# Temperature Measurement: RTD and Thermocouple Selection

> Temperature is the most commonly measured process variable in industrial automation, alongside pressure, level, and flow.

- Canonical URL: https://wiki.aspotomasyon.com/article/temperature-measurement-rtd-thermocouple-selection
- Publisher: ASP OTOMASYON A.Ş. (ASP OTOMASYON Wiki)
- Category: Field Devices & Instrumentation
- Published: 2026-07-21 · Updated: 2026-09-13
- Reading time: 4 min · Language: English

Temperature is the most commonly measured process variable in industrial automation, alongside pressure, level, and flow. Selecting the correct temperature sensor — and understanding its operating principle, accuracy class, and installation requirements — is essential for process control, quality assurance, and safety. The two dominant industrial temperature sensor technologies are **Resistance Temperature Detectors (RTDs)** and **thermocouples (TCs)**.

![A measurement chain from the process to an application. Calibration, loop checks, and process validation answer different questions; retain evidence for each applicable check.](https://wiki.aspotomasyon.com/assets/diagrams/field-devices-instrumentation-context.svg)

*A measurement chain from the process to an application. Calibration, loop checks, and process validation answer different questions; retain evidence for each applicable check.*

## Resistance Temperature Detectors (RTDs)

RTDs measure temperature by exploiting the predictable change in electrical resistance of a metal (typically platinum) as temperature changes. The relationship is defined by the Callendar–Van Dusen equation and is highly repeatable.

### Pt100 — The Industrial Standard

The most common RTD is the **Pt100** — a platinum element with a resistance of 100 Ω at 0 °C. Per IEC 60751, the temperature coefficient (α) is 0.00385 Ω/(Ω·°C), giving a sensitivity of approximately 0.385 Ω/°C. Pt1000 sensors (1,000 Ω at 0 °C) offer higher sensitivity and are used in applications with long cable runs.

### RTD Element Types

| Type | Temperature Range | Accuracy | Characteristics |
| --- | --- | --- | --- |
| **Thin-film** | −50 to +500 °C (encapsulated to 600 °C) | Class A or B | Compact, cost-effective, good vibration resistance. Most common in industrial probes. |
| **Wire-wound** | −200 to +660 °C | Class A or B | Higher accuracy and stability over wide ranges. More fragile than thin-film. |
| **Coiled (strain-free)** | −200 to +850 °C | Class A | Best industrial stability. Wire expands freely, minimizing strain errors. |
| **SPRT (Standard)** | −200 to +500 °C | ±0.03 °C | Laboratory reference grade. Defines ITS-90. Not for industrial use. |

### RTD Tolerance Classes (IEC 60751)

| Class | Tolerance Formula | At 0 °C | At 100 °C |
| --- | --- | --- | --- |
| **AA** | ±(0.1 + 0.0017\|t\|) | ±0.10 °C | ±0.27 °C |
| **A** | ±(0.15 + 0.002\|t\|) | ±0.15 °C | ±0.35 °C |
| **B** | ±(0.30 + 0.005\|t\|) | ±0.30 °C | ±0.80 °C |
| **C** | ±(0.60 + 0.01\|t\|) | ±0.60 °C | ±1.60 °C |

### Wiring Configurations

- **Two-wire** — Simplest but includes lead wire resistance in measurement. Only suitable for non-critical applications.
- **Three-wire** — Industry standard. Uses a Wheatstone bridge to cancel lead resistance. Sufficient for most industrial applications.
- **Four-wire** — Highest accuracy. Separate current source and voltage measurement eliminates all lead resistance effects. Used for precision measurements.

## Thermocouples

Thermocouples measure temperature using the **Seebeck effect**: when two dissimilar metals are joined at two junctions and a temperature difference exists between the junctions, a voltage (thermoelectric EMF) is generated. Thermocouples are self-powered (no external excitation needed) and cover a much wider temperature range than RTDs.

### Common Thermocouple Types

| Type | Positive Leg | Negative Leg | Range (°C) | Sensitivity | Atmosphere |
| --- | --- | --- | --- | --- | --- |
| **Type K** | Chromel (Ni-Cr) | Alumel (Ni-Al) | −200 to +1,260 | ~41 µV/°C | Oxidizing, inert |
| **Type J** | Iron | Constantan (Cu-Ni) | −40 to +760 | ~55 µV/°C | Reducing, vacuum, inert |
| **Type T** | Copper | Constantan (Cu-Ni) | −200 to +370 | ~43 µV/°C | Oxidizing, reducing, vacuum |
| **Type E** | Chromel (Ni-Cr) | Constantan (Cu-Ni) | −200 to +870 | ~68 µV/°C | Oxidizing, inert |
| **Type N** | Nicrosil (Ni-Cr-Si) | Nisil (Ni-Si) | −200 to +1,260 | ~39 µV/°C | Oxidizing, inert (K alternative) |
| **Type S** | Pt-10%Rh | Pt | 0 to +1,480 | ~10 µV/°C | Oxidizing, inert |
| **Type R** | Pt-13%Rh | Pt | 0 to +1,480 | ~10 µV/°C | Oxidizing, inert |
| **Type B** | Pt-30%Rh | Pt-6%Rh | +200 to +1,700 | ~10 µV/°C | Oxidizing, inert |

## RTD vs. Thermocouple Selection Guide

| Criterion | RTD (Pt100) | Thermocouple (Type K) |
| --- | --- | --- |
| **Temperature range** | −200 to +850 °C | −200 to +1,260 °C (K), up to 1,700 °C (B) |
| **Accuracy** | ±0.1 to ±0.3 °C (Class A) | ±1.1 to ±2.2 °C (Standard) |
| **Stability** | Excellent (low drift) | Moderate (drift over time) |
| **Response time** | Slower (1–10 s) | Faster (0.1–5 s) |
| **Signal level** | Resistance (Ω) | Voltage (µV) |
| **Cost** | Higher | Lower |
| **Cold junction** | Not required | Required (CJC compensation) |
| **Best for** | −200 to +500 °C, high accuracy | >500 °C, fast response, cost-sensitive |

## Installation Best Practices

- **Thermowell installation** — Always install sensors in thermowells for process-isolated removal. Thermowells must be rated for process pressure, velocity (vibration), and corrosion.
- **Insertion depth** — Minimum insertion depth should be 10× the sensor diameter into the process for accurate measurement. Center of the pipe is ideal for pipes > 300 mm diameter.
- **Thermowell wake frequency** — ASME PTC 19.3 TW calculates the maximum allowable insertion length based on process velocity to prevent thermowell failure from vortex-induced vibration.
- **Cable routing** — Route thermocouple and RTD extension cables away from power cables to avoid electromagnetic interference. Use shielded cable for RTDs in high-EMI environments.
- **Transmitter location** — Mount temperature transmitters close to the sensor to convert the low-level signal (Ω or µV) to 4–20 mA or digital (HART, Fieldbus) before long cable runs.

> ASP OTOMASYON engineers select, configure, and commission temperature measurement systems for process industries across Turkey, ensuring accurate and reliable temperature control from sensor to control system.

## Apply this to your project

- Confirm the instrument tag, range, engineering unit, process connection, and environmental suitability against approved documents.
- Check the complete measurement chain, including sensor configuration, channel scaling, quality indication, and display units.
- Record reference equipment, uncertainty where relevant, test points, deviations, and the final configuration.

[Follow an ASP project guide](https://wiki.aspotomasyon.com/guides) to connect the fundamentals with implementation and acceptance.

![Engineering review sequence: Specify → Inspect → Compare → Record. Keep the recorded evidence with the accepted configuration.](https://wiki.aspotomasyon.com/assets/diagrams/field-devices-instrumentation-review.svg)

*Engineering review sequence: Specify → Inspect → Compare → Record. Keep the recorded evidence with the accepted configuration.*

## Primary references and further reading

Use the original specifications and product documentation for implementation details. The examples in this guide are illustrative engineering scenarios, not published project results.

- [FieldComm Group — HART technology](https://www.fieldcommgroup.org/technologies/hart)
- [NIST — Policy on metrological traceability](https://www.nist.gov/calibrations/traceability)

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Cite the canonical article: https://wiki.aspotomasyon.com/article/temperature-measurement-rtd-thermocouple-selection (Temperature Measurement: RTD and Thermocouple Selection, ASP OTOMASYON, updated 2026-09-13).