Transmille 8000 Series - Manual (Page 38)

8000 SERIES OPERATION MANUAL
TRANSMILLE LTD. Page 37
Understanding thermal EMF voltages.
Thermal EMF voltages are generated wherever there is a metal to metal junction, an electrical
connection between two different metals. The voltage generated is dependant on the metals and the
temperature of the junction. Voltages up to many 10’s of micro volts can easily be generated by a
simple banana plug connection, each connection being effectively a thermocouple.
Diagram : Metal to metal junction between test lead and terminal
For low level DC voltage measurements these thermally generated EMF voltages are the primary
source’s of error. To avoid such errors it is best to use copper-to-copper connection, avoiding nickel-
plated brass plugs, use gold plated copper plugs where possible. Avoid temperature gradient’s from
any source, soldering iron close to test leads, air condition’s unit blowing cold/hot air across test
leads etc. Even a test lead, which has been previously passing a high current, will be warm and give
errors.
Diagram : Gold plated copper to gold plated copper Diagram : Leads previously heated by high current
to avoid thermally generated EMFs
Diagram : Avoid hot/cold temperature sources when taking measurements
Note that thermal EMF do not only effect voltage measurements but also resistance measurements.