2 ms·

Because......you can't really go below it? So they are calibrated to start at 400ppm, because your ambient air won't have any less than that unless you have act

Because......you can't really go below it? So they are calibrated to start at 400ppm, because your ambient air won't have any less than that unless you have active CO2 scrubbers running, and that's very very rare. Also I have multiple SCD41 sensors and can confirm that they do give you values both below 400ppm and above 5000ppm - they just get less accurate out of that range.

snvzz 4y ago
I can't get below 400ppm even outdoors.
- gambiting 4y ago
  That's the point - globally all air now contains above 400ppm, it's the great tragedy of climate change. You can only see less than 400ppm if you put the sensor in an airtight container and add something that absorbs CO2 to it - then the value will go below 400ppm. But because that's not how most people use these sensors, they are calibrated for 400ppm upwards.
- Robotbeat 4y ago
  I would hope not: https://www.co2.earth/ https://www.co2.earth/
Robotbeat 4y ago
A freshly poured concrete structure with poor ventilation can go well below 400ppm.
- gambiting 4y ago
  Sure, so like I said, you have a situation where a CO2 scrubber(in that case concrete) is present. If you need accurate measurement of CO2 below 400ppm and not just "it's less than 400ppm" then you need to buy a different sensor.
  - Robotbeat 4y ago
    This is a problem if you have a fairly well-sealed greenhouse as well. The CO2 level can go below 400ppm and reduce plant productivity. If you have a suggestion for a good but affordable CO2 sensor that can measure at or below 400ppm reliably, I’m all ears!
    - gambiting 4y ago
      So all Sensiron sensors are tuned for minimum 400ppm, other than this one: https://sensirion.com/products/catalog/STC31 https://sensirion.com/products/catalog/STC31 The problem is that it gives you percentage of CO2 in air instead of ppm, it can go all the way down to 0% though. I guess there must be a way to figure out the ppm based on that? I guess? Here's an example of a product with this sensor already integrated: https://www.sparkfun.com/products/18385 https://www.sparkfun.com/products/18385 Also I want to point out that their other sensors(SCD40 and SCD41) will actually give you a value even below 400ppm - just that their accuracy drops below that value. So it might say it's 300ppm when in reality it's 250ppm. Not sure how important it is for you.

snvzz 4y ago

I can't get below 400ppm even outdoors.

gambiting 4y ago
That's the point - globally all air now contains above 400ppm, it's the great tragedy of climate change. You can only see less than 400ppm if you put the sensor in an airtight container and add something that absorbs CO2 to it - then the value will go below 400ppm. But because that's not how most people use these sensors, they are calibrated for 400ppm upwards.
Robotbeat 4y ago
I would hope not: https://www.co2.earth/ https://www.co2.earth/

Robotbeat 4y ago

A freshly poured concrete structure with poor ventilation can go well below 400ppm.

gambiting 4y ago
Sure, so like I said, you have a situation where a CO2 scrubber(in that case concrete) is present. If you need accurate measurement of CO2 below 400ppm and not just "it's less than 400ppm" then you need to buy a different sensor.
- Robotbeat 4y ago
  This is a problem if you have a fairly well-sealed greenhouse as well. The CO2 level can go below 400ppm and reduce plant productivity. If you have a suggestion for a good but affordable CO2 sensor that can measure at or below 400ppm reliably, I’m all ears!
  - gambiting 4y ago
    So all Sensiron sensors are tuned for minimum 400ppm, other than this one: https://sensirion.com/products/catalog/STC31 https://sensirion.com/products/catalog/STC31 The problem is that it gives you percentage of CO2 in air instead of ppm, it can go all the way down to 0% though. I guess there must be a way to figure out the ppm based on that? I guess? Here's an example of a product with this sensor already integrated: https://www.sparkfun.com/products/18385 https://www.sparkfun.com/products/18385 Also I want to point out that their other sensors(SCD40 and SCD41) will actually give you a value even below 400ppm - just that their accuracy drops below that value. So it might say it's 300ppm when in reality it's 250ppm. Not sure how important it is for you.