How do you tell if a level is accurate?

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How do you tell if a level is accurate?
I have one that shows one thing, and if I reverse it's position (flip it over), it's different. I quit using that one, and bought a new one. But how do I know that the new one is accurate? Just like anything, *new* does not mean it's good. Many products are defective when purchased. There are ways to test electrical items, and machines. Most hand and household tools can be tested by just looking at them and making sure they are not flawed, but how does a person test a level?
I think I have a fairly good "eye" when it comes to something being level of not, and I could see that they did not appear level with that old one I had, but when I flipped it, it appeared level to the eye. How that thing could measure level in one direction and not in the other is still a mystery to me. After all, it's just a bubble in some liquid. All I can figure is that the vial with the bubble was not lined up properly in the tools body....
Either way, I got tired of fighting with it, and just replaced it. I was not worth the hassle for a $12 level.
But is there any way to test them? How are they tested when they are manufactured?
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I just switch them one end to the other. If it is level (plumb when checking the vertical) in both directions, I assume it is correct.
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On Fri, 30 Oct 2015 16:45:35 -0400, "Ralph Mowery"

That' what I was doing with that old level, and what showed "level" in one direction was NOT level in th other direction. That's when I knew it was time to buy a new level.
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On Fri, 30 Oct 2015 14:01:06 -0700, Oren wrote:

the level could be straight as an arrow and still read wrong, dummy
sometimes i think youre half a bubble off
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snipped-for-privacy@none.com wrote:

They can leak. HTH
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On 10/30/2015 4:49 PM, snipped-for-privacy@none.com wrote:

Think about this a moment as you ask how to test a level:
If you KNOW it's not working since reversing it in the same plane results in different results, why then would you think the new level is NOT working if you reverse it in the same plane and get the exact same results either way.
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On Friday, October 30, 2015 at 1:42:34 PM UTC-7, Ralph Mowery wrote:

Since you already knew the answer, why did you ask? You don't even need to check both level and vertical, if it is off in one, it is also off in the the other.
Harry K
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wrote:

Not necessarily, as they are two different "levels" in one stick. Either one could be correctly installed. Either one could be incorrectly installed.
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On Saturday, October 31, 2015 at 4:17:29 PM UTC-7, snipped-for-privacy@snyder.on.ca wrote:

Yep, the devil is in the details and I forgot that one little detail :)
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wrote:

I don't think so. It could be off in one because the surface is not level. If it reads the same when reversed, the surface isn't. Only if it's also off in the other direction and by the same amount, to the extent one can tell, does one know the surface is level to the extent one can tell.

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On Fri, 30 Oct 2015 15:35:54 -0600, snipped-for-privacy@none.com wrote:

reads level both directions it IS accurate. There are no other possibilities.
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On 10/30/2015 06:53 PM, snipped-for-privacy@snyder.on.ca wrote:

+1
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On Fri, 30 Oct 2015 19:53:12 -0400, snipped-for-privacy@snyder.on.ca wrote:

If it reads differently end for end, on a true level surface, it is out of adjustment. If the bubble "centers" on a true level surface it is properly adjusted.
If you don't know if you have a true level surface and you don't know if the level is properly adjusted then you don't have any useful info no mater what the bubble does and you need to adjust it. This can be done even without a true level surface by observing where the bubble is for each of the two positions and adjusting the level so it is "off" the same amount for each.
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wrote:

If it reads differently end for end on *any* surface, level or not, it is out of adjustment.
If it reads the same end for end on *any* surface, level or not, it is properly adjusted.

That simply is not true; see above. If the bubble centers, and remains centered when the level is reversed end for end, you know two things: the surface is level, and the level is accurate. If the bubble does not center, and remains off center by the same distance in the same direction when the level is reversed end for end, you know two things: the level is accurate, and the surface is not level. If the bubble does not center, and remains off center by a different amount and/or in the opposite direction, you still know one thing: the level is not accurate.

First thing you've stated correctly -- and you apparently didn't notice that it contradicts everything else you wrote.
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Float it in the middle of a uniform density piece of wood in a sink. That's about as close to level as you can do/find.
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On Fri, 30 Oct 2015 15:35:54 -0600, snipped-for-privacy@none.com wrote:

I occasionally check against another level - or three. Don't you have more than one? I have 6-7 for different purposes. My favorite is one like this: http://tinyurl.com/np55u6b It's accurate enough for checking my other levels, but most of the time I just use this. It's good for sloping too, like with gutters.
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On Fri, 30 Oct 2015 15:35:54 -0600, snipped-for-privacy@none.com wrote:

That's how you know it's not accurate.

Same way. I would have checked it while in the store, before I bought it.

A $12 level is probably not adjustable, but some are. OTOH, this is not an adjustment that changes over time.

See your top line. Or they may have a known level surface, but I think the first method is better.
But you've only tested the horizonal glass of the level. Don't you have ones for vertical and 45^ too?
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snipped-for-privacy@none.com wrote:

i was going to say float it on a small puddle, but that would imply that the level itself is also manufactured in a balanced manner, which may not be the case, but the surface of a small puddle should be level...
another level that can be rigged up quickly if you have some clear tubing is to put some water in it and then hold the ends apart and upright and the water level of the two spots where the water are at should be level.
songbird
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On Sunday, November 1, 2015 at 7:25:21 AM UTC-8, songbird wrote:

Water levels are a wonderful thing but there are things that can throw them off.
Check the line to be sure there are no bubbles in it. I don't know why that would throw it off but the books say it does.
Be sure the temp of the water is the same through the length. Can be done by running fresh water through it just before taking readings.
Do not have one portion in shade and one in sun.
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On 2015-10-30 5:35 PM, snipped-for-privacy@none.com wrote:

and check the surface again. Any error should at least be equal in both directions.
--
Froz...

Quando omni flunkus, moritati
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