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Stating Best Uncertainties Over a Range of ValuesDavid Deaver3.3 Comparison of Laboratories 5 and 6Using a more Accurate DMMUnderstating the CMC values would be even worse hadthe lab chosen a more accurate DMM. Unfortunately, thisis often the case for laboratories wanting to use their bestequipment when calculating the uncertainties for theirCMCs. Figure 4 plots the DMM accuracy and the CMCvalues for the laboratories for the uncertainties shown inTable 5 except for a DMM specification of0.0005% of Reading + 0.0003% of Range.3.4 Laboratory 7: Analysis at FS and MSwith Linear InterpolationThis laboratory seeks to avoid the pitfall of Laboratory6 but is also reluctant to have to calculate uncertaintiesfor each point it wishes to calibrate like Laboratory 5.It is this author’s recommendation that the straight lineapproximation method of Laboratory 7 be encouragedand Laboratory 6 methods which understate theuncertainty over much of the range be disallowed.Equation 1 is the formula for calculating the CMCvalues using a linear interpolation of the uncertainties120100Expanded Uncertainty (nA)806040Laboratory 5Laboratory 7Laboratory 620DMM Specification00% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%Percent of Full ScaleFigure 4. CMCs for Labs 5 & 6 with DMM specifications of 0.0005% Rdg + 0.0003% FS.120Expanded Uncertainty (nA)10080604020Laboratory 5Laboratory 7Laboratory 6DMM Specification00% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%Percent of Full ScaleFigure 5. Labs 5 & 6 CMCs with nearly equal gain and floor uncertainties.Cal Lab: The International Journal of Metrology28 Jan • Feb • Mar 2013