I've owned several CO2 sensors. In particular, I've had 2 Awair Glow units, 1 first-generation Awair unit, 1 second-generation Awair unit, and a TIM10 CO2 sensor. Of these sensors, the TIM10 is my "gold standard" sensor, but it has the very, very annoying property of requiring AC power. Aranet4 promised to be a battery-powered CO2 sensor that could output data to an app. It delivers, but it requires calibration out-of-the-box, in my opinion.Measured against known good sensors (particularly my TIM10), my Aranet4 sensor read about 75ppm higher than the environmental level out-of-the-box. The app warns you that performing calibration will permanently erase the factory calibration data, which is somewhat scary, but I did it anyway. In order to validate that the calibration was successful, I had my trusted TIM10 sensor outdoors, connected to AC power, out of the sun, with the Aranet4 directly next to it, on a substantially windy day with extremely good air quality (as measured by PM2.5 and PM10 presence from AirNow and Purple Air). The floor, outdoors, in this configuration, as read by the TIM10 unit, was 380ppm of CO2. Before calibration, in the same environment, Aranet4 reported 480ppm. After calibration, Aranet4 reported within 15ppm of TIM10, which I considered a success, particularly given the rarity of having such a low reading. All of the Awair units bottom out at 400ppm.Following calibration, for the last few weeks, I've kept an eye on Aranet4 and TIM10 when they've been in the same room. There's approximately a 10-15ppm disagreement between TIM10 and Aranet4, which I consider a very acceptable deviation given that Aranet4 is battery powered, and TIM10 is AC powered.On the app side, Aranet4 provides a serviceable iOS app. The iOS app itself is perfunctory. It provides graphing, exporting data (as csv, including CO2, pressure, humidity, temperature, and date of sample), and basic reporting features. It doesn't support dark mode, though, as an example of its perfunctory nature. The app developers didn't consider the performance impacts that storing all of the data would have on the graphing component, though, so the app gets laggier the more data the sensor measures. You're also only going to get a week of data for historical measurement.Other details are worth mentioning beyond the sensor accuracy and app integration, though. E-ink, as a display type, is just awesome. It looks very nice, aesthetically. Each refresh blanks the screen temporarily, though. Keep this in mind, if you decrease the measurement interval (you can measure as frequently as once per minute). Also, there's no backlight, which is why the battery life is good, but keep this in mind too, if you want to see it in the dark.I've taken this sensor on an airplane, and it works great there too. It's just an incredible sensor to have -- you can pocket it or bring it somewhere in a backpack and still have CO2 measurements, where most sensors can't go. For travel, or just in general, it's a great device to have. After calibration, it measures very accurately, and I'm happy with my purchase all around. However, I'd say calibration is mandatory.I've included a screenshot of a graph made from the CSV, as well as the CSV data from the unit itself.