If you want to use the Reader and the app with the same Sensor, follow these steps:
You MUST use your Reader to start a new Sensor. A Sensor started with the app cannot be used with the FreeStyle Libre reader
Once the Sensor is started with the Reader, open your app and scan your Sensor with the app
Once the Sensor has warmed up, you can use either the app or the Reader to check glucose for up to 14 days
Scan the Sensor with the Reader or app within an 8-hour time period to avoid gaps in data

Remember that the FreeStyle LibreLink app^◊ and FreeStyle Libre readers do not share data with each other. For example, if you switch between your Reader and the app, you may see gaps in your glucose data, which will affect the reports available through the Reader, the FreeStyle LibreLink app, and LibreView. It is important to continue scanning your Sensor with either the app or the Reader at least once every eight hours so you and your healthcare professional can get a complete picture of your glucose trends and patterns.
If you are using FreeStyle Libre 2 sensors, then you will receive alarms on the FreeStyle Libre 2 reader only when you start the sensor with the reader.

◊ The FreeStyle LibreLink app is only compatible with certain mobile devices and operating systems. Please check the website for more information about device compatibility before using the app. Use of FreeStyle LibreLink may require registration with LibreView.