H2Tech - Q1 2021 - 38

MEASUREMENT AND INSTRUMENTATION

MEMS-based hydrogen sensors improve
measurement accuracy and safety
B. ROGERS, NevadaNano, Reno, Nevada

Hydrogen, more specifically green H2 , is being used increasingly across the world for a wide range of industrial applications. In the U.S., forecasters predict that H2 from lowcarbon sources could supply roughly 14% of the country's
energy needs by 2050, including hard-to-electrify sectors now
dependent on natural gas, such as high-heat industrial processes and manufacturing fertilizer.
However, H2 is a potentially dangerous gas. An H2 leak in
a refinery can rapidly escalate to a major disaster; and even in
small amounts, exposure to H2 poses several different hazards
to those working with it.
For decades, catalytic bead sensors and non-dispersive infrared (NDIR) sensors have been staples in the gas detection
industry and have increased the safety of dangerous working
environments. However, neither of these technologies are effective for use in environments where H2 is present. If pellistor/catalytic bead sensors are calibrated to H2 , then detection
readings are not accurate, delivering a reading of +/-30% of
actual concentration. NDIR sensors cannot detect H2 because
it does not absorb infrared light.
The evolution of gas sensing technology. Until recently,
the gas detection industry has been relatively stagnant for
nearly four decades. It was dominated by the use of Pellistor/
catalytic bead and NDIR sensors. Pellistor/catalytic bead sensors were introduced nearly a century ago and still operate

FIG. 1. Global demand for pure hydrogen, MMt, 1975-2018. Source: IEA.1

38 Q1 2021 | H2-Tech.com

using the same principles found in their initial design. They
measure the temperature difference between two beads-one
inert and one coated in a chemical catalyst. The bead coated
with the catalyst will heat more than the inert bead in the presence of many flammable gases.
Pellistor/catalytic bead sensors are appealing because they
respond to a full range of flammable gases, including H2 , methane, butane, propane, and carbon monoxide (CO). They are
also relatively low-cost. Unfortunately, however, these sensors
have critical limitations: when calibrated to a single gas, they
provide inaccurate readings for all other gases; they must be
calibrated frequently to perform properly; they consume relatively high power; and they do not function in low-O2 environments. However, their most significant deficiency is that they
tend to become " poisoned " when exposed to high concentrations of flammable and combustible gases, or when exposed to
even low concentrations of common chemicals like silicones
present in many cosmetics and car waxes. When this occurs,
the sensors do not work correctly, or at all, even though they
may appear to be functioning normally. This scenario can put
the user in a hazardous situation where they are unaware of
dangerous or toxic gases present in the environment.
NDIR sensors were introduced and became popular for use
in gas detection in the 1960s. NDIR sensors use infrared light,
which is absorbed at certain wavelengths by hazardous gases.
By measuring the intensity of the light transmitted through a
sensing chamber containing gas and comparing it to a reference, the instrument can determine the concentration of hazardous gas present.
NDIR technology represented a huge leap in gas detection technology 40 yr ago. Today, like pellistor/catalytic bead
sensors, NDIR sensors use dated technology that has serious
limitations. The most significant is that they cannot detect H2 ,
since it does not absorb infrared light.
H2 is commonly found in the mining and petroleum industries, and can be incredibly dangerous. Allowing it to go undetected may create hazardous working conditions. NDIR sensors can be calibrated to a single gas, making them ineffective
at detecting gas mixtures.
NDIR sensors are also very sensitive to environmental
changes. Moderate fluctuations in temperature or humidity
can freeze the output reading, leaving the user unable to determine gas presence or concentration. Also, humidity, fog and
ambient light can cause interference with detection if they are
able to permeate the open chamber.

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