SMAR TT400WH, Manual

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TT400 WirelessHART ® - Operation, Maintenance and Instructions Manual
2.4
V2-V1 = [RTD + R]
x
I – R
x
I = RTD
x
I
Figure 2.3 – Tree-Wire Connection
In a 4-wire connection, terminals 2 and 3 are high impedance inputs. Thus, no current flows through
those wires and no voltage drop is caused. The resistances of the other two wires are not interesting
since no measurement is done on them. Hence the voltage V2 is directly proportional to the RTD
resistance. (V2 = RTD
x
I).
Figure 2.4 – Four-Wire Connection
A differential connection is like the two-wire connection and gives the same problem (see Figure 2.5).
The resistance of the other two wires will be measured and does not cancel each other out in a
temperature measurement, since linearization will affect them differently.
Figure 2.5 – Two Sensor Connection
NOTE
The material the gauge and the length should be the same connections of 3 or 4 threads.