The free version can only be calibrated once. To get accurate results just upgrade to the paid version. You can also change Units of weight in paid version and use Super Mode.
As accurate as real digital scales if calibrated and used properly with the phone balanced well.
You can change which sensors are used and also set the sensitivity of the scales. You can set the units of the weighings too.
In the help section you can view two video demonstrations showing how to use the app and some tips.
In the paid version you can store and view upto 4 measurements with their own custom descriptions
With Super Scales you dont need any external third party devices or gadgets. Just a android phone with calibrated sensors.