Standards profile: ISO 376 and the force-proving chain

Every tensile test certificate ultimately leans on a force-proving instrument, and ISO 376 is the standard that disciplines them. It specifies how proving devices — precision load cells and proving rings — are calibrated, classified (00 through 2), and used to verify the testing machines that break specimens for a living.

The standard's texture is in its classification criteria: repeatability, reproducibility with rotation, interpolation error, zero return, and creep each get limits per class, because a load cell can excel at one and fail another. The classes then map to what the device may verify — a class 1 instrument for a class 1 machine verification.

US practice runs on the parallel ASTM E74, with its Class A and AA limits and lower-limit-factor logic; laboratories serving both worlds keep the mapping between the two on hand. Both entries in the standards index link to their official sources, and the force explainer later this month walks the physical chain from deadweight machines downward.