Vernier Caliper and Micrometer Screw Gauge

For lengths too small for a standard ruler, vernier calipers and micrometer screw gauges provide greater precision.

Vernier caliper

Precision: 0.01 cm (0.1 mm)

Range: typically 0 to 15 cm

How to read:

1. Read the main scale to the last whole division before the zero of the vernier scale.
2. Find which vernier scale line aligns exactly with a main scale line.
3. Add this vernier reading (in 0.01 cm) to the main scale reading.

Example: Main scale reads 3.4 cm. The 7th vernier line aligns. Reading = 3.47 cm.

Micrometer screw gauge

Precision: 0.001 cm (0.01 mm)

Range: typically 0 to 25 mm

How to read:

1. Read the main scale (thimble edge position) to 0.5 mm.
2. Read the thimble scale (which line aligns with the datum line on the sleeve) in 0.01 mm.
3. Add the two readings.

Example: Main scale reads 5.5 mm. Thimble reads 37. Reading = 5.87 mm.

Zero error

Both instruments can have a zero error (reading is not zero when jaws are closed). Check before measuring and correct the reading.

• Positive zero error: subtract the error.
• Negative zero error: add the magnitude of the error.

Common errors and how to correct them

• Not checking for zero error before starting.
• Using too much force with the micrometer (use the ratchet to avoid over-tightening).
• Reading the wrong scale line on the vernier.

How to apply this in an exam

State the precision of the instrument. Show the main scale and vernier/thimble readings separately, then add them. Correct for any zero error.