What is Spirit Level? | How To Use Bubble Levels

What is Spirit Level?

A spirit level often simply called a level or bubble level is a fundamental tool for checking whether a surface is truly horizontal (level) or perfectly vertical (plumb).

You’ll find it in the hands of carpenters, bricklayers, stonemasons, metalworkers, and surveyors basically, anyone who needs precision in building or aligning things. Even photographers and videographers sometimes rely on spirit levels to ensure their shots are straight.

What makes a spirit level work? The answer lies in its name: inside the tool, there’s a liquid traditionally mineral spirits which suspends a small bubble. When the bubble sits neatly between two marked lines, you know your surface is either level or plumb, depending on the orientation of the vial.

Good-quality spirit levels are impressively precise, often accurate to within plus or minus 0.5 millimeters per meter, or about 0.005 inches per inch (roughly 0.029 degrees).

The vials themselves are typically a yellow-green hue, not just for looks but because additives help shield them from UV light and allow them to work reliably in temperatures ranging from -20°F up to 130°F.

As for accuracy, some spirit levels are rated a bit less exact, like 0.75 mm per meter, or about 0.043 degrees still quite dependable for most jobs.

One thing that’s changed over the years is what the vials are made from and how they’re shaped. Older levels used glass, but most modern ones use tough acrylic for better durability. You’ll see vial shapes ranging from the familiar barrel to rectangular blocks, or even curved, banana-like tubes designed to help measure slope useful if you need to check the incline or pitch of a surface.

Construction

Early versions of tubular spirit levels featured glass vials that were subtly curved, each maintaining a consistent inner diameter at the points where users would check the level. These vials were partially filled with a liquid most often a colored spirit or alcohol so a bubble of air was left inside the tube.

The gentle upward curve of these vials wasn’t just for looks; it played a practical role. Thanks to this design, the bubble would always find its way to the very center the highest spot in the tube when the level was properly aligned.

Even the slightest tilt would cause the bubble to wander away from the marked center, making it easy for anyone to spot when a surface wasn’t quite level.

But not all leveling jobs are straightforward. In situations where a level might need to work upside-down or sideways, the classic curved vial just wouldn’t cut it.

That’s where the uncurved, barrel-shaped vial comes in. This alternative has a slightly wider diameter at the middle, which helps the bubble perform reliably, no matter how the tool is oriented.

As for the choice of liquid, alcohols like ethanol are typically preferred over water. There are a couple of good reasons for this. Alcohols have a lower viscosity and surface tension, so the bubble glides quickly and settles right where it needs to be, with hardly any sticking to the glass.

Plus, alcohols are less fussy about temperature changes they won’t freeze and crack the glass the way water might during a cold snap.

To make the bubble stand out, a splash of dye such as fluorescein (usually in yellow or green shades) is added to the mix. It’s a small detail, but it makes the bubble a lot easier to spot in various lighting conditions.

On a related note, there’s also the bull’s eye level a circular device with a flat base and a slightly domed glass face, marked with a central circle.

Unlike the linear tubular level, which only indicates level in one direction, the bull’s eye design lets you check for levelness across an entire plane. It’s a clever little tool, and a great example of how leveling technology has adapted to suit different needs.

Calibration

When it comes to checking whether a carpenter’s level is truly accurate, you don’t actually need a perfectly horizontal surface to get started. Here’s how it typically goes: you place the level on any flat, relatively level surface and just take a look at where the bubble lands in the tube.

That initial reading gives you an idea of how parallel your surface is to true horizontal—at least according to the tool you’re testing, even though you can’t be sure about its precision just yet.

Now, here’s the trick: without changing the spot, you flip the level around 180 degrees and check it again. If the level is properly calibrated, you should see the exact same bubble position both times. But if the bubble shifts, you’ve got proof that the level isn’t as accurate as you’d want.

To fix this, you don’t need fancy equipment—just a little patience. You keep rotating the level and, each time, make small adjustments to the bubble tube inside its housing, moving it so it corrects about half of the difference you’re seeing. You repeat this process until flipping the level doesn’t change the reading anymore.

Interestingly, this whole approach isn’t just for basic tools. If you’re working with more advanced equipment like a surveyor’s optical level or a theodolite, you’ll follow a very similar routine.

Every time you set up one of these instruments, you have to make sure that not only the instrument itself but also its rotational plane is perfectly level. This is done by carefully adjusting it in two directions that are at right angles to each other, along with checking the built-in spirit level.

How to Check a Spirit Level for Accuracy

To ensure your spirit level is providing reliable readings, it’s important to test its accuracy using a simple method. Start by placing the level on a flat surface something like a sturdy tabletop works well.

Next, make a small mark at one end of the level, and then another mark directly under the center vial; these reference points help keep everything aligned as you test.

Now, check where the bubble sits inside the vial and note its position. Once you’ve done this, pick up the level, rotate it so the ends are switched (a full 180 degrees), and carefully align it with your original marks. Take another look at the bubble.

If your level is accurate, the bubble should land in exactly the same spot as before. This quick check lets you spot any calibration issues right away.

If you also want to check the vertical vial, the process is nearly the same. Just use a flat, vertical surface like a door frame or wall.

Mark your spots, check the bubble, flip the level end-to-end, and compare readings. Consistency in the bubble’s position confirms your level is in good working order.

How to Read a Spirit Level

• Locate the bottom edge of the level. This rests against the surface you’re trying to level.
• Some models feature magnetic edges, which “stick” to metal surfaces for ease of use.
• Inspect the body of the level and check for grabbing points where you won’t block the vials with your hand(s).
• Most levels feature a punched hole on at least one end for hanging above your workbench.
• The tube vial in the center of the level helps you find the true horizontal.
• Tube vials on the ends finds the true vertical.
• On many torpedo levels and other specialized levels, there is an angled tube vial to find level at 45°.

How to Use a Spirit Level

• Clean the level, removing all buildup and dirt from the edges.
• Mark a line along the bottom edge on the wall.
• Flip the level over so that the bottom becomes the top. Put the new top edge along the marked line. If the bubble is centered, your level is accurate. If not, it is defective.
• Place the level on the surface of the object for which you want to find the true horizontal (the “horizon”). Make sure the spirit tube runs parallel to the object. Allow the bubble to float to the top of the spirit tube.
• Put your eyes at level with the spirit tube. In order to get an accurate reading, close one eye.
• Take note of where the bubble is inside the spirit tube. If it’s centered between the lines on the tube, your object is level. If the bubble is to the right of the lines, your object slopes downward right-to-left. If the bubble is to the left of the lines, your object slopes downward left-to-right.
• To find the true vertical or “plumb,” repeat the same process vertically.

How to Level a Scope with a Bubble Level

• Ensure your rifle is unloaded.
• Place the rifle in a vise, gun cradle, or any other holder keeping the rifle secure.
• Place a bubble level on a flat part of the gun. If you have a second spirit level, use it on another flat part, such as the action, above the chamber or on the rib.
• Tighten the rifle in place when you reach level.
• Place the scope in the rings.
• Tighten the scope snug, but loose enough to be able to shift it slightly.
• Place a bubble level on the scope and level the scope horizontally.
• Rotate the level parallel to the scope and level it vertically.
• Your scope is now level.

Types of Spirit Levels

There are different types of spirit levels for different uses:

• Surveyor’s leveling instrument
• Carpenter’s level (either wood, aluminum or composite materials)
• Mason’s level
• Torpedo level
• Post level
• Line level
• Engineer’s precision level
• Electronic level
• Inclinometer
• Slip or Skid Indicator
• Bull’s eye level

A spirit level is usually found on the head of combination squares.

1. Surveyor’s leveling instrument

In the world of surveying, you’ll often hear about instruments like the tilting level, dumpy level, or automatic level. These are all types of leveling instruments, and they play a crucial role when it comes to measuring height differences across larger stretches of land.

If you’ve ever seen a surveyor at work, you’ll notice their leveling instrument comes equipped with a spirit level fixed to a telescope usually offering about 30x magnification—and cross-hairs to help line up measurements accurately. This whole setup is perched on a sturdy tripod for stability.

The basic process works like this: the surveyor takes readings from two vertical rods, each marked with measurements—one placed behind and the other in front of the instrument’s position. By noting the height on each rod, it’s possible to figure out the difference in elevation between those two points on the ground.

Typically, the survey starts at a location where the elevation is already known. From there, the team works their way across the terrain, moving from one point to the next (often spaced around 100 meters, or 328 feet, apart). Each height difference gets added to the last, allowing surveyors to determine elevation changes over long distances.

And just to give you an idea of how accurate this can be: with precise leveling techniques, surveyors can measure the elevation difference between two points a full kilometer (about 0.62 miles) apart, and still get it right down to just a few millimeters.

2. Carpenter’s level

The classic carpenter’s spirit level looks pretty much like a sturdy, short plank—usually with a wider body for extra stability. This design helps ensure the tool sits securely while you’re measuring, so you get an accurate read every time. Right in the center, you’ll find a small window with a tube inside; that’s where the bubble sits.

If you look closely, there are two notches or rings marking where the bubble needs to settle for the surface to be perfectly level. Some carpenter’s levels even have an extra marking for checking a 45-degree angle, which is handy for certain jobs.

3. Line level

A line level is one of those clever little tools made specifically to hang on a builder’s string line. It has hooks built into its body so you can clip it onto the string and let it dangle. The main thing here is that it’s lightweight and compact—this way, it doesn’t sag the string or throw off your measurements.

When you hang it in the middle of the string, the string itself kind of becomes part of the level’s body, and each end of the string stretches out the level’s plane. It’s a simple but effective way to check if a line is true, especially over longer distances.

4. Engineer’s precision levels

Now, when you need accuracy that goes beyond what a regular spirit level can offer, you’d reach for an engineer’s precision level. These are used in situations where you can’t afford to be even a hair off like when you’re setting up the foundations or machine beds in a workshop.

The whole idea is to make sure that whatever you’re leveling, whether it’s a huge industrial machine or a critical piece of equipment, is perfectly aligned so it can work at its highest level of precision.

How Spirit Levels are Made

Spirit levels can be found crafted from all sorts of materials. You’ll see them made of aluminum for its light weight, plastic for its affordability, classic wood for that traditional touch, rugged cast iron, and even various composite materials for specialized needs.

When it comes to who actually uses spirit levels, there’s a pretty broad range. On the professional side, you have contractors and skilled tradespeople think carpenters, masons, framers, electricians, plumbers, and woodworkers who rely on spirit levels for precision in their daily work.

But it’s not just the pros. Plenty of DIY enthusiasts and homeowners reach for a spirit level when they need to hang a picture straight or tackle a home improvement project.

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