# How Does a Spirit Level Work?

#### beach

Hello all,

New to the forum here. Have a bunch of questions about things I see every day that I'd love to ask mechanical engineers. To start off, how does a spirit level work? (Specifically, why is the air bubble in the center when the level is horizontal?)

Thanks!

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#### phinds

Gold Member
Seems pretty straightforward ... air is lighter than water so if you confine the two, the air rises to the top. Then you make the tube curved and calibrate lines on the tube to show level when the rod is level.

EDIT: you could just as easily calibrate the lines to show 45 degrees or whatever you want.

#### D H

Staff Emeritus
The tube that contains the liquid (and the bubble) looks cylindrical, but it isn't. There's a slight concavity to it. If the level is horizontal, the top of the middle of the tube will be slightly higher than either end, so the bubble will go toward the middle. If the level is slightly tilted, the high point will be a bit off-center.

#### Studiot

Unfortunately, wikipedia is a less than reliable source here as it attributes the invention to Thevenot.

Spirit levels were one of the surveying instruments invented and used by the Roman 'agrimensores'.
Roman spirit levels were up to 30 feet long and used to set out the multi-arch structures they built across river valleys. They used water as the liquid.

#### jehan60188

Unfortunately, wikipedia is a less than reliable source here as it attributes the invention to Thevenot.

Spirit levels were one of the surveying instruments invented and used by the Roman 'agrimensores'.
Roman spirit levels were up to 30 feet long and used to set out the multi-arch structures they built across river valleys. They used water as the liquid.
OP asked how it works, not who invented it.

#### D H

Staff Emeritus
OP asked how it works, not who invented it.
And that wikipedia article doesn't say how it works. It instead hints at the answer with "These vials, common on most ordinary levels today, feature a slightly curved glass tube which is incompletely filled with a liquid, usually a colored spirit or alcohol, leaving a bubble in the tube."

Moreover, how does pasting a link to a wiki article help answer the question? The presumption you should make when answering questions is that the questioner has already looked at common resources such as wikipedia.

#### Studiot

I was only trying to add some interest value as I thought the question was already answered.

If beach would like more technical detail eg equations for the curvature of the vial etc I will happily oblige.

#### jehan60188

And that wikipedia article doesn't say how it works. It instead hints at the answer with "These vials, common on most ordinary levels today, feature a slightly curved glass tube which is incompletely filled with a liquid, usually a colored spirit or alcohol, leaving a bubble in the tube."

Moreover, how does pasting a link to a wiki article help answer the question? The presumption you should make when answering questions is that the questioner has already looked at common resources such as wikipedia.
OP asked an open ended question, giving no background as to how much independent research he/she has already done. Wikipedia provides a jumping off point for the acquisition of further information.

But that's neither here nor there. Let's focus on how a spirit level works.

hydrostatic laws say that if unobstructed a bubble of air (less dense than colored spirit) in a container full of spirit (the tube) will float upwards.
if the tube is level, the highest point will be at its center, and the bubble will settle in there. if not, it heads for higher ground (the high end of the tube)

"How Does a Spirit Level Work?"

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