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INTRODUCTION

Our last blog focused on the difference between ISO 376 and ASTM E74.   That blog was a comparison between the major differences when comparing both standards and can be found here.  What we hope to do with this blog is go into a bit more detail and explain the ISO 376:2011 Metallic materials — Calibration of force-proving instruments used for the verification of uniaxial testing machines standard.  The ISO 376 standard is used worldwide, and it is a requirement for anyone calibrating in accordance with ISO 7500. If ISO 7500 is the requirement, then calibration needs to be performed in accordance with ISO 376 on the force-proving instruments used to certify the tensile machine.  It is also the generally accepted force standard for most of the countries outside of North America for calibration of force-proving instruments such as load cells, proving rings, dynamometers, and other instruments used to calibrate similar types of instruments. 

Characteristics of force-proving instruments

ISO 376 characterizes force proving instruments per section 6.1 Identification by stating “All the elements of the force-proving instrument (including the cables for electrical connection) shall be individually and uniquely identified, e.g. by the name of the manufacturer, the model, and the serial number. For the force transducer, the maximum working force shall be indicated.” The standard then goes on to address the application of force in section 6.2 by stating “The force transducer and its loading fittings shall be designed so as to ensure axial application of force, whether in tension or compression.”   Then in section 7.1 ISO 376 further states to ensure “that the attachment system of the force-proving instrument allows axial application of the force when the instrument is used for tensile testing; that there is no interaction between the force transducer and its support on the calibration machine when the instrument is used for compression testing.”

ISO 376 is stressing the importance of keeping the line of force pure, free from eccentric forces as a key component to calibrating the instrument and giving the end user the best probability of being able to replicate that calibration.


Figure 1 Example of Axial Application of Force


Resolution

As defined in section7.2

Figure 2 Example of an analogue scale on a Morehouse 25,000 lbf Ring Force Gauge

“Analogue Scale -The resolution, r, of the indicator, shall be obtained from the ratio between the width of the pointer and the centre-to-centre distance between two adjacent scale graduation marks (scale interval), the recommended ratios being 1:2, 1:5 or 1:10, a spacing of 1,25 mm or greater being required for the estimation of a tenth of the division on the scale.”