Emerson 644 manual RTD or Ohm Inputs, Sensor Lead Wire Resistance Effect- RTD Input

Reference Manual

00809-0400-4728, Rev AA June 2011

Rosemount 644

RTD or Ohm Inputs

The transmitters will accept a variety of RTD configurations, including 2-wire, 3-wire, or 4-wire. 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

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:	1.5 Ω
Resistance/length (18 AWG Cu):	0.025	Ω/m °C
Temperature coefficient of Cu (αCu):	0.039	Ω/Ω °C
Temperature coefficient of Pt(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:

Imbalance of Lead Wires

Basic Error = -----------------------------------------------------------------

(αPt × Ro)

(αCu) × (ΔTamb) × (Imbalance of Lead Wires) Error due to amb. temp. variation = ------------------------------------------------------------------------------------------------------------------------

(αPt) × (Ro)

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