Fisher 244ER Sensor Lead Wire Resistance Effect RTD Input, Given, Total cable length = 150 m

Rosemount Models 244EH and 244ER PC-Programmable Temperature Transmitters

Table 2-1. Examples of Approximate Basic Error

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.

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

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 (αCu) = 0.0039 Ω/Ω °C Temperature coefficient of platinum (αPt) = 0.00385 Ω/Ω °C Change in Ambient Temperature (ΔTamb) = 25 °C

RTD Resistance at 0 °C (Ro) = 100 Ω (for Pt 100 RTD)

•Pt100 4-wire RTD: No lead wire resistance effect.

•Pt100 3-wire RTD:

Basic Error =	Imbalance------------------------------of-------Lead---------------Wires-------------
	(αPt × Ro)
	(αCu) × (ΔTamb) × (Imbalance of Lead Wires)
Error due to amb. temp. variation = ----------------------------------------			-----(---α----Pt-----)---×-----(--R-----o---)---------------------------------------------
Lead wire imbalance seen by the transmitter = 0.5 Ω
Basic error =	0.5 Ω		= 1.3 °C
	(---0.00385------------------------Ω------/----Ω-------°---C-----)---×-----(---100------------Ω-----)
Error due to amb. temp. var. of ± 25 °C
= (---0.0039----------------------Ω-----/----Ω-------°---C-----)---×-----(--25---------°---C-----)---×-----(---0.5----------Ω-----)-		= ±0.13 °C
(0.00385 Ω / Ω °C) × (100 Ω)

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