Rosemount 848T, Справочное руководство

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Reference Manual
00809-0100-4848, Rev AA
July 2009
Rosemount 848T Wireless
3-4
Sensor Lead Wire Resistance Effect—RTD Input
When using a 4-wire RTD, the effect of lead resistance is eliminated and has
no impact on accuracy. A 3-wire sensor will not fully cancel lead resistance
error because it cannot compensate for imbalances in resistance. Using the
same type and length of wire on all three lead wires will make a 3-wire RTD
installation as accurate as possible. A 2-wire sensor will produce the largest
error because it directly adds the lead wire resistance to the sensor
resistance. For 2- and 3-wire RTDs, an additional lead wire resistance error is
induced with ambient temperature variations. The table and the examples
shown below help quantify these errors.
Table 3-1. Examples of
Approximate Basic Error
Figure 3-4. Sensor Wiring
Diagrams
Refer to “Grounding Practices” on page 3-10 for more information on sensor
grounding practices.
Figure 3-5. Rosemount 68Q, 78
Standard Temperature Range,
and 58C RTD Sensor Lead Wire
Configurations
Sensor Input Approximate Basic Error
4-wire RTD Negligible (independent of lead wire resistance up to 60  per lead)
3-wire RTD ± 1.0  in reading per ohm of unbalanced lead wire resistance
(Unbalanced lead wire resistance = maximum imbalance between any
two leads.)
2-wire RTD 1.0  in reading per ohm of lead wire resistance
848T Wireless Sensor Connections Diagram
+
-
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
2-wire RTD
3-wire RTD
4-wire RTD
Thermocouple, millivolt
Single Element
White (1)
White (2)
Red (3)
Red (4)