Rosemount 248, Справочное руководство

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Reference Manual
00809-0100-4825, Rev BB
August 2005
Rosemount 248
2-10
Figure 2-7. Sensor Wiring
Diagrams
Thermocouple or Millivolt Inputs
The thermocouple can be connected directly to the transmitter. Use
appropriate thermocouple extension wire if mounting the transmitter remotely
from the sensor. Make millivolt input connections with copper wire. Use
shielding for long runs of wire.
RTD or Ohm Inputs
The transmitters will accept a variety of RTD configurations, including 2-wire,
3-wire and 4-wire designs. If the transmitter is mounted remotely from a
3-wire or 4-wire RTD, it will operate within specifications, without recalibration,
for lead wire resistances of up to 60 ohms per lead (equivalent to 6,000 feet of
20 AWG wire). In this case, the leads between the RTD and transmitter
should be shielded. If using only two leads, both RTD leads are in series with
the sensor element, so significant errors can occur if the lead lengths exceed
three feet of 20 AWG wire (approximately 0.05 °C/ft). For longer runs, attach a
third or fourth lead as described above.
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. However, a 3-wire sensor will not fully cancel
lead resistance error because it cannot compensate for imbalances in
resistance between the lead wires. Using the same type 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 2-1. Examples of
Approximate Basic Error
Rosemount 248 Sensor Connections Diagram
* Emerson Process Management provides 4-wire sensors for all single element RTDs. Use these
RTDs in 3-wire configurations by leaving the unneeded leads disconnected and insulated with electrical
tape.
1 2 3 4 2 3 4 1 2 3 4 1 2 3 4 1
2-wire
RTD and
3-wire*
RTD
and
248-0000B01C
T/C
and mV
4-wire RTD
and
Sensor Input Approximate Basic Error
4-wire RTD None (independent of lead wire resistance)
3-wire RTD ± 1.0
Ω
in reading per ohm of unbalanced lead wire resistance
(Unbalanced lead wire resistance = maximum imbalance between
any two leads.)
2-wire RTD 1.0
Ω
in reading per ohm of lead wire resistance