"mike" wrote in message
...

You need to check your levels and discard ones that are off
calibration. Find a rigid flat surface longer than the level. Place
the level on it, check the bubble and note the exact location
(centering is not necessary). Reverse the level (180 degrees). The
bubble should be in exactly the same location.

You sure about that?
Passing your test suggests that the level is ok.
Failing your test suggests that the level is bad OR the test
surface is not level.

The surface doesn't need to be level.

If the bubble is not in the same place in both directions, it's definately
defective (or has debris interfering).

He is not suggesting that the bubble should be centered at all.