Measurement Principles; Thermal Effects - Extech Instruments 380580 User Manual

Measurement Principles

The test current flows through the resistance from the Current+ (C+) terminal to the
Current - (C-) terminal. The P+ and P- (POTENTIAL) terminals measure the voltage drop
across the device under test only, thus eliminating the lead and contact resistances. The
meter displays the resistance based on the test current and the measured voltage; refer to
the equation below:

Thermal Effects

Temperature can have a significant effect on the performance of millohmeter due to the
temperature coefficient of the resistance under test and thermal EMF's across dissimilar
conductors.
Most conductors have a large temperature coefficient of resistance
For example: 0.4%/°C for copper. A cooper conductor that has a resistance of 10.00m ohm at
20°C will increase to 10.40m ohm at 30°C. This should be taken into account..
A current going through a resistance will also elevate the temperature so duration of the test
can also change the resistance.
P+
Volts
P-
Rx = Vx / Is
Where:
Vx is the voltage drop across the device under test;
Is is the test current;
Rx is the resistance of the device under test.
6
C+
Current
C-
Version 2.0 1/05