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The Senseair S8 CO₂ sensor[0] used in this project also uses an infrared LED and works on the principal that CO₂ absorbs certain frequencies of light. So if yo

by breput 4y ago

The Senseair S8 CO₂ sensor[0] used in this project also uses an infrared LED and works on the principal that CO₂ absorbs certain frequencies of light. So if you know the brightness of the LED and the volume of air between it and the sensor, you can calculate the amount of CO₂ by comparing the expected vs. received light intensity. [0] https://rmtplusstoragesenseair.blob.core.windows.net/docs/publicerat/PSP126.pdf https://rmtplusstoragesenseair.blob.core.windows.net/docs/pu...

dragontamer 4y ago
Good to know. So methinks that a CO2 sensor would have to be based off of multiple LEDs + Photodiodes, to measure the absorbed light at _different_ frequencies. (LEDs and Photodiodes typically are only active at one frequency: such as 900nm or 700nm). If "all" frequencies are deflected, its PM2.5 (some particle got in the way of the measurement). If "some" frequencies are deflected, then its CO2. ---------- The question after that comes down to calibration and test data. Is there an easy way to create CO2 density and/or PM2.5 particles of a certain density? --------- There's a big difference. 1. Photodiodes and LEDs are extremely common parts manufactured by a wide variety of companies around the world. By tying yourself to Photodiodes/LEDs, you are less tied to any particular company. 2. Photodiodes and LEDs are well under 10-cents each. Though some more carefully calibrated ones / specialty ones may reach as high as $1 to $2.
- breput 4y ago
  > (LEDs and Photodiodes typically are only active at one frequency: such as 900nm or 700nm). That's not entirely correct. The spec sheet for the LED will give an intensity vs. wavelength chart, which depending on the LED can be 10s to 100s of nanometers wide. As long as the source LED has a sufficiently bright output in one of the CO₂ absorption bands, it doesn't have to be exactly tuned like a laser. NDIR is the technology used in most consumer-grade CO₂ sensors: https://en.wikipedia.org/wiki/Nondispersive_infrared_sensor https://en.wikipedia.org/wiki/Nondispersive_infrared_sensor
- breput 4y ago
  > The question after that comes down to calibration and test data. Is there an easy way to create CO2 density and/or PM2.5 particles of a certain density? Most CO₂ sensors have the ability to calibrate to either 400 ppm or 0 ppm. So if you really care enough, you can fill a small box with nitrogen and calibrate to 0 ppm.

by breput 4y ago

dragontamer 4y ago

Good to know. So methinks that a CO2 sensor would have to be based off of multiple LEDs + Photodiodes, to measure the absorbed light at _different_ frequencies. (LEDs and Photodiodes typically are only active at one frequency: such as 900nm or 700nm). If "all" frequencies are deflected, its PM2.5 (some particle got in the way of the measurement). If "some" frequencies are deflected, then its CO2. ---------- The question after that comes down to calibration and test data. Is there an easy way to create CO2 density and/or PM2.5 particles of a certain density? --------- There's a big difference. 1. Photodiodes and LEDs are extremely common parts manufactured by a wide variety of companies around the world. By tying yourself to Photodiodes/LEDs, you are less tied to any particular company. 2. Photodiodes and LEDs are well under 10-cents each. Though some more carefully calibrated ones / specialty ones may reach as high as $1 to $2.

breput 4y ago
> (LEDs and Photodiodes typically are only active at one frequency: such as 900nm or 700nm). That's not entirely correct. The spec sheet for the LED will give an intensity vs. wavelength chart, which depending on the LED can be 10s to 100s of nanometers wide. As long as the source LED has a sufficiently bright output in one of the CO₂ absorption bands, it doesn't have to be exactly tuned like a laser. NDIR is the technology used in most consumer-grade CO₂ sensors: https://en.wikipedia.org/wiki/Nondispersive_infrared_sensor https://en.wikipedia.org/wiki/Nondispersive_infrared_sensor
breput 4y ago
> The question after that comes down to calibration and test data. Is there an easy way to create CO2 density and/or PM2.5 particles of a certain density? Most CO₂ sensors have the ability to calibrate to either 400 ppm or 0 ppm. So if you really care enough, you can fill a small box with nitrogen and calibrate to 0 ppm.