BME680 measurement results interpretation. About repetability/representativity/accuracy ?

[wtc-peg]

Hello,

I’m running BME680 in ULP mode, using BSEC library on stm32,  on two different sensors, could someone help me to explain behaviors ?
The two sensors are at the same place in a room.
You can see joined pictures of measurements (T,H,VOC,CO2), for 24hours in an office with nobody inside.
Both sensors were running for 3 days before this day of measurements.
The global sensor is also equipped with a CO2 NDIR sensor, that show a quit stable CO2 level all along these 24hours.
At the end of period, for both sensors, you can see arrival of 3 people in the office (CO2 increasing and VOC also).

First of all, it is nice to see that the morphologies of the curves are similar.

But my mains  questions are:

1. Why Both sensors get such an increase of VOC for about 0-4 degrees of temperature increase (around 15h00)... in the worst case (250), and according to bosh IAQ table interpretation, it says to me that air is very very polluted (but nobody inside no, paints or anything like this !!??). Why is this value so big ?
2. Why the top of measurement of two BME680 sensors are so different one from each other : Max IAQ at 250 or 140 around 15h00 ?
3. VOC seems "to much" tied to temperature measurement even using BSEC that should already compensate. Anyway to improve that ?

Please help to interpret these results.

[handytech]

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@wtc-peg wrote:

But if IAQ is automatically "coerced" between 25 and 250 how can we detect presence of nocive gazes in a room (paints or other ?) ?

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The BME680 is a tVOC sensor and cannot tell what is the source of VOC in the environment. In the standard IAQ output, the index is automatically scaled so that 250IAQ corresponds to the "worst air" (highest VOC concentration) observed by BSEC in its history. Alternatively you could evaluate the static IAQ output for stationary applications, which still tracks the baselines but has its sensitivity fixed.

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@wtc-peg wrote:

It seems really T/H dependant as what we can expect form output fresh air is only a very few increase of CO2 by night...

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Not only T/H behavior, but compounds and their concentrations in outside air are significantly different than indoor, therefore for stationary applications we expect the IAQ outputs of BSEC to perform optimally indoor.

[wtc-peg]

Hello,

So what i can conclude is that :

• IAQ is coerced between 25 and 250 and will never show values between 250 and 500.
  Then I'll never be able to distinguish between
  people presence in a room
  and
  people presence + important VOC exceptionnal polution.
  Is that correct ?

• Then, you say that I should use Static-IAQ if my sensor is indoor and static.
  Then:
  
  • What will be the range/values of returned s-IAQ values  ? Will it be 0 to 500 ?
  • Could I find a kind of "application note " about s-IAQ measurement and interpretation in Static Indoor environment, and furthermore using ULP mode ?

In advance thanks

[handytech]

There is no further AppNote/documentation than the BME680 datasheet and documents in the BSEC release package.

The static-IAQ output still tracks air quality history so that 25IAQ corresponds to typ. good air observed, but uses a fixed sensitivity. The range of the sIAQ output has been unlocked above 500IAQ, and you should experience a more linear response/behavior of the output.