How to conduct IAQ to monitor indoor air quality?

Each person inhales approximately 15 kg of air per day, 80% of which is indoor air. Outdoor air quality is generally monitored by government agencies, while indoor air quality (IAQ) monitoring is the responsibility of building operators or occupants – provided they are willing to actually perform. Now, a new generation of small surface mount low-power volatile organic compound (VOC) gas sensors are available, enabling decentralized, local IAQ monitoring with small, low-cost components, making it easier for users to operate buildings Air flow and air filtration equipment. This article describes how the new VOC sensor works and how it differs from an absolute single gas sensor. At the same time, it also describes how sensors provide data, enabling air management equipment to more efficiently and effectively respond to changes in indoor air quality.

How to conduct IAQ monitoring? Currently, professional operators of commercial buildings generally use one or two types of air quality data to control the operation of ventilation and air filtration systems. Most commonly, they use a single gas—usually carbon dioxide (CO2) for absolute measurements. They will also refer to the subjective judgment of the occupants on air quality. Because humans exhale CO2, it is normal for a person to be in a room to increase CO2 concentration over time. Therefore, the more people in the room, the higher the CO2 concentration without adequate ventilation. If the concentration of CO2 in the indoor air is too high, people will feel "squeaky", want to doze off, not concentrate, and cannot make effective decisions. As a result, commercial building management systems equipped with CO2 sensors now regulate, filter and/or ventilate equipment based on measured CO2 concentrations. The goal is to keep the indoor air fresh and comfortable while reducing the heat exchange rate, because artificial heating or cooling of the air is a waste of money and energy. In fact, CO2 concentration is an appropriate way to represent the density of people in a given space. And because humans produce VOCs and CO2, which are scientifically called "bio-emissions," today's building operators generally believe that air-conditioning equipment equipped with a CO2-concentrated function can also adequately regulate the concentration of multiple VOCs in indoor air. . Practical considerations support this assumption. After years of development, CO2 sensor components have been attractively packaged, priced, and power efficient enough to ensure integration into the boards of mainstream building automation equipment. Until recently, the optional VOC concentration measurement method was quite limited. There are several methods for measuring and analyzing VOCs suspended in air, including photoionization, flame ionization, colorimetric tubes, and wavelength absorption, which are relatively lightweight methods. In the laboratory, there is a tendency to combine a method using gas chromatography and mass spectrometry (referred to as GC-MS). However, these methods are not suitable for compact, localized, low-power air quality sensing devices because they are either too bulky or too power consuming. That's why the new generation of metal oxide (MOX) VOC sensors are now available, and now available in surface mount IC packages with milliwatts of power, which is very exciting for IAQ monitoring. These low-cost, compact, low-power VOC sensor components are easily integrated into everyday products such as luminaires, air conditioners, fans, and fan remote controls—even in cell phones. Decentralized local VOC sensing is practical and one of the trends.

Figure 1: Types of VOCs that are commonly found indoors and their sources. Therefore, users of air-conditioning equipment should reconsider whether they are still relying solely on CO2 data. In fact, VOC concentrations do not rise and fall with changes in CO2 concentration for two main reasons:

First, not all VOCs are produced by humans (see Figure 1); secondly, the rate at which humans produce CO2 is sustained and generally fairly stable when inactive. However, human-generated VOCs are fluctuating, for example, rising for a period of time after a meal. According to the report of the National Institute of Standards and Technology (NIST) Building and Fire Research Laboratory, “Many sources of pollution are not only from occupants, but also emissions from building materials, as well as pollutants entering the building from the outside. CO2 concentration. It does not provide data such as the concentration of pollutants emitted by the release source that is not related to the occupant." (AK Persily, 1996) For example, in a room with only one person, the CO2 sensor records lower concentrations of CO2 in indoor air, but New homes and carpets have recently been refurbished, and some fixtures have been glued to the walls and floors of the room. In this case, the air conditioning equipment in the room is typically configured to provide a minimum amount of ventilation in this environment, resulting in a single occupant breathing a large amount of suspended VOC. The high concentration of VOC in the indoor air significantly affects the comfort of the occupants. CO2 is odorless, but the VOC smells very heavy and (mostly VOC) is unpleasant. However, the effects of VOC in the air are not only uncomfortable. The US Environmental Protection Agency (EPA) website lists short- and long-term health effects, noting that these effects may be related to VOCs in indoor air. These effects pointed out by the EPA include: irritation to the eyes, nose and throat; headache, loss of coordination and nausea; damage to the liver, kidneys, and central nervous system; some organisms can cause cancer in animals; some are even suspected or known to cause humans cancer. Therefore, these examples have prompted OEMs to begin using surface mount MOX VOC sensors in IAQ monitoring equipment. The working principle of the MOX gas sensor is shown in Figure 2.

Figure 2: How the chip-type MOX gas sensor works The MOX VOC component itself detects multiple VOCs and provides a relative output that corresponds to changes in VOC concentration. The sensor is also capable of calculating the equivalent relative values ​​of multiple VOCs when equipped with an onboard processor. Since the outputs of these components are relative, no calibration is required. In addition, there is a class of absolute output gas sensors: they are ideal and necessary for safety-critical applications where high concentrations of certain gases pose a direct threat to life or health. Such absolute output components are typically: relatively expensive; only one gas can be detected; periodic calibration is required to provide accurate output data. These factors are clearly unpopular in IAQ monitoring applications. VOC sensors complement this important but limited absolute measurement source: this sensor is capable of detecting multiple VOCs and can therefore be used to detect changes in indoor air quality caused by one or more VOC compounds – and this will affect People inside the building. In IAQ monitoring, wide spectrum sensors such as ams CCS811 (2.7mm x 4mm x 1.1mm, LGA package) or iAQ-CORE (15mm x 18mm integrated sensor module) are not for safety critical applications. A particular gas reports its absolute ppm value, but provides a relative change in the concentration of multiple VOCs in the environment, including but not limited to those listed in Figure 1. In IAQ monitoring applications, MOX VOC sensors can be used with absolute output CO2 sensors to provide an accurate baseline for CO2 concentration. The VOC sensor reinforces the measurement of absolute CO2 and collects additional data about VOC events that are not necessarily directly related to the occupants (usually the main cause of elevated CO2 concentrations), as shown in Figure 3.

Figure 3: Comparison of measured values ​​when the VOC sensor and the CO2 sensor are operating simultaneously after several hours of use of the conference room. In Figure 3, during the VOC sensor indicating a drop in air quality, the CO2 sensor has no movement at all, which may be due to Cleaning chemicals used during the meeting, or emissions from equipment such as printers and photocopiers. Depending on the output of the VOC sensor (instead of the indication of the CO2 sensor), there is usually better ventilation; therefore, in this case, during the VOC event, the air quality in the room will be improved for the occupant.

Calibration of air conditioning system measurement data

VOC sensors such as CCS811 or iAQ-Core can effectively detect changes in VOC concentration in the air over time. But how do you use this data to manage the operation of air filtration or air flow equipment? Today, air management systems are typically configured to respond to the absolute value of the measured CO2 concentration. Thus, iAQ-Core and CCS 811 include a processor that executes the provided algorithm, calculating relative eCO2 (equivalent CO2) values ​​and relative TVOC (total volatile organic compound) values. This allows the air quality management system designer to convert the input from the sensor to the appropriate command, such as increasing the fan speed of the ventilator, or opening the vents a little – or slowing down the filtration after the VOC concentration drops. / or air exchange speed to reduce energy consumption. According to the test, the calculated relative value of eCO2 refers to the appropriate reference value and is mainly caused by human VOC emissions, which is closely related to the actual CO2 concentration change measured by the absolute CO2 sensor (see Figure 4).

Figure 4: Comparing the eCO2 value from the ams MOX gas sensor calculation calibration to the actual value measured by the absolute CO2 sensor over a 7-day period

Integrate new opportunities for IAQ monitoring

Today, the advantages of chip-type VOC sensors in terms of size, cost and power consumption make them a decentralized sensing source for air-conditioning system management areas. Integrating them into air quality control systems is only a matter of time – eventually a A healthier, more enjoyable working, living and entertaining environment. It is also possible to integrate small surface mount VOC sensors in end products that previously did not have air quality sensing capabilities, thus demonstrating the value of such sensors. For example, a wide-spectrum VOC sensor in a cookware hood can detect changes in VOC concentrations in the air produced by raw materials and cooked odors, smoke, detergents, etc., and automatically manage airflow based on these changes. In this way, the chef does not have to manually operate the ventilation device. VOC sensors are very helpful in any interior space, including public transportation, private cars, and public buildings such as hospitals, offices, and shops. Because these sensors are small enough to be mounted on a PCB with other electronic components, they can be integrated with virtually any connected component for easy connection to an air management system.