Tech Briefs Magazine - October 2021 - ST-12

Applications
Sensing Hazardous Gases Can Save Lives
n increase in the number of hazardous
gases poses a severe threat to
humanity in general and to workers in
many industries. These gases could come
from natural or man-made sources like
chemical industries, petroleum refining,
stone, plastic, and food processing. Because
of the risk that they leak into the
environment, safety procedures are necessary
to protect the environment and
the workers. Different types of gas detectors
are used to detect different gases
such as commonly occurring pollutants
like carbon monoxide, hydrogen sulfide,
sulphonyl chloride, phosphine, and nitrosyl
chloride.
A
Catalytic Sensors
Pellistor/Catalytic Bead (CB) sensors,
which have been around for nearly a century,
can respond to flammable gases
such as hydrogen, oxygen, hydrogen sulfide,
methane, butane, propane, and carbon
monoxide. They have two beads: an
active bead coated with a catalyst, that
reduces the temperature at which the gas
surrounding it ignites. As a result of the
combustion, this bead heats up, causing
a temperature difference between it and
a reference bead. The heat causes the
resistance to change as a function of the
type and concentration of the gas. Since
the bead is one leg of a Wheatstone
System Communication Components
BLE
BLE
bridge, this resistance change produces
an output voltage signal. Catalytic bead
combustible sensors are quick and accurate
when used for detecting a single gas.
However, this technology is subject to
sensor poisoning from exposure to silicones
and lead compounds.
Furthermore, in multiple gas detection
sensors, these sensor detectors may
give false readings for all other gases if
they are calibrated to a single gas. The
correct choice of combustible gas for
calibration depends upon the industry
application and the potential gas hazard(s)
of that particular industry. For
example, for a propane distributor, the
hazard is propane; in sewers, the main
hazard is methane.
In a complex environment with multiple
combustible gases, a number of
questions need to be considered when
choosing a sensor, most importantly the
Lower Explosive Limit (LEL) - the lowest
concentration (by percentage volume)
of a gas in air that is capable of
producing a flash of fire in the presence
of an ignition source.
* What are the combustible gases?
* Which gas is the most prevalent?
* What are their concentrations as a percent
of LEL?
* What type of sensor do you need -
single gas or multiple gas?
Molecular Property Spectrometer™
(MPS) Sensors
Molecular Property Spectrometer
(MPS) sensors, a proprietary technology
from NevadaNano (Sparks, NV), can
detect the presence of flammable or
combustible gases, including mixtures,
and classify them as hydrogen, methane,
light gas, medium gas, or heavy gas.
They can quickly detect a wide spectrum
of combustible gases, including hydrogen
and heavy hydrocarbons. MPS sensors,
unlike catalytic bead sensors, cannot
become poisoned because the measurement
is based on physical properties
rather than chemical reactions. They do
not need calibration, and offer a fail-safe
feature and built-in diagnostics to notify
the user if a sensor becomes unusable or
compromised.
The MPS Flammable Gas Sensor's transducer
is a micro-machined membrane
with an embedded Joule heater and resistance
thermometer. The micro-electromechanical
system (MEMS) transducer is
mounted on a printed circuit board and
packaged inside a rugged enclosure open
to ambient air. Presence of a flammable
gas causes changes in the thermodynamic
Zigbee
Zigbee
LTE
Satellite
Provider
Internet
Figure 1. Universal Site Monitoring system communication components. (Courtesy Universal Site
Monitoring)
12
Cov
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Figure 2. Personal Safety Monitor Hero 715 -
ATEX/IECEx certified version. (Courtesy
Universal Site Monitoring)
Sensor Technology, October 2021

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Tech Briefs Magazine - October 2021

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