Emerson Rosemount 644h, Reference Manual

Page: 21 / 78

Reference Manual
00809-0100-4728, Rev HA
December 2002
2-11
Models 644H and 644R
RTD or Ohm Inputs
The transmitters will accept a variety of RTD configurations, including 2-wire,
3-wire, 4-wire, and compensation loop 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
Examples of Approximate Lead Wire Resistance Effect Calculations
Given:
• Pt100 4-wire RTD: No lead wire resistance effect.
• Pt100 3-wire RTD:
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
Total cable length: 150 m
Imbalance of the lead wires at 20 °C: 1.5 Ω
Resistance/length (18 AWG Cu): 0.025
Ω
/m °C
Temperature coefficient of Cu ( α
Cu
): 0.039 Ω / Ω °C
Temperature coefficient of Pt(
α
Pt
): 0.00385
Ω
/
Ω
°C
Change in Ambient Temperature ( ∆ T
amb
): 25 °C
RTD Resistance at 0 °C (R
o
): 100
Ω
(for Pt 100 RTD)
Basic Error
Imbalance of Lead Wires
α
Pt
R
o
× ( )
------------------------------------------------------------------ =
Error due to amb. temp. variation
α
Cu
( ) ∆
T
amb
( ) ×
Imbalance of Lead Wires
( ) ×
α
Pt
( )
R
o
( ) ×
-------------------------------------------------------------------------------------------------------------------------- =