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Why Gas Sensing Is Becoming Localized

Low-cost devices are driving more measurements and new applications.

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Sensors that measure air flow, air quality, and chemical makeup are being deployed increasingly for both indoor and outdoor environmental monitoring, in homes, automobiles, and industrial facilities. But despite a raft of new applications for these devices, the necessary standards needed to calibrate and compare those devices are trailing well behind rapid development of new types and combinations of sensors.

In the past, air quality was measured almost exclusively with heavy duty, accurate equipment that relied on infrared spectroscopy, gas chromatography/mass spectrometry (GC/MS), and chemiluminescent analysis. This new breed of sensors are democratizing air monitoring using , small, inexpensive devices. But questions remain about accuracy, security, and reliability.

Some sensors just measure the flow of air, while others measure what is in the air, including gases and particulate levels. But the types of sensors and their uses are proliferating. The global air quality monitoring system market is expected to grow from $5.02 billion in 2021 to $8.33 billion in 2028, according to Fortune Business Insights.

“Main application fields are where power consumption, size, costs play a major role beside accuracy, said Uwe Guenther, senior director of engineering for Renesas‘ Industrial Automation Business Division. “This gets addressed by the new digital, silicon-based miniaturized MOx based sensors, measuring indoor and outdoor air quality,”

Metal-oxide-semiconductor sensors (also known as MOS or MOx sensors, or SMOx) use the electrical properties of semiconductor metal oxides to detect gas. They are used frequently because they are easy to manufacture using existing processes, and the material is readily available. Nanostructures also can use gallium nitride (GaN), another material showing promise as a gas sensor for different gases, depending on the sensor configuration. These nanostructure eventually may cost less and need less power to run, while being more sensitive than the MOX in some ways.

Other examples include:

  • Chemical sensors — These convert chemical or physical properties of a sample into digital signals. These sensors, which consist of a chemical sensing material connected to a transducer, detect changes as well as quantities. So larger concentrations of chemicals will produce a larger signal. (Analyte is the name used for the substance or chemical that is being tested.)
  • Optical, infrared (IR) point sensors, IR image sensors, nondispersive IR (NDIR), and IR spectroscopy — All of these use infrared spectra to determine the exact absorption wavelength of the analyte (the gas), which is used to both identify the gas as well as the concentration.
  • Acoustic sensors — This type of sensor uses a MEMS microphone to measure pressure. Ultrasonic gas detectors can hear gas leaks.
  • Optical plus acoustic — These sensors disperse light to identify different molecules, and to determine the pressure.
  • Biomimetic sensors — These sensors mimic the effect that an analyte has on a human body, causing a change in the sensor. Another sensor is used to interpret the change . This can be used for identifying carbon monoxide, for example, which in this case is a gel.

MEMS gas sensors are becoming more popular, but they have issues. “MEMS gas sensor products today do not conform to specific or well-understood standards with regard to parameter definitions, testing methods, reliability requirements, packaging, pin configurations, communication interfaces, etc., to name a few,” according to Sreeni Rao, senior director of TDK’s gas and environmental sensors. He is helping to remedy that situation by heading SEMI’s MEMS and Miniaturized Gas Sensing Standards Task Force. SEMI’s Device Working Group also is working on gas sensing standards, among other environmental sensors.

New uses
As devices get smaller, air quality sensors are being combined with other sensors. The indoor uses help maintain air quality in occupied spaces, while turning the HVAC systems off for unoccupied spaces.


Fig. 1: Indoor air quality ratings. Source: Renesas/German Environment Agency.

“We are working mainly on solutions that make the customer aware of the air quality in his or her environment, and either advise the customer accordingly (leave the room, do not go running, open the window), or control with this data actuators like HVAC, air purifiers, ventilation systems, and windows,” said Renesas’ Guenther. “A concrete application example for our low-power, miniaturized digital sensors are wearables that provide detection of harmful gases defined by the EPA, specifically O3 and NOx. Our sensors are calibrated to EPA standards and provide feedback on ever-changing air quality from vehicle emissions and formative gases.”

Networks of gas sensors mounted on and in buildings are now connected to the Internet and used in air quality reports as part of a TV weather report, using data from organizations such as Purple Air.

Gas sensors also are helping protect the rainforest. “You have a smart home, you have a smart city — this is sort of like a smart jungle,” said Rob Conant, vice president of software and the ecosystem for IoT software technologies at Infineon Technologies. Infineon has worked with a rainforest protection non-profit to help improve its detection technology. “They take these compute products for the device that’s a bunch of kinds of components — microphones, gas sensors— and put that on a device with solar panels. And they mount those hundreds of feet up on trees in the rainforest. So you’re connected, and it’s way up there in the rainforest. They point out many miles, and they can send that data back to the Internet.”

Even the medical sector is looking at gas sensors. “Higher levels of acetone — the gas called acetone — in a person’s breath is a clear marker of diabetes,” said TDK’s Rao. “There are lots of instruments that actually make use of that fact that just aren’t as optical as pricking your finger and getting a diabetic reading or blood sugar reading. So that is another angle to this whole breath sensors are our gas sensors. That is also extremely important. What we breathe out has an extremely high medical value.”

Bosch has developed a new gas sensor that includes AI — the BME 688 — which detects volatile organic compounds (VOCs), volatile sulfur compounds (VSCs), and gases such as carbon monoxide and hydrogen in the parts per billion (ppb) range. It uses an AI software GUI to help users train the AI. Detection of VOCs are important in semiconductor fabs, as well as other industrial facilities, to help control the levels of combustible gas.

Security
Sensors can be spoofed, and this can be problematic if incorrect data could lead to an error that might hurt someone. One air sensor going haywire in a network of many sensors either means a high concentration of pollutants in one location, or a faulty sensor. But in an enclosed situation, where the sensor data informs when to mix which chemicals or gases, is potentially dangerous.

Consider a water sensor, for example. A hacker could attack the sensors that measure water quality and impact the chemical levels added into water supplies. “If something happens and all the sensors that monitor that the pH values are still around a neutral level, and people start adding chemicals because the sensors are spoofed, this is honestly really scary stuff,” said Vincent van der Leest, director of product marketing at Intrinsic ID. “Unfortunately, I’m not making it up.”

But physical tampering is the more likely attack route. “The biggest problem with these IoT and edge devices is their accessibility to the public,” said Mitchell Mlinar, vice president of engineering at Cycuity.

Conclusion

One of the big benefits of low-cost sensors is their ability to localize measurements, giving people greater awareness of their immediate environment and its impact on their health. The challenge will be making sure measurements are accurate, so they don’t produce unwarranted panic, and that they are secure enough and reliable throughout their projected lifecycles.

“People around the world are much more sensitive to their environment they live in, mainly driven by the higher and higher air pollution around us,” said Renesas’ Guenther. “This awareness has been accelerated by Covid pandemic. This higher sensitivity will get addressed by a rapid growing of sensors systems, making the user aware of their environmental air quality.”

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