Agilent Technologies U2741A Guide de l'utilisateur télécharger pdf (Page 66)

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

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Agilent Technologies U2741A Guide de l'utilisateur Page 66