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#### Yoyizit

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Find a flat surface.
Put different size drill bits or pieces of paper under one end of the level until it reads "level".
Exchange the level end for end.
If it still reads level, the level is good.

#### hayewe farm

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If the level is facing in exactly the same direction, the drill bit didn't move and is exactly the same distance from the end of the level when it is flipped around.

#### nap

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don't bother with the drill bit.

set the level down. Note where the bubble sets. spin it end for end setting it in the same place. Note where the bubble sets.

rotate top for bottom. note where bubble sets. (setting back in the same place)

spin end for end. note where bubble sets.( set in same place)

If the bubble sets in the same relative position to the line on one side or the other of the bubble (the same side of the bubble each time from the current perspective) the level is accurate.

You can do the same for the plumb bubble.

#### Yoyizit

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don't bother with the drill bit.

set the level down. Note where the bubble sets. spin it end for end setting it in the same place. Note where the bubble sets.

rotate top for bottom. note where bubble sets. (setting back in the same place)

spin end for end. note where bubble sets.( set in same place)

If the bubble sets in the same relative position to the line on one side or the other of the bubble (the same side of the bubble each time from the current perspective) the level is accurate.

You can do the same for the plumb bubble.
My only objection to this is that
the sensitivity of the level
to being out of level
is more when the bubble is between the lines (I think).
Because, the curvature of the tube is not circular, it's probably a parabola.

But I should definitely do the flipping over several different ways.

And with the drill bits you can measure how close you are to the 500 microinch per inch Grainger spec'n.

#### nap

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And with the drill bits you can measure how close you are to the 500 microinch per inch Grainger spec'n.
unless you have microscopic eyes and the bubble is exactly the same width as the distance between the lines, there is always error. If the bubble does not fill the line gap exactly, you will always be guessing how far out of level the level is setting. is the bubble 1/16" from the line; 1/32"; 1/64"; etc.?

Obviously one would be best served by utilizing as level a surface as is available for my tests but the comparison tests will prove the level against itself and unless you have a certified level to check it against, that is as close to checking the accuracy as you can get.

If the bubble locations do not match, there is some part of the level that is not true with the others. Now, if you want to find out where it is, be my guest but at that point, I toss the level. It will never be dependable unless you were to have it machined and calibrated. Not worth the cost unless it is a very large level.

#### Yoyizit

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unless you have microscopic eyes and the bubble is exactly the same width as the distance between the lines, there is always error. If the bubble does not fill the line gap exactly, you will always be guessing how far out of level the level is setting. is the bubble 1/16" from the line; 1/32"; 1/64"; etc.?

Obviously one would be best served by utilizing as level a surface as is available for my tests but the comparison tests will prove the level against itself and unless you have a certified level to check it against, that is as close to checking the accuracy as you can get.

If the bubble locations do not match, there is some part of the level that is not true with the others. Now, if you want to find out where it is, be my guest but at that point, I toss the level. It will never be dependable unless you were to have it machined and calibrated. Not worth the cost unless it is a very large level.
That's my point. I think:
based on the bubble being centered,
if it fails the drill bit test it is bad,
if it passes it is good.

The bits and shims provide you a reference surface.

You can further gauge how far the bubble moves vs. how level the level is.

As a sanity check I'll have to run some tests on this with my levels; 4", 9" and 24" plus the line level. The repeatability of the readings is one limit on the accuracy.

I guess if you have a perfectly uniform-density piece of wood or other uniform-density material that is floating, this is a perfectly level surface but you should still do the end-for-end flips.

#### hayewe farm

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Why not just nail one end of a piece of 1X to the wall put the level on it ,raise the 1X until level and nail the other end. now you can flip the level as many times as you want and even check against other levels.

#### Yoyizit

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Why not just nail one end of a piece of 1X to the wall put the level on it ,raise the 1X until level and nail the other end. now you can flip the level as many times as you want and even check against other levels.
A benchmark. Yes.

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