Universal testing machine calibration: how often, to which standards, and how
A universal testing machine applies controlled tension or compression forces to material specimens to measure properties such as tensile strength, yield strength, and elongation. Because every strength result depends directly on the accuracy of the force-measuring system, periodic force verification against traceable force-proving instruments is required for valid test data and accreditation.
Also known as: UTM, tensile testing machine, tensile tester, compression testing machine, materials testing machine
How often should a universal testing machine be calibrated?
Reverify immediately after relocation of the machine, after repair or adjustment of the force-measuring system, and after suspected overload events, regardless of elapsed calendar time.
Where this number comes from
ISO 7500-1 and ASTM E4 both recommend verifying the force-measuring system at intervals of no more than 12 months, unless machine-specific reasons justify otherwise, as summarized by accredited calibration provider ZwickRoell. Reverification is also required after the machine is relocated, repaired, or adjusted.
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
- Relocation of the machine triggers immediate reverification regardless of elapsed time
- Repairs or adjustments to the load cell, frame, or force indication electronics invalidate the current verification
- High test throughput and frequent operation near full capacity accelerate load cell drift
- Accidental overloads or specimen failures that shock-load the force transducer
- Accreditation requirements (for example an ISO/IEC 17025 scope) that fix the verification cycle
- As-found classification history: repeated results near the Class 1 error limits argue for a shorter cycle
Standards relevant to universal testing machine calibration
Primary international standard defining the force calibration and classification procedure for tension/compression testing machines
North American practice for force calibration and verification of static and quasi-static testing machines, with a plus/minus 1 percent accuracy requirement
Governs the calibration and classification of the reference force-proving instruments (load cells, proving rings, dynamometers) used to verify the machine
Standards are referenced by designation and title. For normative requirements, always work from the current edition of the standard itself.
How a universal testing machine is calibrated
A typical universal testing machine 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.
- Perform a general inspection of the machine and its force application accessories, checking alignment, grips, and indication system condition
- Select a force-proving instrument (reference load cell or proving ring) calibrated and classified per ISO 376, with a class appropriate to the target machine class
- Allow the reference instrument to reach thermal equilibrium and exercise the machine and transducer by preloading to maximum force
- Apply at least three series of increasing test forces at discrete points spanning the verified range, typically from 20 percent (or lower, down to 0.1 percent on wide-range systems) up to 100 percent of each range
- Record machine indication against the reference at every point, capturing as-found errors before any adjustment
- Calculate relative accuracy, repeatability, reversibility, and zero errors and compare against the class limits (for example Class 0.5 or Class 1)
- Adjust or repair the force indication system if errors exceed the required class, then repeat the verification to record as-left results
- Issue a calibration/verification certificate stating the verified ranges, class achieved, and the reference standards used with their traceability
Reference equipment typically used
- ISO 376 classified force-proving instrument (reference load cell, proving ring, or dynamometer)
- Precision indicator/readout for the reference transducer
- Calibrated thermometer to record ambient temperature during verification
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Sources
- ZwickRoell, "ISO 7500-1, ASTM E4 Load Cell Calibration" (accredited calibration service page)
The 12-month interval: states ISO 7500-1 and ASTM E4 recommend an interval of no more than 12 months unless otherwise specified, plus recalibration after relocation; also the measuring points from 0.1 to 100 percent of force range
- ISO 7500-1:2018, Metallic materials - Calibration and verification of static uniaxial testing machines - Part 1, International Organization for Standardization
Existence and scope of the governing calibration/verification standard for the force-measuring system
- ASTM E4, Standard Practices for Force Calibration and Verification of Testing Machines, ASTM International
Scope of force calibration and verification practice and the plus/minus 1 percent accuracy requirement for force-indicating systems
- ISO 376:2011, Metallic materials - Calibration of force-proving instruments used for the verification of uniaxial testing machines, International Organization for Standardization
Requirement that reference force-proving instruments used for machine verification are calibrated and classified per ISO 376