For cell phone, you don't need to trust the app much - it reports raw (X, Y, Z) vector. Turn the phone sideways and see how it's Z went to 0 and it's now Y (or

For cell phone, you don't need to trust the app much - it reports raw (X, Y, Z) vector. Turn the phone sideways and see how it's Z went to 0 and it's now Y (or X) showing -9.8. Spin in the chair and see how the vector's magnitude is increasing. This should be a pretty effective evidence that the vector is not offset, and at most scaled.