Calibrating a hardness tester: Requirements and procedure
A Hardness Tester It should be calibrated regularly to ensure that measurement results remain reliable, reproducible, and comparable. Calibration involves checking and documenting, whether the device operates within the required deviations. Calibration of a hardness tester is particularly important in quality assurance, goods receipt, manufacturing and laboratory, because testing decisions depend directly on the measured hardness values.
the essentials in brief
- The calibration of a hardness tester serves to... To identify and document measurement deviations.
- Depending on the procedure, among other things, Test force, test specimen, measuring system and device function checked.
- For Brinell, Vickers, Button and Rockwell be valid own standard series with requirements for verification and calibration.
- Direct and indirect checks fulfill different tasks in the calibration process.
- The Calibration interval depends on usage, standard requirements, test task, customer (IATF) and internal specifications.
Why does a hardness tester need to be calibrated?
Hardness testers operate with defined test loads, indenters, and measuring systems. Even small deviations can lead to incorrect hardness values. Causes include wear, damaged indenters, altered device settings, or mechanical influences.
Calibration verifies that the hardness tester continues to meet the required accuracy. It creates confidence in the measurement results, fulfills requirements from standards, audits and customer specifications (e.g. IATF 16949) and helps to avoid incorrect decisions in production as well as costly complaints.
What is tested during the calibration of a hardness tester?
The exact scope depends on the testing method and the type of equipment. Basically, it's about checking the key factors influencing the hardness test and documenting the results in a traceable manner.
Typical checkpoints include:
- Inspection personnel
- Preliminary test force (e.g. Rockwell)
- Indenters (damage, validity of indenter test certificate)
- Test cycle / force application times
- Measuring device (optical or depth measuring system)
- Repeatability
- Display error
- Hardness comparison plates (indirect calibration)
- Installation conditions (e.g. temperature, vibration, …)
- Documentation and traceability of measurement results
- Compliance with ISO/ASTM standards of the respective hardness scale
For example, the ISO 6507-2 standard governs the calibration of Vickers hardness testers. It covers the verification of the testing machine, test force, indenter, and optical measuring system. The standard describes both the direct calibration of individual components and the indirect calibration using certified hardness reference blocks.
DAkkS calibration, factory calibration, adjustment and maintenance: What is the difference?
The terms are often used interchangeably in everyday language, but they describe different qualifications and measures.
| Concept | Significance |
|---|---|
| DAkkS calibration | Calibration in accordance with standards (here DAkkS) is performed by a calibration laboratory accredited according to ISO 17025. During this process, the measurement deviations of a device are determined under defined conditions, documented, and traced back to national standards. A quality grade is also assigned. |
| factory calibration | Simplified and usually less expensive calibration with less documentation. Generally, no quality grade is assigned. The quality can be high if the provider is accredited according to ISO 17025. However, since the term is not legally protected, quality can vary significantly between providers. |
| adjustment | Setting up a device to correct detected deviations and restore accuracy. |
| Maintenance | Measures to maintain functionality, e.g. cleaning, testing, lubrication or replacement of wear-related components. |
| repair | Measures to restore functionality after failure, damage or wear. |
Calibration does not automatically mean that the device is modified. Adjustment or repair only occurs if a deviation needs to be corrected.
How often should a hardness tester be calibrated?
There is no standard calibration interval for every hardness tester. The interval should be based on usage, the testing procedure, internal requirements for test equipment monitoring, and specifications from quality assurance or customer requirements.
Many companies use an annual calibration interval. DAkkS prohibits specifying a calibration interval of more than 12 months. Additional calibration may be advisable:
- after repairs or service work
- after a change of location
- in case of unusual or fluctuating readings
- after mechanical overload
- in the case of new testing tasks or changed testing procedures
- before important audits or approvals
It is crucial that the interval is justified, documented, and verifiably determined within the framework of test equipment monitoring.
Which standards are relevant for calibration?
The calibration requirements depend on the Hardness testing method The respective series of standards cover not only the execution of the test, but also the verification of testing machines and reference bodies.
DIN EN ISO 6506-2 It covers the verification and calibration of testing machines for Brinell hardness testing.
DIN EN ISO 6507-2 describes the verification and calibration of testing machines and diagonal measuring systems for Vickers hardness testing.
DIN EN ISO 6508-2 It covers the verification and calibration of testing machines and indenters for Rockwell hardness testing.
DIN EN ISO 4545-2 It covers the verification and calibration of testing machines and indenters for the Knoop hardness test.
ISO / IEC 17025 This standard is relevant for testing and calibration laboratories. It describes requirements for the competence, impartiality, and uniform working methods of laboratories.
For practical application, it is important that the hardness tester is suitable for the respective procedure and that the calibration is tailored to the test areas actually used.
Direct and indirect verification explained briefly
When calibrating hardness testing devices, a distinction is often made between direct and indirect verification.
The direct review It examines individual technical parameters of the device. These include, for example, test force, indenter, measuring system, or travel distance. It shows whether individual components meet the required specifications.
The indirect verification This is done via comparative measurements with certified hardness reference plates. This checks whether the hardness tester delivers plausible and standard-compliant results in practical testing. For Brinell, ISO 6506-2 describes direct and indirect verifications of testing machines and also specifies when these verifications must be carried out.
When is calibration particularly important?
Regular calibration is particularly important whenever hardness values serve as a basis for decision-making. This applies, for example, to material approvals, supplier verifications, complaints, or test reports for customers.
Calibration is also particularly important:
- before audits or certifications
- in tests requiring documentation
- after repairs or interventions on the device
- after a change of location
- in case of unusual measurement results
- during high utilization of the hardness tester
- for safety-relevant components
- in the case of new materials or changed testing requirements
A missing or unclear calibration can lead to measurement results no longer being reliably evaluated afterwards.
What should be included in the calibration certificate?
A calibration certificate documents the conditions under which the hardness tester was tested and the results obtained. The exact wording depends on the procedure, the laboratory, and the scope of the calibration.
Typical details include:
- Identification of the hardness tester
- applied testing procedure
- used references or hardness comparison plates
- Measurement results and observed deviations
- Date of calibration
- Information on traceability
- Environmental conditions, if relevant
- Evaluation according to the applied requirements, if agreed.
The calibration certificate should not only be filed but also professionally evaluated. Only then will it be clear whether the hardness tester is still suitable for its intended testing tasks.
Common errors in the calibration of hardness testing equipment
In practice, problems often arise not from the calibration itself, but from unclear procedures in test equipment monitoring. Typical errors include:
- The calibration interval is not specified or documented.
- The device will not be checked again after repair or relocation.
- The test range used does not match the actual application.
- Hardness comparison plates are unsuitable or no longer valid.
- The penetrator is damaged or worn out.
- Calibration certificates are filed, but not evaluated.
- Regular internal control measurements are lacking.
Proper monitoring of testing equipment reduces these risks and ensures that deviations are detected early.
Consulting on calibration and test equipment monitoring
The calibration of a hardness tester should be tailored to the testing task, the procedure, and the internal quality assurance system. Crucially, it's not just the certificate that matters, but whether the device delivers reliable results within the used force range and with the specific procedure.
Frequently asked questions about calibrating a hardness tester
Why does a hardness tester need to be calibrated?
A hardness tester must be calibrated to ensure that measurement results remain reproducible, comparable, and verifiable. This is particularly important for quality assurance, test reports, audits, and customer requirements.
How often should a hardness tester be calibrated?
The calibration interval depends on usage, testing procedures, internal specifications, and quality assurance requirements. Many companies use an annual interval. Additional calibrations may be advisable after repairs, relocations, or if unusual readings are detected.
What is checked during calibration?
Depending on the method, the test force, indenter, measuring system (optical or indentation measuring system), device function, and comparative measurements with hardness reference plates are among the parameters checked. The exact scope depends on the standard, device type, and test task.
What is the difference between calibration and adjustment?
During calibration, a deviation is detected and documented. During adjustment, the device is set to correct a detected deviation.
Which standard applies to the calibration of hardness testing equipment?
That depends on the method. For Brinell, DIN EN ISO 6506-2 is relevant, for Vickers DIN EN ISO 6507-2, for Knoop DIN EN ISO 4545-2, and for Rockwell DIN EN ISO 6508-2.
