RTD vs Thermocouple: Which Temperature Sensor Should You Use?

How RTDs work

An RTD (Resistance Temperature Detector) measures temperature by exploiting the well-characterised relationship between temperature and electrical resistance in pure metals. As temperature increases, atomic vibration in a metal lattice increases, increasing the resistance to electron flow.

Platinum is the preferred material for precision RTDs because it has a highly repeatable, nearly linear resistance-temperature relationship, excellent chemical stability, and can be produced in high purity. The IEC 60751 standard defines the resistance-temperature curve for platinum RTDs using the Callendar-Van Dusen equation:

For temperatures from 0 °C to 850 °C: R(T) = R₀ × (1 + A×T + B×T²)

Where R₀ is the resistance at 0 °C, and the IEC coefficients are A = 3.9083 × 10⁻³ °C⁻¹ and B = −5.775 × 10⁻⁷ °C⁻². This produces the "European" or "DIN curve", with a mean temperature coefficient of approximately 0.003850 Ω/Ω/°C between 0 and 100 °C.

The PLC or transmitter measures the RTD resistance using a constant-current source (typically 1 mA) and measures the resulting voltage. Higher measurement current increases the signal but also causes self-heating — the current through the RTD element dissipates power (I²R) which raises the element temperature slightly above ambient. This self-heating error is why RTD measurement current is kept low.

PT100, PT1000, and accuracy classes

PT100 has a nominal resistance of 100 Ω at 0 °C, making it the most common industrial RTD. Its 0.385 Ω/°C sensitivity means a 1 °C change produces only a 0.385 Ω change in resistance. This relatively low sensitivity means lead wire resistance (even a few ohms) can cause significant measurement errors — requiring 3-wire or 4-wire connections for accuracy.

PT1000 has a nominal resistance of 1000 Ω at 0 °C. Its 10× higher resistance means lead wire resistance is proportionally less significant, and it can often be used with 2-wire connections for adequate accuracy. PT1000 is common in HVAC and building automation; less common in industrial process control.

IEC 60751 accuracy classes

Class	Tolerance at 0 °C	Tolerance at 100 °C	Typical use
AA	±0.10 °C	±0.25 °C	Laboratory, calibration reference
A	±0.15 °C	±0.35 °C	High-accuracy process, pharmaceutical
B	±0.30 °C	±0.80 °C	General industrial process control
C	±0.60 °C	±1.60 °C	Non-critical industrial, HVAC

Class B (tolerance formula: ±[0.30 + 0.005 × |T|] °C) is the most common in industrial control panels and process plants. Class A is used where accuracy is critical.

RTD wiring: 2-wire, 3-wire, 4-wire

Because the RTD element resistance at 0 °C is only 100 Ω for a PT100, lead wire resistance can be a significant source of error. A typical copper wire has about 0.017 Ω/m at 20 °C, so 10 m of 0.5 mm² wire (each way) adds approximately 0.34 Ω — equivalent to about 0.88 °C error for a PT100.

2-wire connection

The two lead wires are connected in series with the RTD element. The measuring device cannot distinguish between lead wire resistance and RTD resistance. Acceptable only for very short cable runs (under 1–2 m) or when using PT1000 where lead resistance is proportionally smaller. Not recommended for industrial process measurement.

3-wire connection

A third wire is added at one end of the RTD. The measuring device uses a Wheatstone bridge circuit to subtract the lead resistance of one wire from the measurement (assuming all three wires have equal resistance). The assumption of equal resistance is only valid when all three wires are the same gauge and length and at the same temperature. 3-wire connection is the industrial standard for most applications — it provides adequate accuracy without the cost of 4-wire connections.

4-wire connection

Two pairs of wires: a current pair that drives measurement current through the element, and a voltage pair that measures the voltage drop across only the element (not the leads). The measuring device has high input impedance on the voltage pair, so negligible current flows through the voltage leads and their resistance has no effect on the measurement. This is the Kelvin (4-wire) sensing method and provides the highest accuracy, eliminating lead resistance error entirely. Used in laboratory instruments, custody transfer measurement, and pharmaceutical applications.

How thermocouples work

A thermocouple consists of two dissimilar metal wires joined at one end (the measurement junction, also called the hot junction). When there is a temperature difference between the measurement junction and the other ends of the wires (the reference junction, also called the cold junction), a voltage is generated proportional to that temperature difference. This is the Seebeck effect, discovered in 1821.

The voltage produced is small — a Type K thermocouple produces approximately 41 µV/°C. The measuring instrument converts this voltage to temperature using the thermocouple's standardised voltage-temperature tables (IEC 60584). A critical requirement: the instrument must also measure the temperature at the reference junction (cold junction) and add it to the calculated temperature difference. This is called cold-junction compensation (CJC). All thermocouple input modules include CJC circuitry.

If the thermocouple wires are connected to the instrument using standard copper wire instead of matching thermocouple extension wire, the copper-to-thermocouple junctions at the terminal strip create additional thermocouple junctions that introduce errors. This is why correct extension wire is mandatory.

Thermocouple types: J, K, T, E, N, R, S, B

IEC 60584 standardises thermocouple types by letter designation. Each type uses a different pair of alloys with different voltage-temperature characteristics, accuracy tolerances, and suitable temperature ranges.

Type	Materials (+/−)	Range	Sensitivity	Notes
K	Chromel / Alumel	−200 to +1260 °C	~41 µV/°C	Most common general-purpose type worldwide. Susceptible to oxidation above 800 °C.
J	Iron / Constantan	−40 to +750 °C	~52 µV/°C	Common in older US industrial installations. Iron rusts above 500 °C; limited upper range.
T	Copper / Constantan	−200 to +350 °C	~43 µV/°C	Excellent for sub-zero applications. Copper conductor limits upper range.
E	Chromel / Constantan	−40 to +900 °C	~68 µV/°C	Highest sensitivity of base-metal types. Good stability. Less common.
N	Nicrosil / Nisil	−200 to +1300 °C	~39 µV/°C	More stable than Type K at high temperatures. Growing use in demanding applications.
R	Pt13%Rh / Pt	−50 to +1768 °C	~11 µV/°C	Noble metal, high cost. Accurate at high temperatures. Used in glass, ceramics, steel.
S	Pt10%Rh / Pt	−50 to +1768 °C	~10 µV/°C	Historical standard for high-temp calibration (the old 0 °C ice point standard).
B	Pt30%Rh / Pt6%Rh	+100 to +1820 °C	~6 µV/°C	Very high temperatures only; near-zero sensitivity below 100 °C.

For most industrial applications, Type K is the default choice: it has the widest temperature range of the base-metal types, is well-supported by all PLC input modules and transmitters, and thermocouple wire is readily available. Type J is common in existing North American installations and its replacement stock is widely stocked.

Thermocouple wiring and extension wire

Thermocouple wires must be run continuously from the measurement junction to the measuring instrument using either thermocouple wire (the same alloys as the thermocouple) or compensating extension wire (a different alloy with similar Seebeck properties, less expensive). Introducing any other metal at an intermediate junction creates an unwanted thermocouple junction at that point.

Extension wire colour codes vary by country and are specified in ASTM E230 (US), IEC 60584-3 (international), and BS EN 50446 (UK). Always verify the colour code for your region and type combination — colour conventions are not consistent internationally. Label all thermocouple cables with the type (J, K, T) at both ends.

Shielding: thermocouple signals are typically in the millivolt range and are susceptible to electrical interference. Use shielded cable for runs in cable trays with power cables. Connect the shield drain wire to instrument common ground at one end only (to avoid ground loops). Avoid running thermocouple cables parallel to high-voltage power cables.

Side-by-side comparison

Property	RTD (PT100)	Thermocouple (Type K)
Accuracy	±0.3 °C (Class B) at 0 °C	±1.5 °C (Class 1) up to 375 °C
Temperature range	−200 to +850 °C (standard element)	−200 to +1260 °C
Long-term stability	Excellent (platinum is stable)	Drifts with oxidation and grain growth at high temperatures
Response time	Slower (mass of element and protection tube)	Faster (especially bare wire junctions)
Vibration resistance	Poor (platinum wire element is fragile)	Good (wire is robust)
Cost (sensor)	Higher (platinum material)	Lower (base-metal alloys)
Wiring complexity	3-wire or 4-wire required for accuracy	2-wire (matched extension wire required)
Cold-junction compensation	Not required	Required (built into input module)
Self-heating error	Possible if excitation current too high	None (voltage source, not resistive)
Open-circuit detection	Easy (resistance goes to ∞ or 0)	Input module senses open circuit but may read ambient temperature

Decision guide

Choose an RTD when:

• Accuracy is the primary requirement (food, pharmaceutical, chemical, calibration)
• Temperature is below 600 °C
• The sensor will not be subjected to severe mechanical vibration or shock
• Long-term stability without recalibration is important
• You need to measure small temperature differences accurately (RTDs provide ~5× better resolution)

Choose a thermocouple when:

• Temperature exceeds 850 °C (kilns, furnaces, flue gas, steel production)
• The sensor is subject to vibration or mechanical stress that would destroy an RTD element
• Very fast response time is required (bare or grounded junction thermocouple in milliseconds vs seconds for sheathed RTDs)
• Cost per point must be minimised for many measurement locations
• Replacing existing Type J/K installations where wiring is already in place

Frequently asked questions