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Rosemount Models 244EH and 244ER PC-Programmable Temperature Transmitters
2-14
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:
Total cable length = 150 m
Imbalance of the lead wires at 20 °C = 0.5
Ω
Resistance/length (18 AWG Cu) = 0.025 Ω/Ω °C
Temperature coefficient of copper (
α
Cu
) = 0.0039 Ω/Ω °C
Temperature coefficient of platinum (
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)
• Pt100 4-wire RTD: No lead wire resistance effect.
•Pt100 3-wire RTD:
Lead wire imbalance seen by the transmitter = 0.5
Ω
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
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
()×
-------------------------------------------------------------------------------------------------------------------------=
Basic error
0.5 Ω
0.00385 Ω / Ω°C()100 Ω()×
--------------------------------------------------------------------------------- 1.3 °C==
Error due to amb. temp. var. of 25 °C±
0.0039 Ω / Ω°C()25 °C()× 0.5 Ω()×
0.00385 Ω / Ω°C()100 Ω()×
-------------------------------------------------------------------------------------------------------
0.13 °C±==