Precision in force measurement starts with the right standards. But there are quite a few of them, and it’s not always easy to keep track: ISO 376, DKD-R 3-3, ISO 7500—which one is particularly important for a specific application, and how do they differ?

One thing is certain: from internationally recognized calibration standards to industry-specific guidelines, each plays a crucial role in ensuring accuracy, traceability, and comparability. In addition, there are several other approaches that go even beyond conventional standards to achieve even lower measurement uncertainties. One example of this is GTM-RL-003, a calibration procedure developed by us at GTM.

Determining which standard or regulation is relevant and applicable for a specific application is no trivial decision. Whether the focus is on maximum precision, cost-effective calibration, or a standard specifically tailored to the application: understanding the differences makes the actual measurement process easier. And for this reason, we provide here a brief overview of the most important standards and their implications.

ISO 376: The international standard for the calibration of force-proving instruments

ISO 376 is the globally recognized standard for the calibration of force-proving instruments and has been in use for over 60 years in various versions (its origins date back even further). Originally developed—and still used today—to classify materials testing machines under static loading, it serves as the basis for calibration in many fields due to its high level of recognition.

ISO 376 covers:

• Repeatability: Two measurement series under identical conditions in the same mounting position—the result shows how repeatable the sensor is under unchanged conditions.
• Reproducibility: Measurements in three mounting positions, each rotated 120° around the measurement axis. This captures how the sensor responds to different mounting positions—a measure of its reproducibility in real-world applications.
• Reversibility  (hysteresis): Describes the difference in the output signal between rising and falling load—a measure of the sensor’s directional dependence.
• Regression deviation: The deviation between the actual sensor characteristic curve and the mathematical curve fit.
• Creep: The change in signal over time under a constant load—a standalone classification criterion since ISO 376:2011.

In particular, the reproducibility test takes into account the vectorial nature of the force. A calibration is valid for a maximum of 26 months. Earlier recalibration is required if the force transducer has been overloaded or repaired.

The standard is not only used for the calibration of force transducers but is also mandated by ISO 7500, which governs the calibration of materials testing machines. For example, a Class 0.5 testing machine must be calibrated using a Class 0.5 force transducer classified according to ISO 376.

ISO 7500: Verification and Calibration of Testing Machines

ISO 7500 is an international standard that is particularly important in materials testing machines. It ensures that materials testing machines measure the applied forces precisely and reliably. This procedure is highly recognized internationally.

ISO 7500 also stipulates that materials testing machines must be regularly inspected and calibrated to ensure the accuracy of force measurement. The standard recommends a calibration interval of no more than 12 months. Recalibration is also mandatory if the testing machine’s location changes or following repairs that could affect force measurement.

DKD-R 3-3: The German guideline for the calibration of force-proving instruments

For industrial users seeking simpler and more cost-effective calibration procedures, DKD-R 3-3 was developed by the German Calibration Service (DKD) in collaboration with the PTB. This guideline describes simplified calibration procedures compared to ISO 376.

However, there is one important point to note: Due to the simplification, the vector properties of the force may not be fully captured. Furthermore, a simplified procedure must not result in smaller measurement uncertainties. Therefore, DKD-R 3-3 requires that so-called a priori values—for example, from previous calibrations—be taken into account. Important: Force measuring devices calibrated according to DKD-R 3-3 do not meet the requirements for the calibration of materials testing machines.

GTM-RL-003: Manufacturer-specific calibration procedure for reference force transducers in force measurement systems

For applications requiring maximum accuracy and minimal measurement uncertainties, we offer our own in-house developed procedure: GTM-RL-003. This guideline was specifically developed for the calibration of reference force transducers used in force reference standard measurement systems or in interlaboratory comparisons.

Our procedure includes:

• Calibration in four or more mounting positions.
• Two or more measurement series per mounting position.

With GTM-RL-003, smaller measurement uncertainties can be achieved than with calibration according to ISO 376. This procedure is particularly suitable for national metrology institutes and other high-precision applications.

EURAMET Calibration Guide No. 4: Guide to Measurement Uncertainty in Force Measurement

The European guideline EURAMET Calibration Guide No. 4 (current version 3.0, as of February 2022) provides guidance on estimating measurement uncertainty in various areas of force measurement. It contains instructions for calibration, including the selection of suitable reference standards and the consideration of environmental conditions.

It is a valuable supplement for:

• Force transducer calibrations according to ISO 376
• Material testing machine calibrations according to ISO 7500
• Various types of force measurement equipment

The guide is particularly helpful for professionals who need to establish a measurement uncertainty budget for their applications. It complements existing standards and offers practical support in evaluating uncertainties. Furthermore, the guideline serves to harmonize calibration practices across Europe.

... and why are these standards so important?

That concludes our brief overview! Standards and guidelines such as ISO 376, DKD-R 3-3, or GTM-RL-003 are not merely technical standards—they build confidence and traceability in measurement in measurement accuracy and ensure global comparability. Whether in research, industry, or calibration: the right standard can make all the difference.

Author: Daniel Schwind, Technical Director of GTM Testing and Metrology GmbH