Durometers measure the hardness of elastomers, plastics, and other non-metallic materials using Shore scales (A, D, OO, and others). Calibration verifies the spring force, indenter geometry, and indicator accuracy against traceable standards. Accurate durometer readings are essential for rubber compound quality control and material specification compliance.
Inspect the durometer for damage, worn or bent indenter, and legible scale markings. Verify the presser foot is flat and undamaged. Check that the spring mechanism returns the indenter to the zero position when unloaded.
Using optical magnification or a certified indenter standard, verify the indenter tip geometry matches ASTM D2240 specifications. For Shore A, the indenter must be a truncated cone with 0.79 mm diameter flat tip. For Shore D, a sharp cone with 0.1 mm radius tip.
Using a calibrated force tester, verify the spring force at several dial positions (e.g., 0, 50, 100 scale points). Compare to the ASTM D2240 force-deflection specification for the applicable Shore scale.
Take five readings on each certified durometer test block, spacing impressions at least 12 mm apart. Calculate the average and compare to the certified block value. Test blocks should span the operating range (e.g., 30, 50, 70, 90 Shore A).
Compare the average reading on each test block to the certified value. The error at each point must be within the allowable tolerance per ASTM D2240.
Record all data including indenter condition, spring force results, test block readings, and errors. Issue the calibration certificate and apply the calibration label.
Per ASTM D2240, the durometer reading must agree with the certified test block value within ±1 Shore unit for well-controlled conditions. Spring force must match the specification within ±1% at each test point. Indenter geometry must meet dimensional tolerances per the standard.
12 months, with monthly test block verification
Shore A uses a blunt indenter with a lighter spring for softer materials (rubber, elastomers). Shore D uses a sharp indenter with a heavier spring for harder materials (hard plastics, rigid composites). Shore A scale covers soft rubber (20A) to hard rubber (90A). Shore D covers hard rubber (20D) to rigid plastic (80D).
ASTM D2240 requires a minimum sample thickness of 6 mm (0.24 in) for accurate readings. Thinner samples can be stacked, but readings on stacked samples may differ from solid specimens. The sample must be flat and at least 12 mm larger than the presser foot diameter.
Certified durometer test blocks typically have a 1-year calibration interval. Rubber test blocks can drift due to aging, temperature exposure, and UV degradation. Store them in a cool, dark location in their protective case. Replace blocks when they no longer calibrate within tolerance.
CalibrationOS tracks due dates, stores certificates, and generates audit-ready reports.
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