Emerson Rosemount 148 Reference Manual page 18

Installation
June 2019
Figure 2-5: Rosemount 148 Sensor Connections Diagram
Thermocouple
The thermocouple can be connected directly to the transmitter. Use appropriate
thermocouple extension wire if mounting the transmitter remotely from the sensor.
RTD or ohm inputs
The Rosemount 148 will 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 of up to 60 Ω
per lead (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, both RTD leads are in series
with the sensor element, so significant errors can occur if the lead lengths exceed three ft.
of 20 AWG wire (approximately 0.05 °C/ft,). For longer runs, attach a third of fourth lead as
described above.
Sensor lead wire resistance effect-RTD input
For 2- and 3-wire RTDs, an additional lead wire resistance error is induced with ambient
temperature variations. Using the same type of wire on all lead wires will make the
installation as accurate as possible.
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 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
Sensor input
4-wire RTD
3-wire RTD
2-wire RTD
18
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
Reference Manual
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