The ASTM E4, ISO 7500-1 standards describe the fundamental conditions, requirements and verification of load cell calibrations for static, single-axis testing machines. They are used to ensure proper calibration and are highly relevant in a wide range of industries and for many different applications. The standards establish an important foundation for reliable test results.
An essential part of the standard describes the general inspection tasks for the testing machine, according to which calibrations are only to be performed on a materials testing machine if the machine is in perfect working order. To ensure that this is the case, calibrations are usually preceded by regular maintenance according to the inspection plan. This is described in more detail in Annex A of the DIN EN ISO 7500-1 standard.
The general inspection includes the following:
- A visual inspection of the setup and general condition of the materials testing machine
- Inspection of the structure and crosshead drive of the materials testing machine
- Inspection of the actual state and adjustment
- Application of the test forces:
- The minimum requirements for standard compliant force calibration according to ISO 7500-1 includes five measuring points in a range of 20 to 100% of the calibrated force display range. It is important to make sure that the measuring points are distributed as uniformly as possible.
- ZwickRoell calibrates up to nine additional recommended force levels and indicates this in the calibration certificate: 10, 7, 4, 2, 1, 0.7, 0.4, 0.2 and 0.1%. By calibrating up to 14 measuring points, the range of application for your load cell and the reliability of your test results are significantly increased.
Determination of characteristic values q, b and u and evaluation of the force measuring system: The standard also requires determination or calculation of the characteristic values “relative display deviation q”, “relative reversibility v”, and “relative repeat accuracy b”. The force measuring system is classified according to these characteristic values. Among other things, the measurement uncertainty according DIN EN ISO 7500-1 Supplement 4 is also taken into account for this classification.
Documentation is provided in the form of a calibration certificate. The purpose of this certificate is to document proper implementation of the inspection and calibration.
The ISO 7500-1 standard specifies the minimum requirements for general information that must be included in the calibration certificate.
This includes, among others:
- A reference to the underlying standard
- The exact identification of the testing machine and, if available, of the force display (e.g., by means of the manufacturer and type designation and the serial number)
- The installation location and calibration temperature
- The calibration date and the name or mark of the performing body
Standards ASTM E4 and ISO 7500-1 recommend an interval of no more than 12 months, unless otherwise specified according to reasons applicable to the machine. A testing machine, however, must be calibrated if the installation location changes, for example for relocation purposes.
Minimum requirements for standard-compliant calibrations according to ISO 7500-1 include the following 5 measuring points: 20, 40, 60, 80 and 100%. ZwickRoell calibrates up to nine additional normative recommended levels: 10, 7, 4, 2, 1, 0.7, 0.4, 0.2 and 0.1%.*
*Typically, for ZwickRoell type Xforce High Precision+ load cells the range starts at 0.1/ 0.2% of the nominal force; for other types of load cells the range starts at 0.4%. ZwickRoell calibrates servohydraulic materials testing machines starting at 1%.
In general, the ISO 7500-1 standard specifies the calibration and inspection processes of tensile and compression testing machines.
American Society for Testing and Materials