Extensometer calibration: how often, to which standards, and how
An extensometer measures the extension (strain) of a specimen during uniaxial materials testing, feeding elongation data into tensile, creep, and fracture results. Calibration against traceable displacement standards assigns the accuracy class (ISO 9513 Class 0.2, 0.5, 1, 2 or ASTM E83 Class A to E) that test standards require, so out-of-calibration extensometers invalidate strain-based material properties.
Also known as: extensometer system, clip-on extensometer, video extensometer, COD gage, strain extensometer
How often should an extensometer be calibrated?
Calibrate before and after any single test expected to last longer than 18 months (ISO 9513), and reverify after repair, overextension, or relocation of the test system; verification should cover the gauge length and measuring range actually used, as installed.
Where this number comes from
ISO 9513:2012 states that under normal conditions calibration is recommended at intervals of approximately 12 months and that the interval shall not exceed 18 months, except for tests running longer than 18 months, where the system is calibrated before and after the test.
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
- Number of tests run and frequency of mounting/demounting, since clip-on knife edges and arms wear and shift with use
- Type of extensometer system: contacting clip-on units drift differently from video or laser non-contacting systems
- Class required by the test standards in use; tighter classes (ISO Class 0.2/0.5, ASTM Class A/B-1) warrant more frequent verification
- Long-term creep and stress relaxation programs, where ISO 9513 notes a three year interval based on practical experience with ISO 204 testing
- Any suspected mechanical damage, overextension beyond travel, or repair, which triggers immediate recalibration
Standards relevant to extensometer calibration
Primary calibration method and classification standard; it sets the recommended 12 month and maximum 18 month calibration interval.
US practice for calibration, verification, and class assignment (Class A, B-1, B-2, C, D, E) of extensometer systems.
Standards are referenced by designation and title. For normative requirements, always work from the current edition of the standard itself.
How an extensometer is calibrated
A typical extensometer 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 extensometer, knife edges or contact points, cabling, and conditioning electronics; allow thermal stabilization in the calibration environment
- Mount the extensometer on a calibration apparatus that applies known displacements traceable to national length standards, set to the gauge length of interest
- Set the nominal gauge length and verify it against the applicable tolerance for the target class
- Apply a series of known displacement increments over each measuring range used, in increasing and (where required) decreasing directions, recording extensometer readings as found
- Calculate relative error and fixed (absolute) error at each point and compare against the class limits of ISO 9513 (Class 0.2, 0.5, 1, 2) or ASTM E83 (Class A to E)
- Adjust or repair the system if it fails the intended class, then repeat the run and record as-left results
- Assign the classification per range and gauge length, estimate measurement uncertainty, and issue a certificate stating the classes achieved
Reference equipment typically used
- Extensometer calibration apparatus (precision displacement generator)
- Laser interferometer or traceable reference displacement transducer
- Gauge length setting fixtures
- Calibrated thermometer for ambient monitoring
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Sources
- ISO 9513:2012, Metallic materials - Calibration of extensometer systems used in uniaxial testing, ISO
Backs the interval claim directly: recommends calibration at intervals of approximately 12 months, not to exceed 18 months, with the before/after rule for longer tests and the three year note for long-term creep testing.
- ASTM E83, Standard Practice for Calibration, Verification, and Classification of Extensometer Systems, ASTM International
Backs the ASTM classification scheme and the calibration/verification procedure elements.
- NextGen Material Testing, "Understanding ASTM E83 and ISO 9513 Standards for Extensometers and COD Gages"
Backs the class systems (ISO number classes vs ASTM letter classes), relative and fixed error limits, and the requirement to verify as installed at the gauge length and range of interest.
Cite this data
Gaugelog Calibration Interval Reference, v1.0 (July 2026). 68 instrument types, 236 verified sources. Licensed CC BY 4.0.
Download as CSV or JSON. Intervals are typical starting points, not compliance requirements; every row cites its sources.
The interval on this page is one row of the dataset. Browse all 68 types on the calibration interval reference.