The word micrometer has two meanings: it is a unit of length equal to one thousandth of a millimetre (µm), and it is the name of a measuring instrument that reads thicknesses and diameters to hundredths of a millimetre.
As a unit: 1 micrometer = 0.001 mm = 0.000001 m. On site you meet it in the dry film thickness of paints and coatings, in the particle size of fillers, and in the fineness of binders. A paint layer is specified in microns of dry film thickness, and silica fume has particles of roughly 0.1 µm.
As an instrument: the micrometer, or micrometer screw gauge, works on a fine screw thread. One full turn of the thimble moves the spindle a fixed, small distance, and the reading is taken off the scale.
Working with a micrometer gauge
- Measuring range. An external micrometer normally covers 25 mm: 0-25 mm, 25-50 mm, 50-75 mm and so on. Covering a wide range means owning a set
- Resolution. Standard instruments read to 0.01 mm; versions with a vernier scale or digital display reach 0.001 mm
- Measuring force. Always close the gauge using the ratchet or friction thimble, never the thimble itself. That clamps the workpiece with the same force every time and makes readings repeatable
- Zeroing. On a 0-25 mm gauge, check that the scale reads zero when the anvils touch. A shifted zero puts the same error into every measurement
- Temperature. Steel expands. Do not hold the instrument in your hand for minutes before a precision reading, and work at roughly room temperature
Use in construction
For everyday building work a vernier caliper is normally enough: it resolves to 0.1 or 0.05 mm, which is ample for joinery, reinforcing bar and pipework. The micrometer comes in where it genuinely matters — setting out ironmongery, checking sheet thickness in zinc and lead, measuring wear on tooling, and in metalwork and machine shops generally.
| Vernier caliper | Resolves to 0.1 or 0.05 mm; ample for joinery, reinforcing bar and pipework. |
|---|---|
| Micrometer (standard) | Reads to 0.01 mm. |
| Micrometer (vernier scale or digital display) | Reaches 0.001 mm. |
Coating inspection uses a related instrument: the film thickness gauge, which measures dry film thickness in microns without damaging the coating.
Related terms
- Vernier caliper
- Tolerance
- Setting out
- Folding rule
- Silica fume
- Dry film thickness
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