The standard uncertainty uMcal in the mass of the reference weight is usually obtained from the calibration certificate. If an expanded uncertainty UMcal is given, then calculate uMcal = UMcal/k. If the coverage factor k is not given than assume k = 2.0. Look up the value for the degrees of freedom ν corresponding to k (see for example Table G.2 in [3]). For k = 2.0, ν = 60. When several reference weights are used together, the standard uncertainty due to the calibration for the combination is simply the sum of the individual standard uncertainties (because their values are correlated).