There are several reasons:
Beginning with Android 8.0 (Oreo), Google changed the way you’re notified about apps running in the background on your phone. An application which runs background processes should sent a notification to users. μBarometer runs a such process for tracking air pressure data.
You cann't turn off the notification, but can hide it:
The μBarometer app continues to run the tracking process but won't send the notification.
If you hide the μBarometer notification, you’ll receive a different notification from the Android letting you know the μBarometer app is using battery resources. All apps that run in the background affect your phone’s battery life. You can hide this notification as well.
The physical sensor is calibrated at the factory and cannot be changed programmatically. The µBarometer allows you to set a correction value for the reading. For this you need to go to Advanced Settings/Pressure Calibration and enter the value, which will be algebraically (i.e., taking into account the sign) added to the value obtained from the sensor.
μBarometer measures only the current air pressure. Icons are displayed to improve the UI. You can change their position or even turn off the display of icons in the settings.
This needle allows you to "remember" the pressure value at some point in time and compare it with the new value.
Yes. On the source device, open Data table and click the Share button (the leftmost button in the bottom row). A csv file will be generated, which can be transferred to another device using standard Android tools (for example, with using mail). On the target device, open Data table, click the "Import csv" button (second left button in the bottom row) and select the file sent from the first device.
Yes. Using the menu items Export settings / Import settings.