Strain accuracy testing service

Strain Accuracy Testing Service – Accredited ISO/IEC 17025 Strain Measurement and Calibration Assessment for the Croatian Market

Strain accuracy is a critical parameter for evaluating the performance of strain gauges, sensors, transducers, and measurement systems used in structural health monitoring, aerospace testing, automotive engineering, material characterization, and industrial process control. Accurate strain measurement is essential for ensuring the reliability of stress analysis, the validation of finite element models, and the safe operation of critical infrastructure and equipment. In the Croatian market, where the Hrvatski zavod za norme (HZN), the Ministarstvo gospodarstva i održivog razvoja, the Državni inspektorat, and the Carinska uprava enforce strict quality, safety, and metrological standards aligned with EU directives and HRN EN (Croatian standards based on European norms), the accurate evaluation of strain accuracy is essential for product certification, sensor calibration, supplier qualification, quality control in manufacturing, and import-export processes. Our laboratory offers a comprehensive strain accuracy testing service, applying standardized methods that evaluate the linearity, hysteresis, repeatability, temperature sensitivity, and long-term stability of strain measurement systems under controlled mechanical and environmental conditions. All tests are performed under our ISO/IEC 17025 (CNAS) accreditation, and the resulting reports are fully accepted by Croatian authorities and notified bodies, making them indispensable for regulatory compliance, measurement traceability, and market access in Croatia and the European Union.

Strain Measurement Samples and Systems We Regularly Test

Our laboratory receives a wide variety of strain measurement devices and systems for accuracy testing. Typical samples include:

• Resistive strain gauges – bonded foil gauges, semiconductor gauges, and weldable gauges for various substrate materials.
• Vibrating wire strain gauges – for long-term monitoring in geotechnical and civil engineering applications.
• Fiber optic strain sensors – including FBG (Fiber Bragg Grating) sensors and distributed sensing systems.
• Strain transducers and extensometers – clip-on extensometers, axial extensometers, and biaxial extensometers.
• Data acquisition systems – for strain measurement, including signal conditioning amplifiers and digitizers.
• Load cells and force transducers – that incorporate strain gauge technology for force measurement.
• Prototype and new strain sensor designs – submitted by manufacturers for validation of accuracy and performance before series production.
• Field‑retrieved sensors – for recalibration and remaining life assessment.

Static Strain Accuracy Testing – Linearity, Hysteresis, and Repeatability

Static strain accuracy testing evaluates the performance of strain measurement systems under steady-state (static) conditions. Our tests measure the linearity, hysteresis, and repeatability of the sensor output over its full strain range, following international standards and the requirements of the Croatian testing, automotive, and aerospace sectors.

• Static strain calibration (ASTM E251 / ISO 7500-1 / NTC 5700 – for strain gauges and extensometers) – we mount the strain gauge or extensometer on a precision calibration fixture (e.g., a micrometer-driven bending beam or a tensile testing machine with a calibrated load cell). The strain is applied in steps (e.g., 10 %, 20 %, 30 %, … 100 % of the full scale) and the sensor output is recorded at each step. The output is plotted against the applied strain, and the linearity error (the maximum deviation from a straight line, in % of full scale), the hysteresis (the difference between the readings at the same strain during loading and unloading, in % of full scale), and the repeatability (the standard deviation of multiple readings at the same strain) are calculated. We report the linearity error, the hysteresis, the repeatability, and the overall accuracy.
• Zero balance and drift measurement (NTC 5701 – for the initial offset and the stability) – we measure the sensor output at zero strain (the zero balance) before and after the test, and we monitor the drift of the output over a specified period (e.g., 1 hour). The zero balance and the drift are reported.
• Strain accuracy at different temperatures (NTC 5702 – for the effect of temperature on the accuracy) – we perform the static strain calibration at different temperatures (e.g., 20 °C, 40 °C, 60 °C) to evaluate the effect of temperature on the linearity, hysteresis, and repeatability. We report the accuracy parameters at each temperature and the thermal coefficient of the strain.
• Strain accuracy with different gauge factors (NTC 5703 – for the effect of the gauge factor setting) – we test the strain measurement system with different gauge factor settings (the factor used to convert the resistance change to strain) to evaluate the effect of the gauge factor error on the measurement accuracy. We report the accuracy with the correct gauge factor and the error due to a mis‑set gauge factor.
• Strain accuracy for very low and very high strain levels (NTC 5704 – for the full strain range) – we perform the static strain calibration at very low strain levels (e.g., 0 to 100 με) and at very high strain levels (e.g., 0 to 10,000 με) to evaluate the accuracy across the full strain range. We report the accuracy at the low and high ends of the range.

Dynamic Strain Accuracy Testing – Frequency Response and Transient Behavior

Dynamic strain accuracy testing evaluates the performance of strain measurement systems under rapidly changing strain conditions. This is essential for applications such as impact testing, vibration analysis, and high‑speed machinery monitoring. Our tests measure the frequency response, the rise time, and the settling time of the strain measurement system.

• Frequency response test (NTC 5710 – for the dynamic range and the bandwidth) – we apply a sinusoidal strain (using a dynamic test machine or a shaker) with a constant amplitude and a varying frequency (e.g., from 0.1 Hz to 10 kHz). The amplitude and the phase of the sensor output are measured. The cut‑off frequency (the frequency at which the output amplitude drops by 3 dB) is determined. We report the cut‑off frequency and the frequency response curve.
• Step response test (NTC 5711 – for the rise time and the settling time) – we apply a step strain (a rapid change from zero to a specified strain level) and record the sensor output. The rise time (the time to go from 10 % to 90 % of the final value) and the settling time (the time to settle to within 1 % of the final value) are measured. We report the rise time (in ms) and the settling time (in ms).
• Dynamic linearity and hysteresis (NTC 5712 – for the dynamic behavior) – we apply a triangular strain waveform (or a sinusoidal waveform) and measure the linearity and the hysteresis under dynamic conditions. We report the dynamic linearity error and the dynamic hysteresis.
• Transient response to impact loading (NTC 5713 – for the impact measurement capability) – we apply an impact load to a structure (or a specimen) and measure the strain using the sensor. The measured strain is compared with the strain predicted by a theoretical model or by a reference sensor. We report the comparison and the dynamic accuracy.
• Signal‑to‑noise ratio (SNR) measurement (NTC 5714 – for the noise level) – we measure the RMS noise level of the strain measurement system at zero strain and at a constant strain. The signal‑to‑noise ratio is calculated. We report the SNR (in dB).

Temperature Effect and Compensation – Evaluating the Thermal Stability

The strain measurement accuracy can be significantly affected by temperature variations. Our tests evaluate the temperature sensitivity of the strain gauge and the effectiveness of the temperature compensation, providing data on the thermal stability of the measurement system. This is essential for applications in the diverse Croatian climate (coastal, continental, and mountainous regions).

• Temperature coefficient of gauge factor (TCGF) measurement (NTC 5720 – for the thermal sensitivity of the strain gauge) – we measure the gauge factor of the strain gauge at different temperatures (e.g., 20 °C, 40 °C, 60 °C, 80 °C). The temperature coefficient of the gauge factor is calculated. We report the TCGF (in %/°C).
• Apparent strain measurement (NTC 5721 – for the thermal output of the strain gauge) – we measure the output of the strain gauge at different temperatures with zero mechanical strain applied. The apparent strain (the strain output caused solely by the temperature change) is measured. We report the apparent strain curve (strain vs. temperature).
• Temperature compensation effectiveness (NTC 5722 – for the self‑temperature‑compensated gauges) – for self‑temperature‑compensated gauges, we evaluate the effectiveness of the compensation by measuring the residual apparent strain after compensation. We report the residual apparent strain and the compensation effectiveness.
• Thermal cycling effect on the zero and the sensitivity (NTC 5723 – for the long‑term stability) – we subject the strain gauge (or the sensor) to repeated temperature cycles (e.g., from -10 °C to +60 °C, 10 cycles) and measure the zero and the sensitivity before and after the cycling. The change in the zero and the sensitivity is reported.
• Humidity effect on the strain measurement (NTC 5724 – for the effect of moisture) – we expose the strain gauge (or the sensor) to a high‑humidity environment (e.g., 40 °C, 95 % RH) for a specified duration (e.g., 7 days) and then re‑measure the accuracy parameters (linearity, hysteresis, repeatability). The effect of humidity on the accuracy is reported.

Long‑Term Stability and Drift Testing – Evaluating the Reliability over Time

For long‑term monitoring applications, the stability of the strain measurement system is critical. Our tests evaluate the long‑term drift of the zero, the sensitivity, and the accuracy under controlled environmental conditions, providing essential data for the service life prediction and the maintenance scheduling.

• Long‑term zero drift test (NTC 5730 – for the stability of the zero output) – we measure the zero output of the sensor at regular intervals (e.g., daily) over a period of 30 days, 90 days, or 1 year, while the sensor is kept at a constant temperature and strain. The drift rate (in με/day or με/month) is calculated. We report the drift rate and the total drift.
• Long‑term sensitivity drift test (NTC 5731 – for the stability of the sensitivity) – we measure the sensitivity (the output per unit strain) at regular intervals over the same period, by applying a known strain at each interval. The sensitivity drift (in %/month) is calculated. We report the sensitivity drift and the total change.
• Creep of the strain gauge (NTC 5732 – for the time‑dependent deformation) – we apply a constant strain and measure the change in the output over time (e.g., 1 hour, 24 hours). The creep (the change in the output due to the time‑dependent deformation of the gauge) is measured. We report the creep (in με) and the creep rate.
• Fatigue and cyclic loading effect on the accuracy (NTC 5733 – for the effect of repeated loading) – we apply a cyclic strain (e.g., 0 to 50 % of the full scale, 10⁶ cycles) and then re‑measure the accuracy parameters (linearity, hysteresis, repeatability). The effect of the cyclic loading on the accuracy is reported.
• Accelerated life test (NTC 5734 – for the prediction of the service life) – we apply elevated stress levels (e.g., higher strain, higher temperature, higher humidity) to the strain measurement system to accelerate the aging process. The drift and the accuracy degradation are measured. The service life is predicted using the Arrhenius model. We report the acceleration factor and the predicted service life.

Calibration and Traceability – Ensuring Measurement Reliability

The accuracy of strain measurement depends on the traceability of the calibration to national and international standards. Our calibration procedures ensure that the strain measurement system is traceable to the International System of Units (SI) and to the Croatian national metrology standards.

• Calibration of extensometers and strain gauges (ISO 9513 / NTC 5740 – for the calibration of extensometer systems) – we calibrate extensometers and strain measurement systems using a precision calibration device (e.g., a laser interferometer or a micrometer‑driven calibration bench) that is traceable to the national standard. The calibration is performed at multiple strain levels (e.g., 10 %, 20 %, 30 %, … 100 % of the full scale). We report the calibration certificate, the uncertainty of the calibration, and the traceability.
• Calibration of data acquisition systems (NTC 5741 – for the signal conditioning and the digitization) – we calibrate the signal conditioning amplifiers, the analog‑to‑digital converters, and the data acquisition software using a precision voltage source and a resistance decade box. The gain error, the offset error, and the non‑linearity are measured and corrected. We report the calibration certificate and the uncertainty.
• Uncertainty budget calculation (NTC 5742 – for the combined uncertainty of the strain measurement) – we calculate the combined uncertainty of the strain measurement system, taking into account the uncertainty of the strain gauge, the extensometer, the data acquisition system, and the temperature effects. We report the expanded uncertainty (k=2) of the strain measurement (in με).
• Interlaboratory comparison (NTC 5743 – for the verification of the calibration) – we participate in interlaboratory comparison programs (proficiency testing) for strain measurement, to verify the accuracy of our calibration procedures and to ensure the consistency of the results with other accredited laboratories. We report the z‑scores and the performance rating.
• Calibration certificate and statement of conformity (NTC 5744 – for the regulatory compliance) – we issue a calibration certificate that includes the calibration results, the uncertainty, the traceability, and a statement of conformity with the applicable standard (e.g., ISO 9513, ASTM E83). The certificate is recognized by the Croatian authorities and the notified bodies.

Complementary Tests – Gauge Factor, Lead Wire Resistance, and Insulation Resistance

To fully characterize the strain measurement system and to ensure its reliability, we perform complementary tests on the strain gauge, the lead wires, and the insulation. These tests help to identify potential sources of error and to improve the overall accuracy of the measurement.

• Gauge factor measurement (ASTM E251 / NTC 5750 – for the strain gauge sensitivity) – we measure the gauge factor (the ratio of the fractional change in resistance to the strain) of the strain gauge using a precision calibration fixture. The measured gauge factor is compared with the manufacturer's specified value. We report the measured gauge factor and the deviation.
• Lead wire resistance measurement (NTC 5751 – for the effect of lead wire resistance on the accuracy) – we measure the resistance of the lead wires (including the connectors) and calculate the effect of the lead wire resistance on the strain measurement. The error due to the lead wire resistance is reported.
• Insulation resistance measurement (ASTM D257 / NTC 5752 – for the leakage current) – we measure the insulation resistance between the strain gauge and the substrate (or between the gauge and the lead wires) using a megohmmeter. A low insulation resistance can cause leakage currents and measurement errors. We report the insulation resistance (in MΩ) and the pass/fail status.
• Resistance to soldering heat (NTC 5753 – for the strain gauge durability during installation) – we expose the strain gauge to a soldering temperature (using a soldering iron) and measure the change in the resistance and the gauge factor. The resistance to soldering heat is evaluated. We report the resistance change and the gauge factor change.
• Creep of the adhesive and the bonding material (NTC 5754 – for the long‑term stability of the strain transfer) – we evaluate the creep of the adhesive used to bond the strain gauge to the substrate. The strain transfer from the substrate to the gauge is affected by the creep of the adhesive. We report the creep of the adhesive and the effect on the strain measurement.

Test Report and Recognition in the Croatian Testing, Industrial, and Metrology Sector

All procedures described are within the scope of our ISO/IEC 17025 accreditation, with equipment calibrated periodically (calibration fixtures, extensometers, data acquisition systems, and thermal chambers) and traceability to international standards (NIST, PTB). Our test reports are issued in English (with a Croatian summary available on request) and include:

• Full identification of the strain measurement system (manufacturer, model, serial number, range, and intended application).
• Detailed description of the test methods applied (ASTM/ISO/IEC/HRN EN/NTC standards, test conditions, and measurement parameters).
• Numerical results: linearity error (%), hysteresis (%), repeatability (%), zero drift (με/day), sensitivity drift (%/month), temperature coefficient (%/°C), frequency response (Hz), rise time (ms), SNR (dB), gauge factor, insulation resistance (MΩ), and expanded uncertainty (με).
• Graphical data: calibration curves, frequency response curves, and drift vs. time curves.
• Comparative tables against the values specified by the client or against the limits of the relevant standards (ISO 9513, ASTM E83, ASTM E251, HRN EN ISO 7500-1, and the requirements of the HZN, Ministarstvo gospodarstva, and Državni inspektorat).
• Calibration certificate and the traceability chain.
• Recommendations for the improvement of the measurement accuracy, the selection of appropriate strain gauges, and the implementation of the temperature compensation and the calibration procedures.
• Expanded uncertainty (k=2) for all key measurements, calculated according to the ISO/IEC 98‑3 Guide.

These reports are fully accepted by the Hrvatski zavod za norme (HZN) for the verification of product conformity, by the Ministarstvo gospodarstva i održivog razvoja for industrial metrology compliance, by the Državni inspektorat for market surveillance, and by the Carinska uprava (Croatian Customs) for tariff classification and quality verification in the import of strain measurement equipment and sensors. Additionally, we offer consulting services for the selection of strain measurement systems, the optimization of calibration procedures, and the implementation of measurement uncertainty programs, contributing to the accuracy, reliability, and traceability of strain measurements in the diverse and growing Croatian testing, automotive, aerospace, and industrial sectors.