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Checking and Using a Spirit Level
The spirit level is an essential tool for building construction. Rest assured, I know, because I have lost count of the number of times I have held this tool in my hand against posts, walls, floors, shelves etc...and stared at the bubble.
The other reason you can take my word for it: my day job is carpentry (yes, I'm actually licensed) though that hasn't stopped jokers refer to me as impersonating a carpenter for years!
On building sites a level has been known also as the "spiritual level" (to add a bit of serious fun to the day and remind us that this life is a spiritual journey where we must strive to do what's right - otherwise things will fall down!
Also, you might hear trades also call it a straight edge or plumb stick. The latter term, rather crude, because it is a highly accurate and straight instrument that has nothing in common with a stick which is usually crooked!
"Plumb" is a building term which means something is vertical or more specifically it means the thing points to the center of the earth and is in perfect equilibrium leaning neither forward nor back nor left nor right!
How about that?
Level as Straight Edge
Levels are also used to check how straight or bowed something is. All one is doing here is putting the level against the surface in question and seeing at which points the level is touching.
If, for example, either end of a level is touching a wall, but not the middle, you are usually looking at a bowed wall or stud.
If either end of the level is not touching, the bow is in the opposite direction (bulging instead of hollow).
It is at this point, I should mention a common mistake with using a level.
If the piece you are holding the level against is bulging out or curved even slightly watch out! You need to be careful because you might not be getting a true reading depending on which part of the curve or bulge you are placing the level against.
Always use the longest level you can to minimize the chances of making this common mistake.
For example, you might think you are looking at a stud out of plumb but it may actually only be bulging and your undersized level is resting on the upper or lower end of the curve not traversing the full length.
So if the level is touching for the full extent of the work, sweet! The work is straight and the reading will be accurate!
Checking a Level
Before you use or buy a spirit level, check to see it is accurate.
Ideally if you are buying one get the best you can afford. You'll notice the good ones have a very generous bubble between the lines on the gauge.
This means you don't need to estimate the gap is equal either side of the bubble. In other words, when level or plumb, the bubble should be touching or just about touching the lines on either side of it.
To see if the level is accurate (you can also check this if you ever accidentally drop it Aaargh!).
To check for Plumb: Put it against a wall then flip it 180 degrees so the opposite side is now touching the wall. If the reading remains the same as when you first had it against the wall, everything is sweet.
The bubble position 3 (see diagram)- should remain the same whether side 1 or side 2 is placed against the wall.
If not, uh oh. I'm afraid your level is no longer accurate.
To check for level - the same principle as for plumb applies except now you will be checking the middle bubble so use a table or shelf instead. You still need to flip it 180 degrees and check if the reading has remained the same.