ASTM requires that load and strain measuring devices be calibrated annually. If any device has been damaged or is subjected to excessive use, more frequent calibrations should be done. Calibration records should be kept to determine when it’s time to replace or refurbish a device. Each device has its own certified range and, therefore, no results should be reported outside the certified ranges.
According to the E4 Standard Practices for Force Verification of Testing Machines, minimum certifiable force for a given range is 200 times the force resolution. Resolution on dial gages and pen recorders depends on the distance between graduation marks and the width of the needle or pen. Often, with these devices, it is not clear what the resolution should be, and determining it is subject to much interpretation.
Worn machine components can result in misalignment that creates bending stresses, lowering tensile stress readings. Check the test machine’s alignment and play to ensure concentricity of the crosshead over the full travel range.