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. The
table below shows common thermoelectric voltages for connections
between dissimilar metals.
Copper-to-
Approx. µV/° C
Copper
<0.3
Gold
0.5
Silver
0.5
Brass
3
Beryllium Copper
5
Aluminum
5
Kovar or Alloy 42
40
Silicon
500
Copper-Oxide
1000
Cadmium-Tin Solder
0.2
Tin-Lead Solder
5
Noise Caused by Magnetic Fields
If you are making measurements near magnetic fields, you should take
the necessary precautions to avoid inducing voltages in the measurement
connections. You should be especially careful when working near
conductors carrying large currents.
7
Chapter 7 Measurement Tutorial
Measurement Techniques and Sources of Error
251
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