Thermal Emf Errors; Measurement Tutorial - HP 34401A User Manual

Measurement Tutorial

The HP 34401A is capable of making highly accurate measurements.
In order to achieve the greatest accuracy, you must take the necessary
steps to eliminate potential measurement errors. This chapter describes
common errors found in measurements and gives suggestions to help
you avoid these errors.
Thermal Errors
EMF
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.
Approx. m V / °C
Copper-to-
Copper
Gold
Silver
Brass
Beryllium Copper
Aluminum
Kovar or Alloy 42
Silicon
Copper-Oxide
Cadmium-Tin Solder
Tin-Lead Solder
The HP 34401A's input terminals are copper alloy.
198
<0.3
0.5
0.5
3
5
5
40
500
1000
0.2
5