RTD vs Thermocouple: Differences, Accuracy, and Which to Use
RTD vs thermocouple compared — temperature range, accuracy, cost, response time, and wiring — plus how each connects to a PLC or temperature transmitter.
RTDs are more accurate and stable; thermocouples cover a wider temperature range and respond faster. For precision measurement below 600 °C — pharmaceutical, food, HVAC — choose an RTD (typically Pt100 or Pt1000). For high-temperature processes, fast-changing conditions, or cost-constrained installations, choose a thermocouple (typically Type K or Type J).
Both sensors feed a PLC either directly through a dedicated analog input card or through a temperature transmitter that converts the signal to a standard 4-20mA loop.
Quick Comparison: RTD vs Thermocouple
| Parameter | RTD (Pt100) | Thermocouple (Type K) |
|---|---|---|
| Temperature range | −200 °C to +850 °C | −200 °C to +1,260 °C |
| Accuracy | ±0.1 °C to ±0.3 °C (Class B) | ±1 °C to ±2.2 °C typical |
| Linearity | Near-linear over range | Non-linear; requires lookup table |
| Response time | Slower (seconds) | Faster (milliseconds for bare wire) |
| Self-powered | No — requires excitation current | Yes — generates its own voltage |
| Wiring | 2-wire, 3-wire, or 4-wire | 2-wire (extension cable required) |
| Cost | Higher | Lower |
| Durability | Fragile element; vibration-sensitive | Rugged; survives harsh environments |
| Long cable runs | Lead resistance causes error (use 3/4-wire) | Extension wire maintains accuracy |
| PLC input card | RTD input module | Thermocouple input module or mV input |
What is an RTD?
An RTD (Resistance Temperature Detector) is a temperature sensor whose electrical resistance changes predictably with temperature. The sensing element is a fine coil or thin-film of pure metal — most commonly platinum — deposited on a ceramic or glass substrate.
The most common industrial RTD is the Pt100: platinum wire with a resistance of exactly 100 ohms at 0 °C. The resistance increases approximately 0.385 ohms per degree Celsius (for the standard IEC 60751 European curve, often called the DIN curve). A Pt1000 works on the same principle but starts at 1,000 ohms at 0 °C, making it easier to read in environments with significant lead resistance.
Other RTD materials exist — nickel (Ni) and copper (Cu) elements are used in some legacy HVAC and motor-winding applications — but platinum remains the industrial standard due to its chemical stability, repeatability, and the wide range of certified calibrations available.
RTDs do not generate their own signal. A measurement circuit (the input card or transmitter) passes a small excitation current through the element, then measures the resulting voltage drop to calculate resistance. This dependency on an external excitation source is the key hardware difference from thermocouples.
What is a Thermocouple?
A thermocouple is formed by joining two dissimilar metals at one end. When a temperature difference exists between the hot junction (the measuring tip) and the cold junction (the terminal block or transmitter), the sensor generates a small electromotive force (EMF) — typically in the millivolt range. This is the Seebeck effect.
Because a thermocouple is self-powered, it requires no excitation and can operate in environments where wiring a power supply is impractical. The generated voltage is non-linear and small, so the measurement circuit must apply a cold-junction compensation (CJC) correction and a type-specific linearization table to convert the millivolt signal into a temperature reading.
Thermocouple extension cable uses the same alloy pair as the sensing element itself. Using ordinary copper wire for extension introduces an unintended junction and creates measurement errors. Always use the correct compensating or extension cable for the thermocouple type installed.
How Each Sensor Works
RTD operating principle
When temperature rises, lattice vibrations in the platinum crystal structure increase, impeding electron flow and raising resistance. This relationship is described by the Callendar-Van Dusen equation, but for practical purposes, modern input cards and transmitters store the full IEC 60751 lookup table and linearize the output in firmware.
The measurement accuracy depends heavily on the wiring configuration:
- 2-wire RTD: lead resistance is included in the measurement — introduces error that grows with cable length; suitable only for short runs where precision is not critical.
- 3-wire RTD: an extra wire allows the circuit to measure and subtract one lead resistance, assuming the leads are equal. This is the most common industrial configuration and eliminates most of the lead-resistance error.
- 4-wire RTD: separate current-supply and voltage-sense pairs completely eliminate lead resistance from the measurement. Used in laboratory and precision applications where the highest accuracy is required.
For automation applications, 3-wire Pt100 is the default unless the engineering specification states otherwise.
Thermocouple operating principle
The thermocouple produces a voltage proportional to the temperature difference between the measuring junction and the reference junction. In modern transmitters and PLC input cards, an onboard CJC sensor (usually a small precision thermistor or RTD) measures the temperature at the terminal block. The firmware adds the CJC correction to the measured EMF before converting to temperature.
Poor CJC performance is a common source of thermocouple error in practice — if the terminal block is in a drafty enclosure or exposed to sunlight, CJC errors can exceed the inherent thermocouple tolerance.
RTD Wiring in Detail
2-wire configuration
The simplest wiring: two conductors connect the RTD element to the input terminal. The measured resistance includes both lead resistances in series. A 10-meter run of 0.5 mm² copper cable adds roughly 0.7 ohms, which equates to approximately 1.8 °C of error on a Pt100. Acceptable only for short runs or non-critical monitoring.
3-wire configuration
A third conductor is added, typically at the measurement end, connecting to a separate terminal on the input card. The card uses a Wheatstone bridge circuit to measure the resistance of one lead and cancel it from the total measurement. This is the standard configuration for industrial RTD installations. Most PLC RTD input modules expect 3-wire connection.
When wiring a 3-wire RTD, the two leads on the same side of the element (typically colored red-red-white or red-white-white depending on the manufacturer) connect to terminals A and B; the single wire connects to terminal C. Refer to the module manual for the exact terminal assignment.
4-wire configuration
Four separate conductors: two carry the excitation current, two measure the voltage across the element. Because the voltage-sensing terminals draw negligible current, lead resistance has no effect on the reading. Required by IEC 60751 for the highest accuracy classes. Common in laboratory instruments, pharmaceutical validation, and metrology applications.
Thermocouple Types: J, K, T, E
The four types most commonly found in industrial PLC installations:
Type K (Chromel-Alumel) is the general-purpose workhorse. Range −200 °C to +1,260 °C; sensitivity approximately 41 µV/°C. Suitable for oxidizing atmospheres. The most stocked type globally; default choice when no other constraint applies.
Type J (Iron-Constantan) covers −40 °C to +750 °C; sensitivity approximately 55 µV/°C — higher output than Type K in the lower range. Limited to reducing or inert atmospheres because the iron conductor oxidizes above 550 °C in air. Common in older North American equipment and plastics processing.
Type T (Copper-Constantan) covers −200 °C to +350 °C; well suited for cryogenic and food applications. Copper conductor limits the upper range but provides excellent stability at low temperatures.
Type E (Chromel-Constantan) produces the highest output of the four types (approximately 68 µV/°C), improving signal-to-noise ratio. Range −200 °C to +900 °C. Good choice in environments with electrical noise, as the higher signal voltage is less susceptible to interference.
Always verify the thermocouple type matches the PLC input card configuration. Configuring a Type J card for a Type K sensor (or vice versa) introduces a systematic error that increases with distance from 0 °C.
When to Use an RTD vs a Thermocouple
Choose an RTD when:
- Accuracy is the priority — pharmaceutical, food processing, laboratory, cleanroom
- Temperature range is below 600 °C — the Pt100 is well-characterized in this range
- Long-term stability is required — RTDs drift significantly less over years of service
- The process involves slow, steady-state temperatures — where the slower thermal response is not a limitation
- Regulatory compliance demands traceable calibration — IEC 60751 Classes AA, A, B, and C are internationally recognized
Choose a thermocouple when:
- Temperature exceeds 850 °C — molten metal, kilns, furnaces, gas turbine exhausts
- Fast response time is critical — combustion monitoring, safety trips, exothermic reactions
- The environment involves vibration or mechanical shock — thermocouples are more physically rugged
- Cost per point drives the decision — large installations with hundreds of measurement points
- Installation in tight spaces — small-diameter bare-wire thermocouples (0.5 mm) are available for surface or insertion measurements where an RTD element would not fit
For most standard industrial process control loops — reactor jacket temperature, pasteurizer temperature, HVAC duct measurement, chiller supply temperature — a 3-wire Pt100 RTD connected to a PLC RTD input card or a DIN-rail temperature transmitter is the right default. See types of industrial sensors for a broader survey of sensor technologies used alongside PLCs.
Connecting RTDs and Thermocouples to a PLC
Direct PLC input cards
Most major PLC manufacturers offer dedicated temperature input modules:
- RTD input modules supply the excitation current, handle 2/3/4-wire compensation, and linearize the Pt100/Pt1000 (or Ni/Cu) curve in firmware. The module outputs a scaled integer (typically in tenths of a degree or hundredths of a degree) to the process image.
- Thermocouple input modules include onboard CJC, apply type-specific linearization for J/K/T/E/R/S/B types, and output a scaled integer. Some modules support millivolt input for non-standard sensors.
These modules are covered in the PLC programming basics fundamentals guide alongside other analog I/O types.
When using direct input cards, the PLC program reads the raw integer from the input word and applies a scaling block (or simply uses the already-scaled engineering-unit value if the card supports it) to produce a process variable in degrees Celsius or Fahrenheit for the PID or monitoring logic.
Temperature transmitters and 4-20mA output
A temperature transmitter (also called a temperature head transmitter or DIN-rail transmitter) accepts the RTD or thermocouple directly at its input terminals, performs all CJC, linearization, and scaling internally, and outputs a standard 4-20 mA signal proportional to the configured temperature span.
This approach has several advantages:
- The PLC only requires a standard analog input card (0-10 V or 4-20 mA), which is less expensive than a dedicated temperature card.
- Long cable runs between the field device and the control panel carry a robust current signal rather than the low-level millivolt thermocouple signal or the resistance signal susceptible to noise.
- Transmitter configurations (type, range, alarm setpoints) are set via HART or a configuration tool at the transmitter rather than in the PLC program.
- A single input card type serves all field instruments — pressure transmitters, flow transmitters, and temperature transmitters all share the same 4-20 mA infrastructure.
The trade-off is an additional device in the signal chain, adding a potential failure point and a calibration item.
For installations where the temperature measurement point is more than 30 meters from the control panel, or where the cable route passes through electrically noisy areas (variable-frequency drives, power cables), a temperature transmitter with 4-20 mA output is the preferred architecture.
For PLC communication protocols and analog signal integration, including how scaled values are mapped across fieldbus networks, refer to the dedicated communications guide.
Isolation and grounding
Both RTDs and thermocouples can introduce ground loops if the sensor is in contact with grounded metal (process vessels, heat exchangers). Galvanically isolated input cards or isolated temperature transmitters break the ground loop and eliminate common-mode interference. This is a common source of noisy or drifting temperature readings that can be misdiagnosed as a faulty sensor.
Frequently Asked Questions
What is the difference between an RTD and a thermocouple?
An RTD measures temperature by detecting a change in electrical resistance in a metal element (usually platinum). A thermocouple measures temperature by detecting the small voltage generated at the junction of two dissimilar metals. RTDs are more accurate and stable; thermocouples cover higher temperatures and respond faster. Both require an input card or transmitter to convert their raw signal into a usable process variable.
Which is more accurate, an RTD or a thermocouple?
RTDs are significantly more accurate. A Class B Pt100 RTD has a tolerance of ±0.3 °C at 0 °C, tightening to ±0.1 °C for Class A. A standard Type K thermocouple carries a tolerance of ±1.5 °C or ±0.4% of the reading, whichever is greater. For precision applications — pharmaceutical, food safety, calibration — RTDs are the standard. For high-temperature or rugged applications where ±2 °C is acceptable, thermocouples are appropriate.
When should you use a thermocouple instead of an RTD?
Use a thermocouple when the process temperature exceeds 850 °C (above the reliable range of standard Pt100 RTDs), when fast response time is required (bare-wire thermocouples respond in milliseconds), when the installation is subject to vibration or physical abuse, or when the cost of large numbers of measurement points is a constraint. Furnaces, kilns, combustion systems, and molten-metal applications are natural thermocouple applications.
What is a Pt100?
A Pt100 is the most common type of RTD. "Pt" refers to platinum — the sensing element material — and "100" refers to its resistance of 100 ohms at 0 °C. The resistance increases predictably as temperature rises, following the IEC 60751 standard curve. The Pt100 is available in 2-wire, 3-wire, and 4-wire versions, with Class AA, A, and B accuracy grades. The Pt1000 is identical in principle but has a resistance of 1,000 ohms at 0 °C, making it less sensitive to lead resistance and suitable for longer cable runs.
Can I use a standard analog input card for an RTD or thermocouple?
Not directly. Standard 0-10 V or 4-20 mA analog input cards cannot accept the millivolt signal of a thermocouple or the resistance signal of an RTD. You need either a dedicated temperature input card that handles the sensor type natively, or a temperature transmitter that converts the sensor signal to a 4-20 mA output before it reaches the PLC.
What causes a thermocouple to read incorrectly?
The most common causes are: incorrect extension cable (using copper instead of the correct alloy pair), a damaged or corroded junction, cold-junction compensation error (drafty terminal block), incorrect thermocouple type configured on the input card, and ground loops from a grounded sensor in contact with a grounded vessel. Check each of these in sequence before replacing the sensor.
