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By choungyeol
#203381
Hi
I used a Sparkfun CCS811 sensor to monitor VOCs indoor and a few questions arose for interpreting the readings. After 20 min warming up in a clean air room, I placed it in a room where some degree of VOC gases were expected and recorded it for 2 days.

First, I observed the sensor values increased rapidly up to 6000 ppb, as shown in figure 1. This result makes me wonder why the value exceeds the upper limit. As far as I know, CCS811 detection ranges from 0 to 1189 ppb and if it exceeds the limit, the value is clipped. Shouldn't the value remain same once it went beyond the limit? Or Is the sensor capable of detecting the wider range?

Second, as shown in figure 2, there are two distinctive curve trends in the gas concentration versus the sensor resistance: One trend shows the resistance decreases asymptotically as the concentration increases and, however, the other trend shows the resistance is almost flat over the concentration. The first trend was observed before the baseline compensation while the second trend was observed after the compensation. Baseline compensation is shown in figure 3. Why the resistances corresponding to the same concentration are different before and after the baseline compensation? Is the readings after the compensation reliable?

I am new to the sensor technology and it would be greatly appreciated to have your kind help. Thanks alot.
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By TS-Chris
#203392
Hi choungyeol.
First, I observed the sensor values increased rapidly up to 6000 ppb, as shown in figure 1. This result makes me wonder why the value exceeds the upper limit. As far as I know, CCS811 detection ranges from 0 to 1189 ppb and if it exceeds the limit, the value is clipped. Shouldn't the value remain same once it went beyond the limit? Or Is the sensor capable of detecting the wider range?
My guess is that the accurate range is from 0 - 1189 ppb but the sensor can measure above that. The data sheet does mention values above 1189 should be clipped, but it's possible that you may need to configure the chip to enable clipping. SparkFun doesn't make the CCS811 chip, we just make the board that it's on and we don't have detailed knowledge on the inner workings of the chip. You might want to try contacting the manufacturer (AMS) to see if they have any insight as to what might be going on.