Review on GZAIR Temperature Relative Humidity Monitor: Accurate Readings from 0 to 9999PPM by Steve Campbell

Great gauge. Fast, accurate, quick results, the battery is great and is USB powered so you can use the USB battery to extend usage time when you're out of power. You will be amazed at how much CO2 rises in your room when you are in it. The only complaint about the manual. It tells you exactly how a function runs, but not what the function does. You can set an upper/lower limit, I have no idea what does that, why I want to do this.