Levels and Plumb Bobs

Part of Precision Measurement

Making and using levels and plumb bobs to establish true horizontal and vertical references — essential for all construction, surveying, and machinery installation.

Why This Matters

Gravity provides a free, universal reference: straight down is always the same direction everywhere on Earth. A plumb line — any weight on a string — points straight down. A level surface is perpendicular to the plumb line direction. These two references (vertical and horizontal) are the foundation of all construction and most engineering.

Without accurate level and plumb references, walls lean, floors slope, machinery vibrates rather than runs smooth, and water systems fail to flow in the intended direction. A door frame that is out of plumb by 2 degrees will have a door that either swings open or swings shut on its own — a minor annoyance in modern construction, but a significant daily irritation and waste of energy in a community that uses those doors constantly for decades.

The tools required are among the simplest in engineering. A plumb bob is a weight on a string. A water level is a tube of water. A bubble level requires slightly more skill to make but can be fabricated with basic materials. All of them give greater precision in their domain than any amount of naked-eye estimation.

The Plumb Bob

A plumb bob is a pointed weight suspended on a fine string from a fixed point. When the string is still, it hangs perfectly vertical — this is the plumb line.

Making a plumb bob: Any dense object can serve: a piece of lead, a heavy bolt, a steel rod with a pointed end. The key requirements:

• Sufficient weight to keep the string taut (at least 100–200g; heavier is better outdoors in wind)
• A pointed lower end so you can project the plumb point to the ground surface precisely
• A hook or hole at the top for attaching the string

For a precision plumb bob: turn a cone of lead, cast iron, or brass. The cone should have its axis perfectly aligned with the attachment point — any eccentricity causes the bob to hang off-axis.

String: Use the finest available thread that is strong enough to support the weight without breaking. Silk thread is ideal (nearly invisible vibration). Linen thread is good. Avoid twisted cord — it causes the bob to rotate slowly under its own twist, making the plumb line circle rather than hang still.

Dampening: In a light breeze, the bob swings. Damp the swing by submerging the bob in a bucket of water — the water drag quickly stops the oscillation. For indoor work, no dampening is needed.

Applications:

• Setting a corner post vertical: hang the plumb from the post top and check that the string is within 2mm of the post base at all points along its height
• Establishing a vertical reference for laying out a wall
• Transferring a point from floor to ceiling (or vice versa) in a building
• Checking that a water wheel shaft is truly vertical

The Water Level

A water level uses the principle that water seeks its own level — connected columns of water always reach the same elevation. A tube of water with open ends at each location: when the water is still, both ends are at exactly the same height, regardless of the terrain between them.

Making a water level: A length of flexible tube (leather, gut, rubber if available) with a short glass or clear-bottomed cylinder at each end. The tube carries water between the two viewing cylinders. The water surface in each cylinder marks the level elevation.

For a basic version without clear cylinders: mark the outside of two opaque cylinders with the water level when the system is set up at a known reference elevation. Any measurement point where the water reaches the same mark is at the same elevation.

Length: A water level works over any distance the tube reaches — 5 meters, 50 meters. The longer the tube, the more useful it is for site surveys. However, longer tubes take more time to equilibrate (the water must stop sloshing before reading).

Bubble elimination: Air bubbles in the tube break the water column and give false readings. Fill the tube from one end, tilting the tube to work bubbles out as you fill. Check that no bubbles remain by holding both ends up to a light source.

Freezing: In cold weather, the water level freezes. Add a small amount of salt to lower the freezing point, or use a diluted alcohol-water mixture. Either reduces accuracy slightly as the solution density is not quite equal at both ends if temperature differs.

Applications:

• Establishing a level course around a building foundation
• Setting the level of a mill race channel (critical for mill design)
• Checking floor levelness over a long span
• Setting the height of a wall plate at equal elevation on all four walls before rafters are placed

The Spirit Level (Bubble Level)

A spirit level uses a curved vial of liquid in which a bubble moves to indicate level. The bubble rises to the highest point in the curved vial — which is the center when the vial is truly horizontal.

The spirit level reads faster than a water level and requires only one person to operate, but it requires more skill to make and must be calibrated regularly.

Making a spirit level vial: The vial is the most difficult part — it requires a slightly curved tube, sealed at both ends, with a small air bubble in an otherwise liquid-filled interior. Traditional vials were made from glass; viable alternatives:

• A curved piece of horn or thin bone, waterproofed and sealed with wax
• A tightly stoppered glass bottle with a slight curvature

The liquid should be slightly viscous for stability — a mixture of alcohol and water (mostly alcohol for low-temperature performance) is ideal. The bubble size determines sensitivity: a larger bubble is more visible but less sensitive; a smaller bubble is more sensitive but harder to see.

Radius of curvature: Determines sensitivity. For a vial with radius of curvature R, an inclination of angle θ (in radians) moves the bubble through an arc of length R × θ along the vial. For a 10mm bubble movement to indicate 1mm per meter of slope (0.057 degrees = 0.001 radians): R = 10mm ÷ 0.001 = 10,000mm = 10m. A very gentle curve.

Mounting: The vial is mounted in a wooden or metal body with reference edges on its bottom face. The bottom face must be accurately flat, and the vial must be adjusted so the bubble centers when the reference face is level.

Calibration: Reverse the level on a flat surface — if the bubble is not in the same position when reversed, the vial is not centered on the reference face. Adjust by shimming under the vial mounting screws until the bubble reads identically in both orientations.

The Builder’s A-Frame Level

Before bubble levels were common, builders used an A-frame level — a triangular frame with a plumb bob hanging from the apex. When the frame is level, the plumb string hangs exactly over a marked center point on the crossbar.

Construction: Cut two equal-length legs and a crossbar to form an isosceles triangle. The apex must be directly above the midpoint of the crossbar — check by construction, not estimation. Hang a plumb bob from the apex. Mark the plumb’s resting position on the crossbar with great precision — this is the center mark.

Use: Place the A-frame on a surface. If the plumb hangs over the center mark, the surface is level. Works over the length of the A-frame legs.

The A-frame level is easy to make, robust, requires no maintenance, and never needs calibration once constructed correctly. Its limitation is that it only works for surfaces as long as the frame’s base — typically 0.5 to 2 meters.