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In three-wire connections, one wire is connected to one end of the resistance probe, and
two wires are connected to the other end. Probe resistance can be measured using
many different methods, but ail of these require that at least two of the conductors in the
cable are identical. Some methods even require that conductor resistances be "tuned"
to specific constant values.
The voltage across the probe can also be measured with three wires. The voltage drop
in the current circuit is measured and cable effects are compensated for on the
assumption that the voltage drop in the feed conductor is identical to that in the return
conductor.
ip{
Constant current
IR
R = E-2
X
E|r
Figure 5-3. Pt100 Measuring Circuit
If the probe is connected to the CL521 using three conductors, the instrument
automatically detects the lack of the fourth conductor and begins to operate in the three
wire mode. The fourth conductor should, however, be used whenever possible.
Figure 5-4. Connecting a 3-Wire Resistance Probe to the CL521
16
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