SMAR TT302, Installation, Operation & Maintenance Instruction Manual

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TT302 – Fieldbus Temperature Transmitter
2-4
For correct measurement of RTD temperature, it is necessary to eliminate the effect of
the resistance of the wires connecting the sensor to the measuring circuit. In some
industrial applications, these wires may be hundreds of meters long. This is particularly
important at locations where the ambient temperature changes constantly.
The
TT302
permits a 2-wire connection that may cause measuring errors, depending on
the length of connection wires and on the temperature to which they are exposed. (See
Figure 2.3 - Two-Wire Connection
).
In a 2-wire connection, the voltage V2 is proportional to the RTD resistance plus the
resistance of the wires.
V2 = [RTD + 2 x R] x I
Figure 2.3 - Two-Wire Connection
In order to avoid the resistance effect of the connection wires, it is recommended to use a
3-wire connection (See
Figure 2.4 - Three – Wire Connection ) or a 4-wire connection
(See Figure 2.5 - Four - Wire Connection ).
In a 3-wire connection, terminal 3 is a high impedance input. Thus, no current flows
through that wire and no voltage drop is caused. The voltage V2-V1 is independent of the
wire resistances since they will be cancelled, and is directly proportional to the RTD
resistance alone.
V2-V1 = [RTD + R] x I - R x I = RTD x I
Figure 2.4 - Three – Wire Connection
TT2EM204.CDR
TT2EM203.CDR