Spirit level: setting it up and changing the bit or blade

Learn to check a spirit level is accurate and read it correctly before trusting any line as level or plumb.

SL-01Know the parts of the levelA spirit level is a straight metal or plastic bar with one or more small glass or plastic tubes set into it, called vials. Each vial is filled with liquid and has a bubble in it. The bubble sits between two lines when the surface is level (flat and horizontal) or plumb (perfectly upright, vertical). Most levels have one vial for level and one for plumb.

SL-02Check the level is accurate firstBefore you trust any level, test it. Put it on a flat surface like a worktop and note exactly where the bubble sits between the lines. Now turn the level end to end (flip it 180 degrees) and put it back in the same spot. If the bubble sits in exactly the same place both times, the level is accurate. If it moves, the level is out and will give you false readings.

SL-03Rest it fully on the surface, don't tilt itLay the level flat along the surface you're checking, with the whole underside touching. Don't let it rock on a bump or hang off the edge. If you're checking something vertical, like a shelf bracket, hold the level flush against the face with light, even pressure top and bottom.

SL-04Read the bubble straight onGet your eye level with the vial, not looking down at an angle. Looking from an angle makes the bubble appear off-centre even when it isn't, this is the mistake almost everyone makes at first. The surface is level or plumb when the bubble sits exactly between the two lines, not just roughly near them.

SL-05Check twice from different anglesFor anything that matters, like a shelf, check level along the length, then turn the level 90 degrees and check level across it too. A shelf can look level from the front and still slope side to side. Also check it in a second spot along the same line, small dips in a surface can trick you if you only check once.

SL-06Confirm you've got it rightYou've done it correctly if the bubble sits centred between the lines when viewed straight on, in more than one position along the surface, and the level itself passed the flip test in step SL-02. If any of those don't line up, don't trust the reading, fix the surface or re-check the level.

SL-07About 'changing the bit or blade'A standard spirit level has no bit or blade to change, it's a solid bar with sealed vials, there's nothing to swap out. If you're using a digital or laser level instead, the only thing you'll ever replace is the battery, following the maker's instructions for that model. If a vial cracks or the bubble leaks, the level itself needs replacing, it can't be repaired.

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