Digital Caliper Calibration: Operating Procedure and Readings Grid (NF EN ISO 13385-1 / ISO 17025)

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The digital caliper is the most common dimensional measuring instrument in the workshop. Its specific source of uncertainty—the Abbe error—fundamentally distinguishes it from the micrometer. This procedure covers its complete calibration according to NF EN ISO 13385-1 and ISO 17025 requirements.

1. Environmental Conditions

  • Temperature: 20°C ± 2°C — acclimatization 1 hour minimum
  • Humidity: < 60%
  • Work surface: granite surface plate or clean reference plane

2. Required Equipment

  • Grade 1 gauge blocks with valid certificate
  • Lint-free cloth + isopropyl alcohol
  • Cotton gloves

3. Control Points (NF EN ISO 13385-1) | MPE ±0.020 mm

Nominal Point Measurement 1 Measurement 2 Measurement 3 Average Deviation Status
10.000 mm 10.008 10.009 10.008 10.008 +0.008 ✅ COMPLIANT
50.000 mm 50.012 50.013 50.012 50.012 +0.012 ✅ COMPLIANT
100.000 mm 100.015 100.016 100.015 100.015 +0.015 ✅ COMPLIANT
150.000 mm 150.018 150.017 150.018 150.018 +0.018 ✅ COMPLIANT
200.000 mm 200.010 200.011 200.010 200.010 +0.010 ✅ COMPLIANT

4. Abbe Error

u_Abbe = L × sin(θ) / √3

5. Thermal Correction

u_therm = L × α × ΔT / √3 = 200 × 11.5×10⁻⁶ × 2 / √3 = 0.0027 mm

6. Uncertainty Budget

Source Value (k=1)
u_rep 0.0035 mm
u_Abbe 0.0058 mm
u_cale 0.0010 mm
u_therm 0.0027 mm
u_res 0.0029 mm
Expanded U (k=2) 0.015 mm

⚠️ ILAC-G8 at 150 mm point: |0.018| + 0.015 = 0.033 mm > MPE 0.020 mm → Gray zone — Case 2: NON-COMPLIANT.

7. ISO 17025 Compliance

  • §5.6: Gauge blocks with COFRAC traceable certificate
  • §7.6: Expanded uncertainty U declared on the test report
  • §7.8.6: ILAC-G8 rule applied
  • Frequency: 12 months (extendable to 24 months via OIML D10)

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