing for hydrogen stations or hydrogen vehicles in the future. Additionally, it can be utilized for quality checks of hydrogen.”
Their findings were presented in “Optimization for hydrogen gas quantitative measurement using tunable diode laser absorption spectroscopy,” published in Optics and Laser Technology. Scientists from Chiba University, the Mitsubishi Electric Corporation, Kyushu University, Shikoku Research Institute, and Tokai University conducted the research.
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