BS EN IEC 62812:2019 pdf free download – Low resistance measurements – Methods and guidance
4.4 Variation of resistance with temperature
One of the reference conditions prescribed in IEC 601 1 5-1 for measuring the resistance is the reference temperature of 20 °C. For practical reasons, however, most tests and measurements are permitted to be executed under standard atmospheric conditions for testing as defined in IEC 60068-1 , which includes a permissible range for the ambient temperature from 1 5 °C to 35 °C. If measured with sufficient accuracy, a resistor measured at 1 5 °C or at 35 °C will not show the same resistance as when measured at 20 °C. In fact, there is a variation of resistance with temperature for almost every type of resistor, which typically does not follow a linear relationship. The slope and the amount of variation depend substantially on the technology and manufacturing of the resistor and in some cases also on the actual resistance.
A real resistor not only consists of the resistive element, made of a specific material, but also incorporates conductors on both sides in order to establish the electrical connection. In some cases, these conductors are fixed in shape and effective length and therefore should be included in an overall specification of the resistor. In other cases, such as the axial leaded resistors shown, for example, in Figure 3, the conductors are supplied with generous lengths of lead-wire, of which typically only a part is used in the circuit, requiring a suitable specification of mandatory points of resistance 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.