IEC 62812-2019 pdf download.Low resistance measurements – Methods and guidance.
Thermoelectric e.m.f. is superimposed upon the voltage generated on the unknown resistor by the sourced current, see Figure 7, and, therefore, can seriously affect the accuracy of a resistance measurement. Less obvious is that the distribution of currents in a node will also be affected if thermoelectric e.m.f. affects only one of the branches. This issue of thermoelectric e.m.f. may be of static or of dynamic nature, depending on whether the temperature on the specimen and on the assembly or fixture has reached the steady state or not. Clause 5 discusses methods with a potential of eliminating the influence of a resulting thermal e.m.f. from the resistance measurement result.
4.6 Peltier effect Another cause for a temperature difference, the Peltier effect, will remain unavoidable, even if the symmetrical heat flow proposed in 4.5 would be able to balance the internal sources of thermoelectric e.m.f. The Peltier effect is creating a temperature difference when a d.c. current is passed through the joint of two different materials. Hence, the variety of materials sequentially joined together in a resistor is likely to result in a sequence of temperature differences when such a resistor is subjected to a d.c. current, for example for measuring its resistance. The heat generated by the Peltier effect is proportional to the d.c. current and the thermoelectric e.m.f. of the joined pair of materials. The temperature difference rises exponentially with a time constant to a terminal value, which are both determined by the design of the resistor and by the ability of the test fixture to dissipate heat to the environment. The resulting temperature difference is reversed when the d.c. current is reversed. EXAMPLE 1 A 1 00 µΩ resistor, manufactured by welding copper terminations to each side of a strip of CuNi44 resistive alloy, is measured in a suitable fixture with a current of 300 A in order to achieve a measurable voltage drop of 30 mV. The measuring current by means of the Peltier effect generates heat on the material joints, which have been observed to result in a temperature difference of 30 K after being loaded for 3 min.
This temperature difference generated by the Peltier effect in turn generates a thermoelectric e.m.f. through the Seebeck effect, which again is likely to cause a significant error to the resistance measurement. EXAMPLE 2 As a consequence of the conditions shown in the above Example 1 , the temperature difference of 30 K between the two resistor terminals leads to a thermoelectric e.m.f. of −1 ,2 mV, which adds to the intended voltage drop and thereby causes the resulting voltage and thus the resistance result to deviate by 4 %.
5 Methods of measurement
5.1 General The discussion in Clause 4 has illustrated that – aside from more general low-signal measurement uncertainties – the measurement of low resistance is likely to be affected by a number of very basic phenomena. These phenomena, each by itself, are likely to have detrimental effects on the achievable accuracy of the resistance measurement. These effects may result in substantial measuring errors, which could finally render a low-resistance measurement useless. An obvious first measure to avert the detrimental influence of lead and contact resistance as demonstrated in 4.2 is to apply the offset correction provided by most current meters. This method requires the contact points to be connected directly, without any resistance between them, which may not be applicable in a real test fixture without imposing further influence. Also, since the resistor under test is not involved in this method, it is not possible to compensate for any influence by its materials and of self-heating under the actual measurement. Clause 5 describes a variety of methods with a potential to eliminate the effect of one or more of the described phenomena and discusses the particular background of each method. It also identifies limitations which still can persist for and possible conditions under which such limitations can be waived.
5.2 Four-wire resistance measurement A very common method for the elimination of lead and contact resistances from the actually measured low resistance is to apply fourwire sensing, also known as Kelvin sensing. In this method, a dedicated pair of force wires is used to impress the current into the specimen, while another dedicated pair of sense wires connects the specimen to the voltmeter, as shown in Figure 8. The four-wire method is a widely used method and, therefore, also serves as the basis for many other methods that also aim to eliminate further detrimental effects.IEC 62812 pdf download.