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Max values for TVOC and ECO2 (CCS811)

Sensors
#1

I have SCK 2.1 deployed outdoors in an industrial part of a city where TVOC and ECO2 values occasionally flat-line and, I assumed, max out. In referring to the AMS datasheet for CCS881 sensor (p. 12), I found the following:

• “The equivalent CO2 (eCO2) output range for CCS811 is from 400ppm up to 29206ppm. “
• “The equivalent Total Volatile Organic Compound (eTVOC) output range for CCS811 is from 0ppb up to 32768ppb.”

Yet, the max for my eCO2 is 32768ppb while the maximum value for eTVOC on my sensor is 29206 ppm. Furthermore, the minimum for my ECO2 is 400 ppm. At first I thought maybe the smart citizen web interface had values reversed, but it doesn’t appear to be that simple, as my EC02 range is 400ppm to 32678 ppm (not 0 – 32678 ppm nor 400 – 29206 ppm).

Do you have any idea whether this is a mistake on the datasheet or an issue in the smart citizen web interface?

While the 2 values are indeed, highly correlated, they do often peak at different times. As I am trying to relate their values to some other air quality data in the area, I’d just like to confirm the values on the web interface are indeed coming from their stated sensor. Thanks!

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#2

I cant reply directly to your query except to say that the SC team have gone to a lot of trouble to calibrate the sensors leading to trusted and consistent results.
But generally to gain some more trust, this sort of issue lends itself to parallel measurement with a different device that “ought” to get similar results. The underlying issue is calibration from a known accurate source.
For my part I already have a Bosch Sensortec BME680 sensor board (from Seeed) that also happens to measure eCO2 and eVOC and it seems to have good specification/accuracy so I will be able to see what correlation there may be between the SC2.1 Kit sensor and the BME680 sensor.
I’ll let you know my results in a few weeks… (after my kit arrives). :smiley:

#3

I just received a BME680 sensor too and I was planning to do the same thing! Unfortunately, the SCK that I have access to right now seems to have succumbed to the elements (looks like a bad urban sensor board) and I don’t have another handy.

BUT, my question isn’t so much about calibration to other sensors as it is verification of the information about the installed sensors.

The maximum value I record for ECO2 (32768 ppm) is well beyond the range of the reported maximum value for the sensor (29206 ppm from the datasheet). It appears that the max value for ECO2 is actually the maximum value reported on the datasheet for the TVOC sensor. That’s why I thought maybe the sensors were mismatched with their reporting platform on smartcitizen.me.

However, my minimum value for each sensor DOES indeed match the datasheet (ECO2: 400 ppm and TVOC: 0 ppm).

This leads me to believe it’s an issue with the sensor and not smart citizen sensor programming. What do you think? Are the maximum values reversed in the datasheet or are the sensors actually mismarked from the manufacturer?

This would matter in those instances where folks were trying calibrate to another sensor (see viric’s post here: Trying to improve CCS811 CO2 reads)

Thank you!

#4

The CCS811 sensor includes a computer and a firmware. In the post of mine you linked I posted my github forks, from which you can read the CCS811 firmware version and update it.

I never checked the max values of my sensor, that runs the latest firmware I could find. I didn’t see any communication error that could mix values.

In any case, I could not find much correlation between CO2 levels from a more proper sensor and this CCS811 reads, so I don’t use it.

#5

I have had another look at this because of another question.
The CCS811 sensor directly measures a wide range of Volatile Organic Compounds. The context is indoor air quality. It gives the readings in units of equivalent CO2. I think perhaps it means the amount of CO2 that would produced if the Organic compound was oxidised (burned).
VOC in units of parts per billion, and CO2 in parts per million.
The current level of CO2 in air (global average) is around 403 parts per million.
I may be math-challenged but a reading of circa 32123 PPB is around 32.123 ppm I think.
Perhaps these observations are helpful in your interpretation.

#6

The link below should be informative for this topic.
Of interest to me is the fact that the range of readings for eCO2 and VOC lie far outside those documented. I do wonder if the device was (at the time the topic was started) burned in as recommended.

strong texthttps://cdn.sparkfun.com/assDocs for CCS811