Universal Testing Machine Force Measurement Error: Sources, Testing Methods and Calibration Standard

Force measurement error in universal testing machines directly affects the validity of tensile, compression and flexural test results, making accurate error determination essential for laboratory accreditation and product quality control. This article provides a comprehensive technical guide to the sources of force value deviation, verification methods according to ISO 7500-1, ASTM E4 and JJG 139, and practical solutions for minimizing measurement uncertainty in static uniaxial testing systems.

Technical Overview

Force measurement error refers to the deviation between the force value indicated by a universal testing machine (UTM) and the true force applied to the specimen, typically expressed as a relative percentage error. In modern electromechanical and servo-hydraulic testing systems, the force signal chain consists of a load cell (force transducer), a signal conditioning amplifier, an analog-to-digital converter, and display or software. Errors can arise at any point in this chain, as well as from mechanical factors such as friction, misalignment, and frame deflection.

International and national standards classify universal testing machines into accuracy classes based on permissible error limits. According to ISO 7500-1 and ASTM E4, the most common classes for metallic materials testing are Class 0.5, Class 1, and Class 2, corresponding to maximum permitted relative indication errors of ±0.5%, ±1.0%, and ±2.0% respectively. A machine found to exceed its designated class limits during verification is considered non-conforming and must be adjusted, repaired, or reclassified before it can be used for accredited testing.

Understanding the root causes of force measurement error is the first step in controlling it. The major categories of error include systematic errors, random errors, and drift:

• Systematic errors: Load cell nonlinearity, zero offset, amplifier gain deviation, and mechanical friction produce consistent bias in one direction.
• Random errors: Electrical noise, vibration, and slight variations in loading conditions cause scatter between repeated measurements.
• Hysteresis (reversibility error): The difference between force readings during increasing and decreasing loading, caused by elastic after-effects in the load cell and mechanical friction in the load train.
• Creep and drift: Time-dependent output change under a sustained load or at zero load, often related to temperature effects and transducer stability.

Testing Samples

Force value verification of universal testing machines is performed using reference force measurement standards rather than material specimens. The selection of the reference device depends on the capacity and accuracy class of the machine under test.

Reference Force Transducers

High-precision strain-gauge load cells calibrated in accordance with ISO 376 are the most widely used transfer standards. A Class 00 or Class 0.5 reference transducer provides calibration uncertainty typically better than 0.1% of applied force, sufficient for verifying Class 0.5 and Class 1 machines. Reference transducers must be used with a dedicated high-resolution indicator, and the combined system must be calibrated as a unit.

Proving Rings and Elastic Calibration Devices

Mechanical proving rings with dial or digital micrometers remain in use for older hydraulic machines and field verification. Their deflection-to-force relationship is established by a force standard machine and documented in a calibration certificate with class-specific uncertainty values.

Deadweight Force Standards

Direct loading of known calibrated masses provides the lowest uncertainty and is used mainly by national metrology institutes and primary calibration laboratories. Deadweight systems are limited in maximum force and are typically employed for lower-capacity machines or for calibrating the reference transducers themselves.

Multi-Point Reference Systems

For machines with wide force ranges, a set of reference transducers of different capacities may be required so that the reference uncertainty remains below one third of the permissible error at every calibration point, satisfying the uncertainty ratio requirement.

Testing Items

A complete force verification evaluates several distinct error quantities at each calibration point. These are the core testing items specified in ISO 7500-1, ASTM E4, and the Chinese verification regulation JJG 139 for tension and compression testing machines.

• Relative indication error (q): The deviation of the force indicated by the machine from the reference force, calculated as q = (F − F_i) / F × 100%, where F is the force applied by the reference device and F_i is the corresponding machine indication.
• Repeatability error (b): The maximum spread between repeated indications at the same calibration point under the same loading conditions, reflecting short-term random error and mechanical friction stability.
• Reversibility error (h): The difference between indications obtained with increasing and decreasing forces at the same point, quantifying hysteresis behavior.
• Interpolation error: For machines using calibration curves or tables between verified points, the deviation of intermediate values from the fitted calibration function.
• Zero error and zero drift: The residual indication after complete unloading and the change in zero indication over a specified time interval, typically evaluated over 30 minutes.
• Resolution and noise: The smallest verifiable force increment of the indicating device and the fluctuation of the indication at zero load.
• Return-to-zero behavior: The ability of the indication to return to zero within the specified limit after removal of the maximum force.

Testing Methods

Force verification is conducted using the direct comparison method, in which the reference force measurement device is mounted in the machine in place of a specimen, and forces are applied through it. The procedure below follows the general requirements of ISO 7500-1 and JJG 139.

Preparation and Preloading

Allow the machine and the reference device to equilibrate in the test environment, typically at 10–35°C with a stable temperature (variation not exceeding 2°C during the test). Install the reference transducer in correct axial alignment with the loading axis, ensuring that no bending or lateral forces are introduced. Apply the maximum force as a preload at least three times to seat the load train and stabilize the transducer output.

Calibration Point Selection

At least five calibration points shall be selected, approximately evenly distributed over the verification range, beginning at no less than 20% of the range for Class 0.5 and Class 1 machines. Points should correspond to the nominal forces used in routine material testing, for example 20%, 40%, 60%, 80%, and 100% of the machine range.

Measurement Sequence

Perform the following series at each calibration point:

• Two series of increasing forces (ascending series) to determine indication error and repeatability.
• One series of decreasing forces after preconditioning, pausing 30 seconds at each point before reading, to determine reversibility error.
• Additional series for machines equipped with multiple force ranges, verifying each range separately.
• Zero indication recorded before loading and after complete unloading, plus a 30-minute zero drift observation where required.

Error Calculation and Class Assignment

For each calibration point, compute the relative indication error q, the repeatability error b, and the reversibility error h. Compare each value with the maximum permissible errors for the target accuracy class. The machine satisfies a given class only if all quantities at all points remain within the limits for that class; the class achieved is the lowest class for which all criteria are met. Common acceptance limits per ISO 7500-1 are:

• Class 0.5: indication error ±0.5%, repeatability 0.5%, reversibility ±0.75%.
• Class 1: indication error ±1.0%, repeatability 1.0%, reversibility ±1.5%.
• Class 2: indication error ±2.0%, repeatability 2.0%, reversibility ±3.0%.
• Class 3: indication error ±3.0%, repeatability 3.0%, reversibility ±4.5%.

Uncertainty Considerations

The expanded uncertainty of the reference measuring system shall not exceed one third of the maximum permissible error of the machine being verified. Uncertainty contributors include the reference device calibration uncertainty, its temperature coefficient, alignment-induced bending, indicator resolution, and repeatability of the loading process. A documented uncertainty budget is mandatory for ISO/IEC 17025 accredited calibration laboratories.

Applications

Force value error determination is essential wherever universal testing machines are used to generate reportable material property data:

• Metallic materials testing: Determination of yield strength, tensile strength, and proof strength per ISO 6892-1 and ASTM E8 requires a machine of Class 1 or better, making routine force verification indispensable.
• Third-party testing and certification laboratories: CNAS, ILAC, and national accreditation schemes require documented traceability of the force scale, verified at defined intervals through calibrated reference standards.
• Aerospace and automotive manufacturing: High-consequence components demand Class 0.5 or Class 1 machines with tight control of force error, including reversibility, for fatigue preloading and static strength verification.
• Construction materials testing: Concrete compression, rebar tension, and fastener testing machines are verified against JJG 139, EN 12390, and related standards for regulatory acceptance of structures.
• Plastics, rubber and textiles: Force accuracy directly influences modulus and strength results per ISO 527, ISO 37, and ISO 13934, particularly in low-force ranges where resolution and zero drift dominate.
• In-house quality control: Manufacturers performing incoming material inspection and production monitoring use periodic force verification to maintain SPC data integrity.

FAQ

What is the acceptable force measurement error for a universal testing machine?

The acceptable error depends on the machine accuracy class. For a Class 1 machine, which is required for most metallic tensile testing, the relative indication error must remain within ±1.0%, repeatability within 1.0%, and reversibility within ±1.5% at every verified point. Class 0.5 machines must meet ±0.5% indication error and are used for high-precision applications.

How often should force values of a universal testing machine be verified?

ISO 7500-1 and most national regulations recommend verification at intervals not exceeding 12 months. Additional verification is required after relocation of the machine, major repair or replacement of the load cell, amplifier, or software, after overload events, or whenever test results appear anomalous. High-usage machines in production environments often benefit from semi-annual intervals supplemented by daily or weekly checks with a working reference load cell.

What are the most common causes of excessive force indication error?

Frequent causes include load cell zero drift due to temperature change, amplifier gain drift or poor calibration, mechanical friction from worn grips or bearings, bending moments caused by misaligned or eccentric loading, electrical noise or grounding problems, and incorrect range selection where readings are taken below 20% of the force range. Moisture ingress and cable damage in the load cell circuitry are also common contributors.

Which standards govern force verification of universal testing machines?

The primary standards are ISO 7500-1 (verification and calibration of static uniaxial testing machines), ASTM E4 (practices for force verification of testing machines), JJG 139 (Chinese metrological verification regulation for tension/compression testing machines), and ISO 376 (calibration of force-proving instruments used as reference standards). EN ISO 7500-1 and JIS B 7721 are regional adaptations of the same methodology.

Can a machine with force error outside its class limits still be used?

Yes, under controlled conditions. If the machine exceeds the limits of its intended class but meets the limits of a lower class, it can be reclassified and used for applications where the lower class is acceptable. Alternatively, a correction factor or calibration curve may be applied if permitted by the relevant test standard and the laboratory quality system, provided the correction is documented and the corrected values are traceable.

How can laboratories reduce force measurement error in daily operation?

Recommended practices include allowing adequate warm-up time, verifying zero and a single intermediate force point with a working standard each week, keeping the load cell and cables in stable environmental conditions, avoiding operation below 20% of the selected force range, performing regular grip alignment checks, and scheduling preventive maintenance of the loading system and electronics before accuracy deteriorates.