The second part of OP's post maybe, but not the first part about needing to place the device outside to set the ambient baseline. They basically can't measure a

The second part of OP's post maybe, but not the first part about needing to place the device outside to set the ambient baseline. They basically can't measure at all without that. My Aranet4 was more like 1000 ppm off before putting it outside for awhile, despite being labeled as calibrated from the factory. It was going absolutely ballistic thinking my apartment was full of smog. After sitting outside to get a taste of ambient air, suddenly the inside of my apartment was fine.