User`s guide

48 U2741A User’s Guide
3 Measurement Tutorial
DC Measurement Considerations
Thermal EMF errors
Thermoelectric voltages are the most common source of
error in low- level DC voltage measurements. Thermoelectric
voltages are generated when you make circuit connections
using dissimilar metals at different temperatures.
Each metal- to- metal junction forms a thermocouple, which
generates a voltage proportional to the junction temperature.
You should take the necessary precautions to minimize
thermocouple voltages and temperature variations in
low- level voltage measurements. The best connections are
formed using copper- to- copper crimped connections, as the
digital multimeter's input terminals are copper alloy. The
table below
shows common thermoelectric voltages for connections
between dissimilar metals.
Ta bl e 3 - 1 Thermoelectric voltage for dissimilar metals connections
Copper to - Approx. mV / °C Copper to - Approx. mV / °C
Cadmium-Tin Solder 0.2 Aluminum 5
Copper <0.3 Tin-Lead Solder 5
Gold 0.5 Kovar or Alloy 42 40
Silver 0.5 Silicon 500
Brass 3 Copper-Oxide 1000
Beryllium 5
User's Guide.book Page 48 Friday, August 8, 2008 9:11 AM