Calibrate accelerometer?

I'm developing an app that is supposed to measure vertical accelerations, and in the process I've discovered that the accelerometer seems to be uncalibrated. When the phone is lying perfectly horizontal, the Z axis acceleration should be 9.81, but it is about 8.81 (X and Y being roughly 0.0). I can of course offset the error from within the app by letting the user "calibrate" the app while the phone is lying still. But it seems odd that there is no method within Android to do this - sometimes Google Maps prompts me to calibrate the compass by moving the phone in figure eights - but I have not seen any way to calibrate other sensors... is it possible? A further problem is that if I turn the phone so the top is facing straight up (up/down in line with the direction of gravity), the offset of the axis' are different from what is expected, and different from the offsets I get when the screen is facing up. This all seems to indicate that the accelerometer is not as accurate as one might think, and when adding rotation to the mix the results get even more skewed...