Bests & Worsts Reviews from Amazon

I bought this unit and almost immediately was unsure of it's accuracy since it was reading 1200 PPM inside and over 800PPM outside. I asked for a replacement from Amazon to confirm the accuracy and sure enough it's usually over a 600PPM difference between the two. So of course, having two very different outputs I asked for ANOTHER replacement, expecting that it would match the readings of at least one of the units and I would know which is good/bad. But it didn't. It's closer mind you, but it's still between 100 and 200PPM different than the replacement and 400+PPM different than the original.