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Tip 2: Removing errors due to Thermal EMF (often due to leads)
Thermal EMF caused by dissimilar metals can create a parasitic voltage VEMF in the measurement circuit. The additional voltage will create a measurement error, R=(V+ VEMF)/i. Thermoelectric voltages are generated internally in the resistor or when you make circuit connections due to dissimilar metals at different temperatures. Junctions to consider are at the DUT, relay (multiplexers), and to the multimeter. Each metal-to-metal junction forms a thermocouple, which generates a voltage proportional to the junction temperature. Using all copper connections can minimize errors.
Offset compensation can further minimize thermal EMF errors. The diagram below illustrates the two measurements used in offset compensated measurements, one with the current source and a second without the current source.
The actual resistance is determined by subtracting the second measurement from the first and dividing by the known current source. When using offset compensation the multimeter will make the two measurements and report just the compensated resistance. Offset compensation can be used in both two and four-wire measurements. Using offset compensation will improve measurement accuracy but will reduce measurement speed.
