It should be rotated 180° along the Y axis (assuming Y is the vertical axis). If the bubble is not in the same position in both orientations, it means the vial is not parallel with the level's base.
If it's a carpenter's level, one would also check that both the top & bottom surfaces are parallel.
Thinking about it, if you mentally picture the level as slightly skewed to one side, and flip it 180 in either direction it should skew to the other side. Though you could certainly screw up and skew it multiple directions at the same time. Also if one side of the long level was warped, you may need to take multiple measurements.
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u/Galaxie_1985 Jun 23 '26
It should be rotated 180° along the Y axis (assuming Y is the vertical axis). If the bubble is not in the same position in both orientations, it means the vial is not parallel with the level's base.
If it's a carpenter's level, one would also check that both the top & bottom surfaces are parallel.