Emerson Rosemount 248 Reference Manual page 22

Installation
May 2019
Figure 2-10: 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, and use shielding for long runs of wire.
RTD or ohm inputs
The transmitters accept a variety of RTD configurations, including 2-, 3-, and 4-wire
designs. If the transmitter is mounted remotely from a 3- or 4-wire RTD, it will operate
within specifications, without recalibration, for lead wire resistances up to 60 ohms per
lead (or the equivalent to 6,000 ft. of 20 AWG wire). In this case, the leads between the
RTD and transmitter should be shielded. If using only two leads, the RTD leads are in series
with the sensor element, so significant errors can occur when the lead lengths exceed
three feet of 20 AWG wire (approximately 0.05 °C/ft [9.8436 °F/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 does not impact
accuracy. A 3-wire sensor will not fully cancel lead resistance error since it cannot
compensate for imbalances in resistance between the lead wires. Using the same type of
wire on all three lead wires makes a 3-wire RTD installation the most accurate. A 2-wire
sensor produces the largest error since 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 on
2-1 help quantify these errors.
Table 2-1: Examples of Approximate Basic Error
Sensor input
4-wire RTD
3-wire RTD
2-wire RTD
22
Approximate basic error
None (independent of lead wire resistance)
± 1.0 Ω in reading per ohm of unbalanced lead wire resistance
(Unbalanced lead wire resistance = maximum imbalance between
any two leads.)
1.0 Ω in reading per ohm of lead wire resistance
Rosemount 248 Temperature Transmitter Reference Manual
Reference Manual
00809-0100-4825
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