Hydrogen has extensive applications in many sectors. However, its safe and widespread use necessitates reliable sensing methods, according to researchers at Chiba University, Japan. While tunable diode laser absorption spectroscopy (TDLAS) is an effective gas sensing method, researchers said detecting hydrogen using TDLAS is difficult due to its weak light absorption property in the infrared region. Addressing this issue, Chiba University researchers developed a calibration-free technique which they said in a study enhances the accuracy and detection limits for sensing hydrogen using TDLAS.

According to researchers, hydrogen gas is lightweight, storable, energy-dense, and environmentally friendly compared to fossil fuels, producing no pollutants or greenhouse gas emissions. As such, it has extensive applications across different fields, including transportation, architecture, power generation, and industries. However, hydrogen is highly flammable, and therefore its safe and widespread use requires reliable methods for detecting leaks and ensuring its purity, researchers said. The need for reliable detection methods has necessitated the development of trace-gas sensing techniques. However, detecting low concentrations of hydrogen with TDLAS is difficult because hydrogen has weaker absorption in the infrared region compared to other gases.

To address this issue, a research team from Japan led by Associate Professor Tatsuo Shiina from the Graduate School of Engineering, Chiba University, developed a method for precise hydrogen gas measurement using TDLAS. The team comprised Alifu Xiafukaiti and Nofel Lagrosas from the Graduate School of Engineering, Chiba University, Ippei Asahi from the Shikoku Research Institute, and Shigeru Yamaguchi from the School of Science, Tokai University.

TDLAS technology has gained attention for detecting various gases. Researchers said TDLAS offers several advantages, including non-contact measurement, in situ detection, high selectivity, rapid response, low cost, and multi-component, multi-parameter measurement capabilities. It works on the principle that gases absorb light at a specific wavelength, resulting in a dark line in the absorption spectrum, known as the absorption line. By measuring the amount of laser light that has been absorbed at this wavelength, researchers said the concentration of the gas can be determined.

“In this study, we achieved highly sensitive detection of hydrogen gas through meticulous control of pressure and modulation parameters in the TDLAS setup. Additionally, we introduced a calibration-free technique that ensures the adaptability to a wide range of concentrations,” Professor Shiina said.

In TDLAS, laser light is passed through a pressurized gas cell called a Herriott multipass cell (HMPC) containing the target gas, researchers explained. The laser’s wavelength is modulated or oscillated around the target absorption line of the gas at a specific frequency to remove any environmental noise. The pressure in HMPC can significantly influence the absorption line width and consequently the modulation parameters under TDLAS.

The researchers said they analyzed the width of hydrogen’s strongest absorption line at different pressures. Through simulations, the researchers identified the optimal pressure for a broader absorption line width and the most effective modulation parameters within this line width. Their calibration-free technique involved using the first harmonic of the modulated absorption signal to normalize the second harmonic through their ratio, instead of just relying on the second harmonic signal as in conventional TDLAS systems. Additionally, they said they employed a high-pressure gas cell containing pure hydrogen as a reference to fine-tune the modulating parameters of the laser signal.

Through this approach, researchers said they achieved accurate measurements of hydrogen concentrations in a wide detection range from 0.01% to 100%, where 0.01% equals a concentration of just 100 parts per million (ppm). 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. However, beyond 30 seconds the minimum detection limit increased.

“Our system can significantly improve hydrogen detection systems for safety and quality control, facilitating wider adoption of hydrogen fuel. For example, this system can be reliably used for the detection of leakages in hydrogen fuel cell cars,” Prof. Shiina explained about the potential applications of the study.

To summarize, researchers said this technique could help pave the way for a sustainable future and boost the implementation of hydrogen as an eco-friendly fuel.