Rationale

Existing 8.1 permits the use of "interpolated zero force readings" under method (b) for a series of applied forces, but does not define how the interpolation is performed. Practice across calibration laboratories has historically applied a linear correction proportional to the ratio of applied force to the maximum force in the series, but the absence of any worked example in the standard has produced documented inconsistencies in deflection calculation, particularly where zero drift is non-trivial relative to the lower force limit of the verified range. The proposed insertion provides one mathematical example of how the interpolation may be performed, identifies the assumption of linear zero drift on which the example depends, and acknowledges that alternative weighting schemes are mathematically valid and produce different deflection values. The example is informative rather than prescriptive: the laboratory retains responsibility for selecting and documenting an interpolation method appropriate to the calibration. Because the calculation has only two data points (the initial and final zero readings), all viable interpolation methods are linear; polynomial, spline, and similar higher-order methods are not applicable without intermediary return-to-zero points. The viable methods differ only in their weighting scheme — that is, in how the total drift V(0final) - V(0initial) is distributed across the loaded series. The proposal, therefore, does not constrain the laboratory to a single calculation; it provides one defensible example to help those who struggle with various interpolation methods.