Three ways to reduce thermoelectric voltages

When you connect a voltmeter to a low-impedance circuit that produces no voltages, the meter should read zero. Thermoelectric voltages, however, produce unwanted signals, and they can adversely affect low-level voltage and resistance measurements, such as those across bridge circuits, relays, and connectors.

Thermoelectric voltages (VEMF1 through VEMF4) produce error voltages that are significant when you need to measure millivolts.

• Keep connections at the same temperature. Keep the test circuit away from overhead heating or cooling systems, as either can produce unstable temperatures in the circuit. Wrapping the circuit in insulating material can minimize the effects of unstable and different temperatures.
• Don't let the circuit produce excess current. Current can cause circuit components to self-heat and raise the temperature of the overall circuit. The resulting temperature difference will cause unwanted thermoelectric voltages.
• Use the same material for all conductors. Connections made by crimping copper sleeves or lugs on copper wires result in cold-welded copper-to-copper junctions that generate minimal thermoelectric voltages. Keep connections clean and free of oxides. Copper-to-copper (Cu-Cu) connections may have Seebeck coefficients of <0.2 µV/°C, while copper-to-copper oxide (Cu-CuO) connections may have a coefficient as high as 1000 µV/°C.

If these preventative techniques don't eliminate errors caused by thermal voltages, your test equipment may be able to compensate for them. Many bench, VXI, and PXI DMMs can apply offset voltages to your circuit that can cancel offset errors.