by Henry Zumbrun
Differing opinions are found within the ASTM E28 sub-committee regarding the proper treatment of the trailing zero following the removal of an applied force. The ASTM E28 committee is responsible for the ASTM E74 standard, which is the generally accepted standard for calibration of force-measuring instrumentation in the United States.
Prior to the existence of ASTM E74, there was a split-out sub-set from E4 that hardly resembled what is currently found in ASTM E74. The ASTM E74 standard was published in 1974, and the current revision is ASTM E74-13a.
The current standard allows for two different methods—Method A and Method B—for the treatment of the ending zero. Method A defines the deflection calculation as the difference between the deflection at an applied force and the initial deflection at zero force. Method B defines deflection as the difference between the deflection at an applied force and a zero value derived from either an average zero (if the loading sequence is zero, load, zero) or an interpolated zero (if a series of forces are applied before return to zero force).
Morehouse Instrument Company conducted an analysis consisting of 46 various measuring instruments and analyzed the effect on the Lower Limit Factor, which is the standard deviation of the differences from the predicted response, multiplied by a coverage factor of 2.4.
