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Portable Raman analyzer detects hydrogen leaks from a distance

Researchers have developed a new portable Raman analyzer that can accurately measure very low concentrations of hydrogen gas in ambient air. The instrument could be useful for detecting hydrogen leaks, which pose serious safety risks due to the gas's flammability and tendency to accumulate in confined spaces.

"As hydrogen gains recognition as a promising fuel for transportation, heating and power generation, ensuring safety and minimizing losses becomes even more crucial," said research team leader Andreas Muller of the University of South Florida.

"Hydrogen can leak through minute cracks in pipelines or storage tanks, and in industrial settings, detecting these leaks often requires approaching the source closely. Our Raman analyzer can sense small changes to the ambient hydrogen concentration to identify a leak from a distance."

in the journal Applied Optics, the researchers showed that the instrument can, in a matter of minutes, detect hydrogen concentrations well below 1 part per million in ambient air and thus identify a leak of a few hundred milliliters per minute from a source several meters away.

"Our versatile instruments could be useful in a variety of industrial and scientific applications, including exploration of the significant hydrogen resources that exist beneath Earth's surface," said Muller. "The system could also be optimized to detect many other analytes at equally low concentrations, which might be useful for medical applications, for example."

Portable trace gas detection

Measuring very low concentrations of hydrogen gas in ambient air is difficult because hydrogen is a lightweight, colorless and odorless gas that can quickly disperse. Additionally, the sensitivity and precision of the detection method must be extremely high to pick up trace amounts without interference from background gases. Today's portable hydrogen sensors tend to rely on indirect electrochemical or electromechanical changes in chemically labeled surfaces and can't measure very low concentrations accurately.

To overcome these limitations, the researchers developed a new portable instrument based on Raman scattering, a technique that uses light to produce unique optical signals from a material, enabling noninvasive identification and measurement of substances.

Although Raman scattering is used commercially to analyze liquids and solids, applying it for the detection of low concentrations of gas requires a way to enhance the Raman scattering signal, which is typically difficult to implement outside the laboratory.

In the new work, the researchers designed a Raman analyzer that uses multipass cavity enhancement, a type of enhancement that is uniquely forgiving of disturbances and does not require specialized lasers or stabilization techniques to function.

"We previously studied Raman trace gas analyzers in the laboratory, but implementing a portable version required several engineering innovations," said Muller. "Our new Raman analyzer can handle outdoor environments and map out minute hydrogen concentration changes around a source with reasonable form factor and power consumption."

Taking Raman analysis outside

To build the portable Raman gas analyzer, the researchers used a laser emitting at 442-nm with a high output power of several watts and a narrow linewidth of less than 0.1 nm. They also created a multipass cavity that didn't experience misalignment due to temperature changes in the environment. Crucially, the laser and spectrometer had to consume less than 10 watts each for sufficient field autonomy.

Charuka Arachchige, graduate student at the University of South Florida, led the effort of evaluating the stability and adaptability of the instrument across various environments by mapping hydrogen concentrations over time and distance from a hydrogen source.

After the build-up and decay of hydrogen within a laboratory setting was characterized, he measured it as a function of distance, starting from 8 meters away from the source, across three different locations—a laboratory room, an atrium, and an outdoor open space—under various environmental conditions to assess the instrument's sensitivity and stability.

The findings demonstrated consistent variations in hydrogen concentration across all locations, influenced by the prevailing environmental factors. They were also able to detect hydrogen levels just 63 parts per billion higher than the surrounding level from several meters away, even in outdoor conditions. This demonstrated that small leaks could be detected from a distance.

The researchers are continuing to improve the instrument's limit of detection, speed of recording and compactness.