## 4 Discussion  
The results of this research establish a methodology to develop CRs of bifilar, quadrifilar, and octofilar geometries with appropriate TC and stability. Therefore, low type A uncertainties are achieved in DC calibration with high-accuracy measurement systems, which allows the CR to be employed as references at different frequencies. Three methods join Evanohm-S wire and copper terminals were evaluated. The results of the spot welding method offer measured wire resistance values without instability or drift over 0.05 \(\upmu\Omega/\Omega/\)day. The same results were obtained using this method with Evanohm terminals and with Isaohm wire. Furthermore, the use of round supports ensures a CR's geometry with an appropriate mechanical strain on the wire. Therefore, the models of Equations (1)-(3) and are utilized to determine the change in the CR's resistance value as a function of the frequency. Previous CR research makes a profound description of the model for the change in resistance in functions of the frequency. However, none of them describe the construction methodology to achieve the appropriate TCs and stabilities to use the CR with uncertainties contributions of less than 0.05 \(\upmu\Omega/\Omega\) detailed in this research. A methodology to perform heat treatment to Evanohm-S and Isaohm wires were presented. The results indicate that it is possible to use it in any NiCr alloy wire to decrease its TC to less than 0.5 \(\upmu\Omega/\Omega/\)\({}^{\circ}\)C. However, it is essential that before starting the heat treatment tests, the method to join the NiCr wire to a terminal allows measuring the temperature coefficient without instabilities. As seen in Figure 5, with the dimensions of Table 1, the change in the resistance value of the CRs at frequencies under 2 kHz could be considered as equal to the DC value with an uncertainty of less than 0.01 \(\upmu\Omega/\Omega\). Nevertheless, it is better to use the relative difference between the measured and  
Figure 11: Graph of the relative difference between the measured and the calculated frequency dependence of the equivalent parallel resistance of CR-1k-D, as a function of frequency. Curve 1 using the CR-10k-D5e as reference (ratio 10:1, black points), Curve 2 using the Haddad1000 1201 as reference (ratio 1:1, red points) [24]. calculated frequency dependence between different CRs as the uncertainty due to the change in the resistance value. The frequency dependence of CENAM's 10 k\(\Omega\) octofilar CR and METAS's 1 k\(\Omega\) coaxial CR are in good agreement with the calculated frequency dependence. These results allow using the RC 10K-D5e as a reference in the traceability chain between the QHR and the capacitance measurements at CENAM. However, the results obtained with the RC-1k-D bifilar CR show unexpected linear behavior. The cause for this behavior can be due to damage in the wire to terminal connections during transport from Mexico to Switzerland. Therefore, the wire in RC-1k-D will be replaced to evaluate if the linear frequency dependence disappears. Nevertheless, the spot-welding process, as the cause of the linear frequency performance, cannot be discarded. For this reason, before replacing the RC-1k-D wire, the parameters of the process, like the power application time and the mechanical strain between electrodes need to be controlled. The authors are working on the development of a coaxial impedance bridge, similar to the one used at METAS for the frequency evaluation of the CRs. This measurement system will allow us to rule out the spot welding process as a cause for the linear frequency behavior. Conceptualization, F.L.H.-M., L.M.C.-M. and A.H.P.-E.; methodology, F.L.H.-M. and A.H.P.-E.; validation, F.L.H.-M., A.H.P.-E., C.D.A., L.M.C.-M., C.D.-G., H.A.-B.