Quick Answer
Hardness tester calibration uses NABL-certified reference hardness blocks (test blocks) to verify the tester's accuracy across 3 hardness scales. The calibration checks that the tester measures within ±2% of the certified value on each reference block, per ISO 6508-2, ISO 6507-2, and ISO 6506-2.
Key Takeaways
- Hardness tester calibration uses certified reference hardness blocks as working standards.
- Calibration per ISO 6508-2 (Rockwell), ISO 6507-2 (Vickers), ISO 6506-2 (Brinell).
- Reference hardness blocks are the key consumable — must be NABL-certified and renewed periodically.
- Indenter (diamond or ball) condition must be verified as part of calibration.
- Prism calibrates Rockwell, Brinell, Vickers, Knoop, and Shore hardness testers.
Hardness Testing Methods and Their Calibration
| Method | Indenter | Load Range | Application | Standard |
|---|---|---|---|---|
| Rockwell (HRC, HRB, HRA) | Diamond cone / steel ball | 60–150 kgf | Steel, cast iron, alloys | ISO 6508-2 |
| Vickers (HV) | Diamond pyramid | 1 gf – 100 kgf | All metals, coatings, case depth | ISO 6507-2 |
| Brinell (HBW) | Tungsten carbide ball | 1–3000 kgf | Rough surfaces, castings, forgings | ISO 6506-2 |
| Knoop (HK) | Diamond pyramid (elongated) | 1–1000 gf | Thin coatings, brittle materials | ASTM E384 |
| Shore A/D (Durometer) | Truncated cone / sharp cone | Spring-loaded | Rubber, elastomers, soft plastics | ISO 7619-1 |
Hardness Tester Calibration Procedure
Indenter Inspection
Verify diamond cone geometry (120° cone angle, 0.2 mm tip radius for Rockwell) using calibrated optical microscope. Chipped or worn indenters must be replaced before calibration.
Load Verification
Verify test force using calibrated precision force gauge. Rockwell HRC load = 150 kgf ± 0.3 kgf.
Reference Block Test
Make 5 indentations on NABL-certified reference hardness block at each hardness scale. Calculate mean and standard deviation.
Error Calculation
Compare mean measured hardness with certified value on the reference block. Error must be within ISO tolerance.
Repeatability Check
Standard deviation of 5 indentations must be within reproducibility limit per ISO 6508-2.
NABL Certificate
Issue NABL calibration certificate with hardness test results, expanded uncertainty, and reference block certificate number.
Gujarat Industries Requiring Hardness Tester Calibration
- Automotive component manufacturers (gear, shaft, bearing hardness verification)
- Tool and die makers (HRC hardness of tool steel, dies, molds)
- Steel rolling mills (hardness QC per IS/ASTM standards)
- Casting and forging companies (Brinell hardness per IS 1500)
- Rubber and elastomer manufacturers (Shore A/D durometer)
- Heat treatment shops (case depth verification via microhardness)
Frequently Asked Questions
How are hardness testers calibrated if they require a reference block?
The reference hardness blocks (e.g., HRC 60 block, HRC 30 block) are themselves NABL-calibrated at a higher-level reference laboratory. The blocks carry NABL certificates with certified hardness values and uncertainty. These blocks then serve as working standards to calibrate hardness testers.
How often should hardness testers be calibrated?
Annual indirect calibration (using reference blocks) per ISO 6508-2 is required. Daily verification with a reference block check is good practice for production hardness testers.
Can Prism calibrate portable hardness testers (Leeb/Equotip type)?
Yes. Prism calibrates portable Leeb hardness testers (Equotip, TIME, Proceq types) using NABL-calibrated Leeb reference hardness blocks.
Does calibration of hardness tester require re-certification of reference blocks?
Yes. Reference hardness blocks have a limited service life — typically 5 years or after a defined number of indentations. After this, they must be re-certified by a NABL primary hardness laboratory.
What is the NABL uncertainty for Rockwell hardness calibration?
Prism's NABL accreditation allows Rockwell HRC calibration with expanded uncertainty of ±0.3 HRC (k=2, 95% confidence), which satisfies the requirements of ISO 6508-2.
Written by
Er. Parthiv Kinariwala
Managing Director · Prism Calibration Centre · NABL CC-2480 · Ahmedabad
Er. Parthiv Kinariwala founded Prism Calibration Centre in 2004 and has over 20 years of hands-on experience in calibration engineering, NABL accreditation, and industrial compliance. His team performs 10,000+ calibrations annually from the Vatva GIDC laboratory, serving 5000+ industries across Gujarat.
Prism Calibration Centre — Vatva GIDC, Ahmedabad
Prism Calibration Centre
F-101, Rudraksh Complex 2, Phase 3, GIDC Vatva, Near Jasoda Nagar Cross Road, Ahmedabad — 382445, Gujarat, India
Phone: +91 98245 26444
Email: info@prismcalibration.com
NABL: CC-2480 · ISO/IEC 17025:2017
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