Question 1

What are the different air quality parameters & their levels as measured by AQM9?

Accepted Answer

The chart below shows the differnt parameters measured along with their levels. The levels are categorised based on WHO guidelines and industry best practices.

https://static.wixstatic.com/media/cad19c_bdfd545265514997badf48c47fe5a150~mv2.png

Question 2

How AQM9 Measures VOCs, NOx and other harmful gases?

Accepted Answer

AQM9 uses the Sensirion SGP-41 sensor to detect VOCs and other harmful gases including Nitrogen oxides which can be very harmful to human health. 
SGP-41 is sensitive to the following:
 • VOCs
 • Hydrogen (H2)
 • Carbon Monoxide (CO)
 • Methane (CH4)
 • Formaldehyde (HCHO)
 • Ethanol (C2H5OH)
 • Ammonia (NH3)
 • Nitrous Oxide (NO)
 • Nitrogen Dioxide (NO2)
 • Ozone (O3)
How are the harmful gases measured?​
The SGP-41 measures VOCs and harmful gases using an index which is a scale from 0 to 500.
Measuring gas concentrations in parts per billion (PPB) is only accurate in laboratory settings and often wildly inaccurate in real world settings.
The Sensirion TVOC (Total Volatile Organic Compounds) Index works by comparing current VOC levels to a moving average over the past 24 hours, similar to how a human nose perceives odors.

A TVOC Index above 100 indicates more VOCs than average, while below 100 means fewer VOCs. The VOC Index adapts its gain based on recent VOC events, allowing for consistent action mapping (e.g., triggering an air purifier at a specific index).

Similar to the TVOC index, there is a NOx index for detecting Nitrogen oxides gases.
Note:
Gas sensors used for continuous indoor harmful gases measurement incorporate a metal oxide sensor (MOX or MOS). A MOX sensor measures the presence of reducing or oxidizing gases based on changes in electrical resistance. Measurements can be performed up to several times per second.

While the sensitivity to individual gases can be characterized in well-controlled laboratory experiments, in real life MOX sensors typically provide a non-selective single output which is a sum of individual gases present in the air. In other words, metal oxide sensors cannot differentiate between the various gases present in the air. 
For more information on MOX sensors please check out chapter 1.3 from Sensirion’s Documentation that explains the limitations of MOX sensors and how Sensirion’s TVOC index is a far better indication for harmful gas concentration in indoor air.
Helpful Sensirion Documentation Links:
 • Compliance of Sensirion’s VOC Sensors with RESET and WELL (https://sensirion.com/media/documents/4B4D0E67/6520038C/GAS_AN_SGP4x_BuildingStandards_D1_1.pdf)
 • Is it possible to map the output of a MOX sensor to a norm? (https://sensirion.com/media/documents/D27F03C0/6294E00F/Info_Note_Output_MOX_Sensor.pdf)
 • What is a metal oxide (MOX) sensor? (https://sensirion.com/media/documents/0083CDF4/6294DFEA/Info_Note_MOX_sensor.pdf)
 • What are oxidizing gases? (https://sensirion.com/media/documents/A00A17D5/6294E01E/Info_Note_Oxidizing_Gases.pdf)
 • What are reducing gases? (https://sensirion.com/media/documents/8CF4440E/6294E02F/Info_Note_Reducing_Gases.pdf)

Question 3

How AQM9 Measures Particulate Matter (PM)?

Accepted Answer

AQM9 uses the PMS7003 from Plantower. This sensor operates by detecting laser light scattering by airborne particles. It can detect ultra fine particles with great accuracy and consistancy.
The PMS7003 can detect particles of the following sizes:
 • PM10 μm (Micrometers)
 • PM2.5 μm
 • PM1 μm 
 • PM0.3 μm 
The detection range for PM2.5 is from 0 - 1000 μm/m³.​
The world health organisation’s 2021 air quality guideline (AQG) states that annual average concentrations of PM2.5 should not exceed 5 µg/m3, while 24-hour average exposures should not exceed 15 µg/m3 more than 3 - 4 days per year.

Question 4

Measuresing Temperature, Humidity & Pressure

Accepted Answer

The AQM9 uses the Bosch BME-280 sensor for temperature, relative humidity and barometric pressure.
The operating ranges are:
 • Pressure: 300...1100 hPa
 • Temperature: -40…85°C with decimal point accuracy
 • Humidity : 0-100%
How can measuring air pressure be useful?
Changes in air pressure can give early signs of changing weather conditions.
High pressure in an area often means low humidity and clear skies. Whereas low pressure indicates cloudy or stormy weather is on the way.

Question 5

How AQM9 Measures Carbon Dioxide (CO2)?

Accepted Answer

AQM9 uses the MH-Z19E sensor to detect the CO2 concentration.
MH-Z19E uses Non-Dispersive Infrared (NDIR) light and works by measuring the amount of infrared light absorbed by the CO2 molecules at a particular wavelength. This absorption is directly related to the concentration of the CO2 gas. 
The sensor can detect CO2 levels in parts per million (ppm) from 400-10000 ppm.
High CO2 levels above 800ppm indicate stale air due to poor circulation of fresh air in the indoor environment. 
CO2 levels above 1000 ppm have been linked to various health issues and discomfort, including headaches, drowsiness, and reduced cognitive function.

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