As technology improves, competition heats up for smaller, less power-hungry gas sensors based on relatively inexpensive chips.
The gas sensor market is rapidly expanding, driven by increased concerns about indoor air quality and better technology for detecting and measuring it.
Indoor air quality sensing targets gases and concentrations that differ — with some overlap — from outside. But until the pandemic, which largely coincided with a flood of new research about what is toxic to people and animals, this has been a relatively quiet market. That’s changing as large, clunky systems are replaced by much smaller, cheaper, and more accurate sensors and modules.
The challenge system designers are facing now is understanding which sensors comply with which standards, and what’s actually available with sufficient quantity and quality to achieve accurate results.
“I don’t have to tell you that carbon monoxide is a killer,” said Sreeni Rao, senior director at TDK’s Gas and Environmental Sensing products and business, who is working on SEMI standards for gas sensors. “But there are lots of other gases that can kill a person if not properly handled or ventilated. Carbon dioxide could kill a person in a room in concentrations up to like 50,000 ppm.”
The good news is the number of sensors is ramping quickly. Gas sensors are starting to show up on smartphones, for example. But while standards are plentiful, they also are in flux, so knowing which standards to follow, and then getting the sensor/module certified by a reliable third-party lab, are essential ingredients for a sensor design.
No one size fits all for air quality monitoring, according to a recent survey conducted by SEMI’s MEMS and Sensors Industry Group (MSIG). “Pollutants for indoor and outdoor are different, and certain technologies can be better than others at detection,” according to a paper authored by Radislav Potyrailo, principal scientist at GE Research and chair of MSIG’s Device Working Group; Ryotaro Sakauchi, senior manager at Robert Bosch LLC responsible for consumer market business development of Bosch Sensortec’s MEMS sensors; TDK’s Rao; and Christian Meyer, senior product marketing manager at Renesas, specializing in gas sensors.
Bulky and expensive gas sensing and air quality systems approved by government agencies for official air quality monitoring are not being replaced. But they are being supplemented by smaller and less power-hungry versions, which are democratizing and micro-localizing air quality readings.
One of the issues for sensor and systems designers, along with consumers, is sorting through a plethora of air quality standards. Some of those standards are still being formed and updated, based on new science of what is considered unhealthy. So certifying that a system is based on a real standard, or a series of standards, is important. In addition, these devices need to be calibrated on an ongoing basis, and the algorithms that are “interpreting” the data need to be updated.
Standards can vary from one region to the next. Every country sets its own indoor air quality (IAQ) standards, and some standards are being set by building and health/safety organizations. So rather than follow a government and building-industry group standards, some sensor manufacturers define their own rules, according to Christian Meyer, senior product marketing manager at Renesas who specializing in gas sensors.
New standards based on studies are finding what affects people in an indoor environment. Those standards are evolving as the amount of data gleaned from health studies gets integrated into the design process.
Three important science-based indoor air quality (IAQ) standards come from UBA, WELL, and RESET.
These three IAQ standards are informed by the World Health Organization and other standards. IAQ in public spaces, including offices and factories, is regulated, but residential IAQ often is not. For the most part, you can’t force someone to install a carbon monoxide sensor in a private home, although there can be regulations in building codes for new construction or remodeling inspections.
“There are a lot of other standards,” said Renesas’ Meyers. “For example, ASTM has a standard. SEMI has a standard. UL has a standard. So there are a lot of other standards with different emphasis and different focuses.”
The U.S. Environmental Protection Agency (EPA) monitors outside air, looking at levels of four gases — ozone (O3), NO2, SO2, and CO — and particle concentrations in the air using EPA-approved instruments to get an Air Quality Index (AQI) number.
Indoor air
Gas detection technology can be divided into two camps, those that use physical methods and those using physio-chemical methods. Physical methods are acoustical, thermal, optical, separations, and electronics. Physio-chemical are electro-chemical, ionizing, and chemi-resistive. The IAQ sensors detect some different compounds from the outdoor sensors. Total volatile organic compounds (TVOC) can off-gas from furniture, paint, and carpet, in addition to CO2 and CO. Ozone (O3) and NOx detection are included.
Fig. 1: Gas measurement technologies fall under either physical or physico-chemical methods. Source: Renesas
Absolute measures
Although the human nose is very sensitive to contaminants, it also adapts quickly, which is why people repeatedly exposed to certain smells cease to sense them. It is not limitless, though. The nose and brain adapt to their environment within a certain threshold. For example, the people sitting in a crowded conference room eating their lunch may not smell the food anymore, but the odor is evident to someone who just enters the room. Likewise, residents of Los Angeles cannot smell the ozone because they are used to it.
Absolute measures are important because you can’t trust the human nose to detect a gas after a while. An absolute measure is a number rather than range.
“It does not mean that the air is good, just because we can’t smell it,” said Meyer. “But we want to get one step further. We don’t want to measure the absolute concentration. We also want to tell you what it’s relates to, for cognitive scores for health effects, because this is what the end is counted for. If we feel well, can we prove we have a productive work environment? Or do we sit at home and feel tired all the time? We do not make up our own standards, because we’re not doctors. We are not people who do social studies and see how cognitive scores are. But we do refer to standards already out there.”
The concentration levels to be measured are a good indicator of the type of sensor needed for a job. If detecting parts per million (ppm), the sensor sensitivity can be lower than when detecting parts per billion (ppb). Chemists look at concentration of gas, using mg/m3. Engineers talk in parts per million or per billion.
Also affecting the absolute measure are temperature and humidity, which means sensors do need to be calibrated. “Humidity and temperature always have effects — always,” said Meyer. “Ask what’s the influence of the temperature, because temperature always has an influence for everything, including sensors. That’s why we need to compensate for temperature and humidity.”
To simplify this process, humidity and temperature sensors are starting to be built into air quality monitors. “The typical monitor that you already have for the airport has a temperature/humidity sensor inside. Even some smartphones have it already,” said Meyer. “There are a few dozen smartphones available that have temperature/humidity sensors. The new thing is to include air quality sensors on the smartphone. There are two smartphones available in the world that have an air-quality sensor.”
Fig. 2: Gas sensor technologies. Source: Renesas
Threshold vs. relative measurements
Machine learning also is being built into sensors to make better sense of the data in the context of other variables. Using a threshold number and measurement can set off an alarm that a relative number is at a dangerous level.
“You have three things that are unparalleled when it comes to air quality,” Meyer said. “The first thing is to keep in mind the general thresholds. Threshold means some specific gas has a certain concentration and it’s getting unhealthy or toxic. Think carbon monoxide, for example, which you have in open fires. It’s common in British homes to have a chimney inside a fireplace, where the ventilation is not very good. So you have carbon monoxide or nitrogen oxide, which is another example. Nitrogen dioxide (NO2) and nitrogen monoxide (NO) have lower thresholds limits that should not be exceeded. In public space like in an office environment or industrial environment they are regulated, so a certain cannot be exceeded. For carbon monoxide, this is 50 ppm. As soon as 30 ppm is reached, then you have to sound an alarm. You have to do a counter action — you have to inform the people to leave the building, leave the room.”
Conclusion
Indoor gas sensor ICs and modules are on the cusp of a renaissance of types, materials, and form factors, with more innovations still in the research phase. System-level firmware and hardware improvements, and machine learning algorithms are making it easier for systems designers to adopt gas sensors in systems that automate buildings air quality readings and adjustments, and report the air quality. The data pulled from these sensors is as accurate as required (sensing to parts per million or billion), based on the use case.
As these sensors proliferate, there likely will be more options and capabilities available, and possibly many more standards to which sensors must adhere. Being able to measure small quantities of gases inexpensively and accurately is merely the first step.
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