[snip strongly diverging measurements between separate devices] @BSTRobin wrote: Hi huytyskland, how about the accuracy value during that time? First of all thank you for your quick reply! With rising VOC reading the accuracy may drop to Medium for relatively short periods of time. However, it never reverts to Low or Unstable. At the moment, I have two devices running side by side (the nrs. 1 and 3 above), and after giving them another night, readings seem rather better (accuracy given between brackets [H/M/L/U]) Baseline, clean air: #1: VOC: 0.61 ppm [H], IAQ 38.35 [H] #2: VOC: 0.69 ppm [H], IAQ 39.41 [H] -> this seems pretty good. Exposure to 100 ppm acetone, after 20 seconds: #1: VOC: 10.61 ppm [H], IAQ 219.16 [H] #2: VOC: 299.61 ppm [M], IAQ 282.22 [M] -> device 2 shows a reading that is 30 times the value of device 1, with Medium accuracy.The switch from [H] to [M] happens somewhere around a reading of 130 ppm. Another noteworthy detail is the fact that IAQ values are far less divergent. Then we tested a dilution of ~30 ppm acetone, after first returning to baseline levels: #1: VOC: 4.58 ppm [H], IAQ 175.64 [H] #2: VOC: 62.11 ppm [H], IAQ 216.88 [H] -> both devices now report High accuracy, yet there is still a discrepancy by a factor of 10 to 15 - way too much to be of any use. After removing the sensors from the acetone vapor, readings revert to slightly above baseline levels: #1: VOC: 0.90 ppm [H], IAQ 63.75 [H] #2: VOC: 1.00 ppm [H], IAQ 52.93 [H] The overall values are slightly higher than baseline probably because some acetone is absorbed in the materials and slowly released. We also have some questions that you may be able to shed some light on: What is the best calibration procedure for these devices? Are there ways to make these devices 'learn' about gas levels apart from the current BSEC automatic calibration? Can we somehow prevent measurements from reverting from [H] to [M] or lower? At the moment, I'm working on a dual-channel logging application for logging most internal parameters second by second, so that responses of the two sensors can be examined and compared with the highest possible resolution. If you think that this information could be useful, I will post (a link to) the log files here as soon as I have this finished. I also experimented with 'dumb' hot-plate sensors such as the (now obsolete) CCS801, so I fully appreciate the huge difficulties in calibrating and using these devices; still, we had hoped that these Bosch sensors would be a bit easier to deploy (also based on accuracy figures given in the datasheet). As you can see above, we have not yet succeeded in monitoring vapor concentrations with any accuracy at all; even 'High accuracy' measurements can differ by well over an order of magnitude. Anyway, thanks already for any suggestions on how to improve performance here, because at the moment, things are not usable. And if it turns out that these sensors are not supposed to measure ppm's with any accuracy at all, then please let us know, so we can go look for other devices. Best regards, Richard
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