Minimizing Thermal Emfs - Keithley 2750 User Manual

E-4 Measurement Considerations

Minimizing thermal EMFs

To minimize thermal EMFs, use only copper wires, lugs, and test leads for the entire test
setup. Also, it is imperative that all connecting surfaces are kept clean and free of oxides.
As noted in
high as 1mV/°C.
Even when low-thermal cables and connections are used, thermal EMFs can still be a
problem in some cases. It is especially important to keep the two materials forming the
junction at the same temperature. Keeping the two junctions close together is one way to
minimize such thermal problems. Also, keep all junctions away from air currents; in some
cases, it may be necessary to thermally insulate sensitive junctions to minimize tempera-
ture variations. When a Cu–Cu connection is made, sufficient pressure must be applied to
ensure the connection is gas tight to prevent future oxidation.
In some cases, connecting the two thermal junctions together with good thermal contact to
a common heat sink may be required. Unfortunately, most good electrical insulators are
poor conductors of heat. In cases where such low thermal conductivity may be a problem,
special insulators that combine high electrical insulating properties with high thermal con-
ductivity may be used. Some examples of these materials include hard anodized alumi-
num, sapphire, and diamond.
Nulling residual thermal offsets
Even if all reasonable precautions are taken, some residual thermal offsets may still be
present. These offsets can be minimized by using the Model 2750 Relative feature to null
them out. To do so, place the instrument on the 3mV range and short the end of the con-
necting cable nearest the measured source (first disconnect the cable from the source to
avoid shorting out the source). After allowing the reading to settle, press the front panel
REL button to null the offset. Select the appropriate range, and make your measurement as
usual.
Table E-1, copper-to-copper oxide junctions can result in thermal EMFs as
Model 2750 Multimeter/Switch System User's Manual