How-To Guide  ·  Temperature Measurement  ·  Sensor Selection

RTD vs. Thermocouple: Types, Accuracy, Wiring & When to Use Each

Modules: 5069-IY4, 1769-IR6, 1769-IT6  ·  Studio 5000 Logix Designer

1. How RTDs Work

An RTD exploits the physical property that a metal’s electrical resistance increases as its temperature rises. The relationship between resistance and temperature is highly predictable, repeatable, and nearly linear — especially for platinum.

Operating Principle

1. The PLC’s input module sends a small excitation current (typically 0.5 mA or 1.0 mA) through the RTD element.
2. The module measures the voltage drop across the RTD.
3. Using Ohm’s Law (R = V ÷ I), it calculates the element’s resistance.
4. The resistance is converted to temperature using a standard linearization curve (IEC 60751 for platinum, or manufacturer-specific for nickel and copper).

The key specification is the temperature coefficient of resistance (α), which defines how many ohms the resistance changes per degree Celsius. For a standard Pt100 (Platinum 385):

α = 0.00385 Ω/Ω/°C   →   A 100 Ω element changes ~0.385 Ω per °C

At 0 °C, a Pt100 reads exactly 100.00 Ω. At 100 °C, it reads approximately 138.51 Ω.

2. RTD Types: Platinum, Nickel & Copper

Platinum RTDs

Platinum is the most common RTD element because of its excellent linearity, wide temperature range, and long-term stability. Two alpha values are used worldwide:

Supported Pt types per Rockwell Automation publication 5069-TD001 (page 29–30) and 1769-TD006 (page 43–44).

Nickel RTDs

Nickel RTDs have a higher temperature coefficient than platinum (more sensitive), making them useful where high resolution is needed over a limited temperature range. However, nickel is less linear and less stable than platinum.

Copper RTDs

Copper RTDs are the most linear of all RTD types but have the lowest temperature range and are susceptible to corrosion.

RTD Accuracy Classes (IEC 60751)

RTD sensors are sold in tolerance classes defined by IEC 60751 (Class AA, A, B and C, from tightest to loosest). The exact tolerance values are in the standard and the sensor manufacturer’s datasheet; they are not reproduced here. Choose the tighter classes for precision work and Class B for general industrial use.

3. RTD Wiring: 2-Wire, 3-Wire & 4-Wire

RTD wiring configuration is critical to measurement accuracy. The wires connecting the sensor to the module have their own resistance, and this lead resistance adds directly to the RTD reading, causing a positive temperature error.

2-Wire Configuration

The simplest connection. The module measures the RTD resistance plus the lead wire resistance. There is no way to compensate.

• Error: ~2.6 °C per ohm of lead resistance (for Pt100)
• Use when: Cable run is very short, or using Pt500/Pt1000 where lead resistance is proportionally small
• Module support: 5069-IY4 (2-wire mode, or 3-wire mode with terminals Input x− and RTD x jumpered together), 1769-IR6 (with jumper)

3-Wire Configuration

Adds a third wire that runs alongside one of the original two. The module measures the resistance of this third wire and subtracts it from the RTD measurement, assuming both lead wires have equal resistance.

• Error: Compensates for most of the lead resistance (assumes matched leads)
• Use when: Most industrial applications — the standard configuration
• Cable: Belden 9533 or equivalent 3-conductor shielded
• Module support: 5069-IY4, 1769-IR6

4-Wire Configuration

Uses two pairs of wires: one pair carries the excitation current, the other pair senses the voltage drop across the RTD. Since the sense wires carry virtually no current, their lead resistance has zero effect on accuracy.

• Error: Zero lead resistance error
• Use when: Laboratory measurement, long cable runs, or when accuracy is critical
• Cable: Belden 83503 or equivalent 4-conductor shielded
• Module support: 1769-IR6 (the 5069-IY4 supports 2-wire and 3-wire only)

Lead Resistance Impact by RTD Type

4. How Thermocouples Work

A thermocouple generates a voltage (the Seebeck voltage) at the junction of two dissimilar metals. This voltage is proportional to the temperature difference between the measurement junction (where the metals are welded together in the process) and the reference junction (where the wires connect to the PLC module).

The Three Key Principles

Measurement Equation

V_total = V_measured + V_TC(T_CJC)  →  T_measurement = T_TC(V_total)

The module measures the thermocouple voltage, converts the CJC sensor temperature to its equivalent thermocouple voltage and adds it to the measured voltage, then converts the total to temperature using the NIST ITS-90 linearization for that thermocouple type (1769-TD006, p. 45).

5. Thermocouple Types: J, K, T, E, N, R, S, B & C

Base Metal Thermocouples

Lower cost, good for general industrial use. Most common in automation applications.

Noble Metal Thermocouples

Made from platinum-rhodium alloys. Expensive but essential for high-temperature and precision applications.

Refractory Metal Thermocouples

All thermocouple types and ranges sourced from Rockwell Automation publication 5069-TD001 (page 30) and 1769-TD006 (page 45).

6. Thermocouple Wiring & Cold Junction Compensation

Wiring Best Practices

1. Use thermocouple-grade extension wire rated for the specific TC type. Use shielded twisted-pair cable.
2. Ground the shield at the module end only. Do not ground at the sensor end.
3. Keep cable runs short — thermocouple signals are in the millivolt range and susceptible to noise. The 1769-IT6 tech data recommends installing the module at least two slots away from AC power supplies to reduce electrical noise (1769-TD006, page 46).
4. Avoid running TC cables alongside power wires. Maintain at least 12 inches (30 cm) of separation.
5. Observe polarity. Reversing thermocouple wires will give erroneous readings (temperature will appear to decrease when it should increase).

Grounded vs. Ungrounded Thermocouples

Cold Junction Compensation (CJC)

The CJC sensor is built into the module (or into the terminal block). Its accuracy directly contributes to total measurement error:

CJC accuracy from 5069-TD001 (page 30) and 1769-TD006 (page 46).

7. Head-to-Head Comparison

8. Decision Guide: Which Sensor to Use

9. Allen-Bradley Temperature Modules

5069-IY4 / 5069-IY4K — Compact 5000 Universal Input

The most versatile temperature module in the Allen-Bradley lineup. Each of its 4 differential channels can be independently configured for current, voltage, RTD, or thermocouple input. You can mix signal types on a single module.

Specifications from Rockwell Automation publication 5069-TD001, pages 27–30.

1769-IR6 — Compact I/O RTD/Resistance Input

A dedicated 6-channel RTD module with 2-wire, 3-wire, and 4-wire support. The only module in the 1769 or 5069 lineup that supports 4-wire RTD connections for maximum accuracy.

Specifications from Rockwell Automation publication 1769-TD006, pages 42–44.

1769-IT6 — Compact I/O Thermocouple/mV Input

A dedicated 6-channel thermocouple module with two CJC sensors on its terminal block and millivolt input capability.

Specifications from Rockwell Automation publication 1769-TD006, pages 45–46.

10. Studio 5000 Configuration

RTD Configuration (5069-IY4)

1. In the I/O tree, double-click the 5069-IY4 module. Go to the Channel Configuration tab.
2. Set Input Type to RTD for each channel connected to an RTD sensor.
3. Select the RTD Type (e.g., “100 ohm Platinum, alpha=385”).
4. Select the Wiring Mode (2-wire or 3-wire).
5. Set the Temperature Units for the channel (°C or °F). The module returns a REAL value in those units.
6. Configure the Notch Filter frequency (50 or 60 Hz to match your mains frequency).

Thermocouple Configuration (5069-IY4)

1. Set Input Type to Thermocouple for the desired channel.
2. Select the Thermocouple Type (e.g., Type K).
3. Set the Temperature Units for the channel (the module returns a REAL value in those units).
4. Leave both cold-junction channels (CJCh00 / CJCh01) enabled; the thermistors on the RTB14CJC terminal block provide the compensation (5069-UM005, CJChxx tags).
5. Important: When connecting at least one thermocouple to the 5069-IY4, you must use the 5069-RTB14CJC terminal block (spring or screw type) — the standard 5069-RTB18 does not have CJC thermistors.

Reading Temperature in Ladder Logic

The 5069-IY4 and the 1769 temperature modules return temperature data, but in different formats. The 5069-IY4 returns a REAL value already in °C or °F, so no SCP scaling is needed. The 1769-IR6 and 1769-IT6 return INT data: in Engineering Units x1 the value is in tenths of a degree (1000 = 100.0 °C), in x10 it is whole degrees, so divide x1 data by 10 (or use SCP) before comparing to a setpoint in degrees (1769-TD006, pp. 43, 45). This example reads the 5069-IY4:

// Read RTD temperature from 5069-IY4 channel 0
MOV  Source:   Local:2:I.Ch00.Data    // Temperature in °C (REAL)
     Dest:     Oven_Temperature        // Your program tag

// High-temperature alarm
GRT  Source A: Oven_Temperature
     Source B: 450.0                    // Alarm setpoint (°C)
     OTE  High_Temp_Alarm

11. Installation Best Practices

RTD Installation

1. Use 3-wire minimum for all Pt100 installations. Reserve 2-wire for Pt500/Pt1000 or very short cable runs.
2. Match all three leads in 3-wire connections — use the same gauge, length, and routing for all three conductors.
3. Thermowell insertion depth: The RTD sensing element should be fully immersed in the process. Follow the sensor manufacturer’s minimum insertion depth.
4. Avoid self-heating: Configure 0.5 mA excitation current when possible, especially for Pt100 in still air or gas.
5. Cable impedance limit: The 5069-IY4 specifies a maximum cable impedance of 25 Ω for 3-wire mode at specified accuracy (5069-TD001, page 30).

Thermocouple Installation

1. Use the correct extension wire type. Type K extension wire for Type K thermocouples, Type J for Type J, etc. Mixing types creates an unknown error at every transition.
2. Minimize the number of junctions. Every connection point is a potential error source.
3. Use ungrounded junctions unless fast response is specifically required. Grounded junctions can create ground loops.
4. Keep the module at uniform temperature. Temperature gradients across the terminal block degrade CJC accuracy. Avoid mounting near heat sources, VFDs, or in direct sunlight.
5. Use a thermocouple-rated terminal block. The 5069-IY4 requires the 5069-RTB14CJC for thermocouple inputs; the 1769-IT6 uses the two CJC sensors fitted to its terminal block (CJC 0 / CJC 1); keep them installed.

12. Related Guides & Resources

Related PLC Exchange How-To Guides

Rockwell Automation Reference Documentation