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Chapter 7 Measurement Tutorial
Measurement Techniques and Sources of Error
Offset Compensation
A resistance measurement involves measuring a voltage (E) induced
across the resistance by a known current source.
Thermal EMF caused by dissimilar metals can create a parasitic voltage
in the measurement circuit (V
thermal EMF). The thermal EMF can be caused by the input lead
connections or internally in resistor R. In general, this voltage will not
change with the current applied to the resistor.
Ideal
Meter
The voltage measured, and so the resistance calculated, is in error by
V
. Using offset compensation can reduce the errors caused by V
EMF
To make an offset compensated measurement, the meter makes two
voltage measurements, one with the current source on and one with the
current source off, and subtracts the two measurements. The actual voltage
drop across the resistor, and the calculated resistance are obtained by:
First Reading
Offset compensation can be used in 2-Wire or 4-Wire ohms measurements.
Ideal
Meter
EMF
Ideal
E=IR+V
EMF
Meter
Second Reading = ( I
). (See page 251 for a description of
R + V
)
V
EMF
E=(0*R)+V
= V
EMF
EMF
.
EMF
= I
R
EMF
7
259
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