Durometer calibration: how often, to which standards, and how
A durometer measures the indentation hardness of rubber, elastomers, and plastics on Shore scales such as A and D by pressing a spring-loaded indenter into the material. Spring force, indenter geometry, and indicator accuracy all drift with use and mishandling, so periodic calibration against force and displacement standards plus checks on reference blocks are needed for reliable Shore readings.
Also known as: Shore durometer, Shore hardness tester, durometer hardness gage, Shore A durometer, Shore D durometer
How often should a durometer be calibrated?
Spot-check readings on certified reference blocks before critical measurement campaigns, and recalibrate immediately after any drop, impact, or repair or when readings become inconsistent.
Where this number comes from
No interval is mandated in the test method standards, so this is a manufacturer recommendation: Rex Gauge, a major durometer manufacturer, states the recommended calibration interval for all its durometers and test block kits is 1 year. Heavily used instruments are commonly calibrated more often.
Calibration intervals are a risk-based decision for the instrument owner, not a fixed rule: guidance documents such as ILAC-G24 and OIML D 10 describe how to set and adjust them from usage, criticality and calibration history. Treat the interval above as a starting point for your own quality system, not a compliance requirement.
What shortens or lengthens the interval
- Frequency of use: production-line durometers used many times per day drift faster than lab units used occasionally
- Drops and impacts: a dropped handheld durometer can bend the indenter or shift the spring and needs immediate recalibration
- Deviations observed during routine checks on certified reference blocks justify shortening the cycle
- Indenter wear from testing abrasive or filled compounds changes geometry and readings
- Test block kits used for routine checks also age and need their own annual recalibration
Standards relevant to durometer calibration
Primary test method defining durometer types (A, D, and others), apparatus requirements, and spring force calibration requirements
International Shore hardness method for A, D, AO, and AM scales, defining the instrument requirements the calibration must satisfy
Dedicated calibration and verification procedure for durometers of types A, D, AO, and AM and IRHD instruments
Standards are referenced by designation and title. For normative requirements, always work from the current edition of the standard itself.
How a durometer is calibrated
A typical durometer calibration, in an accredited lab or in-house, follows this outline. The exact points, tolerances and paperwork come from the applicable standard and your own procedure.
- Inspect the indenter under magnification and verify its geometry (cone angle, tip radius or flat, and extension) against the dimensional tolerances of ASTM D2240 or ISO 48-4
- Verify the indicator zero: with the indenter fully extended and unloaded the display should read 0
- Verify full scale: press the presser foot onto a flat glass plate so the indenter is fully depressed and confirm the reading is 100 within tolerance
- Calibrate the spring force at several indenter displacements using a calibrated balance or force measurement fixture, comparing measured force to the linear force-versus-reading relationship required by the standard
- Record as-found values, adjust the spring or indicator if any point is out of tolerance, and record as-left values
- Verify overall performance on certified rubber reference blocks spanning the working range and confirm readings fall within the block tolerance
- Issue a calibration certificate documenting indenter geometry, spring force results, display checks, and traceability of the reference equipment
Reference equipment typically used
- Durometer calibrator or calibrated balance/force fixture for spring force measurement
- Certified rubber reference test blocks for the relevant Shore scale
- Flat glass plate for full-scale (100) verification
- Optical measuring equipment for indenter geometry verification
Tracking durometer calibrations in a spreadsheet?
Gaugelog is calibration management software for quality managers who’ve outgrown Excel: instrument register, schedules, due-date alerts and certificates in one place. It launches in 2026. Until then, you can generate a clean calibration certificate PDF with our free tool, no account needed.
Sources
- Rex Gauge Company, "Service / Calibration" (manufacturer calibration page)
The interval: 'The recommended calibration interval for all Rex Gauge durometers and durometer test block kits is 1 year.'
- ISO 18898:2016, Rubber - Calibration and verification of hardness testers, International Organization for Standardization
Existence of a dedicated calibration and verification procedure covering type A, D, AO, and AM durometers
- ISO 48-4:2018, Rubber, vulcanized or thermoplastic - Determination of hardness - Part 4: Indentation hardness by durometer method (Shore hardness), International Organization for Standardization
Shore hardness scales covered (A, D, AO, AM) and instrument requirements
- ASTM D2240, Standard Test Method for Rubber Property - Durometer Hardness, ASTM International
The governing US test method for durometer hardness and its apparatus requirements