Emerson Rosemount 248 Reference Manual page 25

Reference Manual
00809-0100-4825, Rev CB
RTD or ohm inputs
The transmitters accepts 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 up to 60 ohms per lead (or
the 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, 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). 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. However, a 3-wire sensor will not fully cancel lead resistance error 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
errors.
Table 2-1. Examples of Approximate Basic Error
Sensor input
4-wire RTD
3-wire RTD
2-wire RTD
Examples of approximate lead wire resistance effect calculations
Given:
Total cable length:
Imbalance of the lead wires at 20 °C:
Resistance/length (18 AWG Cu):
Temperature coefficient of Cu (
Temperature coefficient of Pt(
Change in Ambient Temperature (T
RTD Resistance at 0 °C (R


Error due to amb. temp. variation
Installation
Approximate basic error
None (independent of lead wire resistance)

± 1.0 in reading per ohm of unbalanced lead wire resistance
(Unbalanced lead wire resistance = maximum imbalance
between any two leads.)

1.0 in reading per ohm of lead wire resistance
):
Cu
):
Pt
):
o
Pt100 4-wire RTD: No lead wire resistance effect.
Pt100 3-wire RTD:
Imbalance of Lead Wires
----------------------------------------------------------------- -
=
Basic Error

=
150 m
0.5 
0.025 /m °C
0.039 / °C
0.00385 / °C
): 25 °C
amb
100  (for Pt 100 RTD)
  
R
Pt o
     T   
Imbalance of Lead Wires
Cu amb
------------------------------------------------------------------------------------------------------------------------ -
    
R
Pt o
Section 2: Installation
Table 2-1help quantify these


March 2014
17