Helium Sensor Could Predict Volcano Eruptions - IEEE Spectrum

Volcanic eruptions can be fast, deadly, and destructive. That’s why every bit of data counts when predicting them.

Sensing company INFICON, based in Bad Ragaz, Switzerland, has developed the first portable helium sensor that can withstand a harsh volcanic environment. They’ve deployed it to monitor Vulcano and Stromboli, two volcanic islands in Italy. The hope is that such on-site, real-time monitoring could predict eruptions before they become disasters.

When magma is closer to the surface or tectonic plates are moving around, helium levels in the ground increase. Previously, scientists would take gas samples from the volcanoes and send them to a lab for testing, resulting in a measurement every few weeks. But the new sensors are portable and can take measurements every 20 seconds, says Josef Grenz, an engineer at INFICON. If you want fast helium measurements on site, “there is no alternative to our sensors,” says Grenz.

Called ‘He-Man’ sensors, they have two parts: a thin membrane permeable only to helium, and a vacuum chamber that ionizes the helium atoms and takes pressure measurements.

First, helium atoms are singled out when they pass through a membrane permeable only to that element. The membrane is made of a very thin layer of glass on top of a silicon chip, which has micrometer sized holes etched into it. Glass already lets helium though, but the thinner the glass the faster helium can cross. The He-Man sensor’s glass layer is just 6.5 micrometers thick, about one tenth of the thickness of a human hair. The silicon layer is needed to support the extremely thin glass during manufacturing.

After the helium atoms pass through the membrane, they arrive in a vacuum chamber. This vacuum chamber contains a Penning Trap, an electromagnetic device that traps them in a circle, using an electric and a magnetic field. The atoms oscillate in a circular pattern, and become ionized by the trap. Once they are ionized, the charges are accelerated onto a charge collector plate which measures the pressure inside the vacuum chamber, detecting the amount of helium present.

“We are now able to make such a small system... approximately the size of the shoebox. Before that you have to have a much larger, much more expensive system.” —Josef Grenz, INFICON

This technological setup is about 20 years old, says Grenz, but it remained confined to the laboratory or as a part of a larger, multi-component device until recently. It required calibration against a reference point by applying a known helium concentration with a constant flow to assess the sensitivity of the sensor. If that reference point was not measured, slight changes in the sensor over time would lead to errors in the system’s output.

INFICON engineers found a way to make this comparison in a scaled-down system. Knowing that the permeability of the glass membrane is temperature dependent, they designed the He-Man to make two measurements at two different temperature values. By comparing those values, they can algorithmically find the reference point instead of having to measure it directly.

“We are now able to make such a small system... approximately the size of the shoebox,” says Grenz, “before that you have to have a much larger, much more expensive system.”

INFICON’s innovation is not the only new helium sensor in development. A helium sensor based on a micro-electromechanical (MEMS) device was recently invented by a group at the University of Nebraska. Their design relies upon the fact that helium has a lower thermal conductivity than air, so “if helium exists, it will cool down the temperature of a MEMS faster than air,” explains Fadi Alsaleem, a professor of architectural engineering at the University of Nebraska.

Alsaleem and his team created a structure that deforms based on temperature, so it deforms less in the presence of helium. The structure acts like a switch in their sensor. “If there is air, the switch will be on. If there’s helium, the switch will be off,” he says. “As simple as that.” The sensor must be tuned to a specific level of helium to detect it, because it transmits a binary value. Alsaleem’s group reported their research in IEEE Sensors Journal in August.

Alsaleem foresees the MEMS sensor being used to detect leaks in nuclear waste storage.

INFICON, in contrast, is focused on developing their He-Man sensor to detect volcanic eruptions and other natural disasters. Increased levels of helium can also be an early warning sign for earthquakes, says Grenz, and every indicator matters when keeping a population safe.