Thermocouple Types Explained: J, K, T, E, N, R, S, B

There are eight letter-designated thermocouple types in common industrial use — J, K, T, E, N, R, S, and B — each defined by a specific pair of alloys, a calibrated millivolt output curve, and a usable temperature range. Choosing the wrong type for your process will produce inaccurate readings or premature sensor failure, and configuring the wrong type on your PLC input card will silently corrupt every temperature value in your program.

This guide explains each type in detail, covers color codes by region, compares accuracy classes, explains extension and compensating wire, and walks through the controls-side considerations that most sensor datasheets leave out — including matching your selection to a TC input card or temperature transmitter and managing cold-junction compensation.

If you are deciding between a thermocouple and an RTD, see RTD vs Thermocouple for a direct comparison.

What Is a Thermocouple?

A thermocouple is a temperature sensor made by permanently joining two dissimilar metal wires at one end. That joined end — the hot junction or measuring junction — is placed at the point where temperature needs to be measured. The other end — the cold junction or reference junction — terminates at the measuring instrument, transmitter, or PLC input card.

When a temperature difference exists between the two junctions, the circuit generates a small electromotive force (EMF) in the millivolt range. This phenomenon is called the Seebeck effect. The magnitude and polarity of the voltage depend on the specific alloy combination and the temperature difference between the two junctions. The measurement instrument converts that millivolt signal into a temperature value using a type-specific lookup table defined by international standards (IEC 60584 and ASTM E230/E1751).

Because a thermocouple is self-powered and requires no excitation current, it is inherently simple and rugged. It has no moving parts, withstands high temperatures, and tolerates vibration that would damage a more fragile RTD. The trade-off is lower accuracy than a Pt100 RTD and the need for type-specific extension cable on every run.

How Thermocouple Types Differ

All thermocouples operate on the same Seebeck principle. What distinguishes one type from another is the alloy pair used for the two wires. Different alloy combinations produce different millivolt output curves, different usable temperature ranges, different sensitivity (how many microvolts per degree), and different immunity to oxidation, reducing gases, moisture, and nuclear radiation.

The eight standard types split into two groups:

• Base metal thermocouples (J, K, T, E, N) use relatively common metals and alloys — iron, copper, nickel-chromium, nickel-aluminium, nickel-silicon. They are inexpensive, available in many sheath diameters, and cover the vast majority of industrial processes up to approximately 1,260 °C.
• Noble metal thermocouples (R, S, B) use platinum and platinum-rhodium alloys. They are significantly more expensive, offer higher accuracy and long-term stability at very high temperatures, and are the standard choice for furnaces, glass, ceramics, and laboratory calibration applications above 1,000 °C.

Thermocouple Types: Ranges, Materials, and Applications

The table below lists the eight standard types with their positive and negative leg materials, usable temperature range, approximate sensitivity at mid-range, and primary application areas. All ranges are for mineral-insulated metal-sheathed (MIMS) construction under IEC 60584; bare-wire ranges differ.

Type	Positive leg	Negative leg	Range (continuous)	Sensitivity (approx.)	Typical use
J	Iron (Fe)	Constantan (Cu-Ni)	−40 °C to +750 °C	~52 µV/°C at 300 °C	Older machinery, plastics, reducing atmospheres
K	Chromel (Ni-Cr)	Alumel (Ni-Al)	−200 °C to +1,260 °C	~41 µV/°C at 600 °C	General purpose; most widely used type
T	Copper (Cu)	Constantan (Cu-Ni)	−200 °C to +350 °C	~47 µV/°C at 100 °C	Cryogenics, food, HVAC, laboratory
E	Chromel (Ni-Cr)	Constantan (Cu-Ni)	−40 °C to +900 °C	~68 µV/°C at 400 °C	Highest output of base types; cryogenics
N	Nicrosil (Ni-Cr-Si)	Nisil (Ni-Si)	−200 °C to +1,260 °C	~39 µV/°C at 600 °C	High-temp alternative to K; more stable
R	Pt-13%Rh	Platinum (Pt)	0 °C to +1,480 °C	~10 µV/°C at 1,000 °C	Furnaces, glass, calibration
S	Pt-10%Rh	Platinum (Pt)	0 °C to +1,480 °C	~9 µV/°C at 1,000 °C	Furnaces, primary calibration standard
B	Pt-30%Rh	Pt-6%Rh	+200 °C to +1,700 °C	~9 µV/°C at 1,000 °C	Very high-temperature furnaces, glass

Type J — Iron / Constantan

Type J is one of the oldest thermocouple types and remains common in legacy equipment — particularly older injection-moulding machines, extruders, and presses. The iron positive leg oxidises rapidly above 500 °C in air, limiting continuous service to 750 °C. It cannot be used above 760 °C because the iron undergoes a crystallographic phase change that permanently alters its calibration.

Type J is well suited to reducing atmospheres (low-oxygen environments) where Type K degrades. The high iron content makes it unsuitable for moist or humid environments due to corrosion risk. Match the PLC input card or transmitter to Type J specifically — its millivolt curve differs enough from Type K that a misconfigured card will read 20 °C to 40 °C low in the 200 °C to 400 °C range.

Type K — Chromel / Alumel

Type K is the most widely used thermocouple type in industrial automation. Its combination of broad range (−200 °C to +1,260 °C), reasonable accuracy, good oxidation resistance, and low cost makes it the default choice for general-purpose applications — kilns, boilers, HVAC systems, engine testing, food processing, and process heating.

The primary limitations of Type K are short-range ordering (a reversible metallurgical change that causes calibration drift in the 300 °C to 600 °C range on repeated cycling) and susceptibility to green rot — a form of preferential oxidation that occurs in low-oxygen, high-temperature environments. For long-term stable measurement in the 800 °C to 1,200 °C range under cycling conditions, Type N is a better choice and uses the same input card calibration class on most modern instruments.

Type T — Copper / Constantan

Type T covers −200 °C to +350 °C and is the preferred thermocouple for low-temperature applications: cryogenic research, freeze tunnels, pharmaceutical cold storage, HVAC ducts, and food processing. The copper positive leg is highly conductive and the output curve is well-characterised at sub-zero temperatures. Because both leg materials are common metals, Type T is inherently immune to most industrial electromagnetic interference.

The upper limit of 350 °C is a firm ceiling — the copper leg softens and anneals above this temperature, permanently altering calibration. Do not use Type T in any application that could approach or exceed this limit, even transiently.

Type E — Chromel / Constantan

Type E produces the highest millivolt output of any base-metal type — approximately 68 µV/°C at 400 °C compared to 41 µV/°C for Type K. This higher sensitivity makes Type E easier to read with basic instrumentation and reduces the impact of electrical noise on the signal. Type E is non-magnetic, which makes it suitable near strong magnetic fields, and it performs well at cryogenic temperatures.

Its range of −40 °C to +900 °C suits many industrial heating and drying applications. Type E is less common than K or J, so stock availability for replacement sensors and extension cable must be confirmed before specifying it for a new installation.

Type N — Nicrosil / Nisil

Type N was developed specifically to address the calibration drift problems of Type K. The addition of silicon and chromium to both legs suppresses the short-range ordering and preferential oxidation that affect Type K at high temperatures. The result is a thermocouple with a range identical to Type K (−200 °C to +1,260 °C) but significantly better long-term stability above 800 °C.

Type N is the correct upgrade path when Type K sensors in a high-temperature cycling application require frequent recalibration. Many modern PLC TC input cards and temperature transmitters carry a Type N setting alongside Type K — verify this during instrument selection.

Type R — Platinum-13%Rhodium / Platinum

Type R is a noble-metal thermocouple rated to +1,480 °C for continuous service. The positive leg contains 13% rhodium. Type R produces a slightly higher millivolt output than Type S and is more common in Europe and Asia. It is used in glass melting furnaces, heat treatment furnaces, semiconductor processing, and wherever stable, accurate measurement above 1,000 °C is required.

Noble-metal thermocouples must be protected from contamination — metallic vapours, carbon, and hydrogen all cause permanent calibration drift. Use appropriate ceramic protection tubes and avoid all contact with base metals or organic materials at temperature.

Type S — Platinum-10%Rhodium / Platinum

Type S is historically significant: it was the primary international calibration standard for temperatures between the gold point (1,064 °C) and the silver point (961 °C) before being superseded by the International Temperature Scale of 1990 (ITS-90). It remains in use as a laboratory calibration reference and in demanding industrial furnace applications.

Type S has a range identical to Type R (0 °C to +1,480 °C) and very similar output. The positive leg contains 10% rhodium rather than 13%. For most new process installations, Type R and Type S are interchangeable in terms of application, but the millivolt curves are not the same — you must configure the correct type on any instrument or transmitter.

Type B — Platinum-30%Rhodium / Platinum-6%Rhodium

Type B covers the highest temperature range of any standard thermocouple: +200 °C to +1,700 °C continuous, with some special constructions rated to +1,820 °C. Both legs contain platinum-rhodium alloy, which gives it exceptional stability but also results in a very low Seebeck coefficient — approximately 0 µV/°C near room temperature.

This near-zero output at low temperatures is actually useful: Type B reads essentially zero below about 50 °C regardless of cold-junction temperature, which means cold-junction compensation errors have minimal effect on accuracy at operating temperature. Type B is the standard choice for very high-temperature processes such as glass tank furnaces, platinum-rhodium alloy casting, and the semiconductor industry.

Thermocouple Color Codes

Thermocouple lead wires and extension cables are color-coded to identify type and polarity, but the color standards vary by region. Using the wrong extension cable — or misidentifying polarity — introduces an error junction and corrupts readings.

The four main standards are IEC 60584-3 (international, used in Europe and most of Asia), ANSI/ASTM (United States), BS 1843 (United Kingdom, now largely superseded by IEC), and JIS C1610 (Japan).

Type	IEC 60584-3 (EU) overall / + / −	ANSI (US) overall / + / −	BS 1843 (UK) overall / + / −
J	Black / White / Blue	Black / White / Red	Black / Yellow / Blue
K	Green / Green / White	Yellow / Yellow / Red	Red / Brown / Blue
T	Brown / Brown / White	Blue / Blue / Red	Blue / White / Blue
E	Violet / Violet / White	Violet / Violet / Red	Brown / Brown / Blue
N	Pink / Pink / White	Orange / Orange / Red	—
R / S	Orange / Orange / White	Green (R/S) / Black / Red	Green / White / Blue
B	Grey / Grey / White	Grey / Grey / Red	—

Key point: in the ANSI standard, the negative leg is always red regardless of type. In IEC, the negative leg is always white. Verify the standard applicable to your equipment before assuming any color. When extension cable marking is worn or absent, use a millivoltmeter to confirm polarity before connecting to an input card.

Accuracy Classes

IEC 60584-2 defines accuracy classes for thermocouples. The class specifies the maximum permissible deviation from the reference millivolt curve over the stated temperature range.

Class	Accuracy (whichever is greater)	Applicable types	Temperature range
Class 1	±1 °C or ±0.4% of reading	J, K, T, E, N, R, S	Narrower range within type limits
Class 2	±2.5 °C or ±0.75% of reading	J, K, T, E, N, R, S, B	Full range per type
Class 3	±5 °C or ±1.5% of reading	B, E, J, K, N, R, S, T	Extended low-temperature range

Class 2 is the standard industrial grade for most applications — it is what you receive unless you specify otherwise. Class 1 thermocouples cost more and are worth specifying for precision temperature control, material testing, or processes where a 2 °C error has measurable quality impact.

Noble-metal types (R, S, B) are available as Special Limits of Error (SLE) or Class 1 sensors for calibration work, typically ±0.6 °C or ±0.1% of reading.

Extension and Compensating Wire

A thermocouple circuit must use the correct type of extension or compensating cable from the sensor head to the measuring instrument. Using ordinary copper wire creates an unintended thermocouple junction at the transition point and introduces a measurement error proportional to the temperature at that junction.

There are two cable categories:

• Extension wire uses the same alloys as the thermocouple itself. It is the most accurate solution and is mandatory for Types T and E. Extension wire is color-coded to match the thermocouple type standard.
• Compensating wire uses lower-cost alloys that approximate the thermocouple's millivolt output over a limited temperature range (typically 0 °C to 200 °C). Compensating cable is used with expensive noble-metal thermocouples (R, S, B) where using the actual platinum-rhodium alloy for the entire cable run would be prohibitively expensive.

Critical rule: compensating cable is only valid within its rated compensation range. If the cable route passes through an area hotter than the compensating range, use extension wire or a head-mount transmitter to convert to a 4-20 mA signal at the sensor, and run instrument cable to the PLC.

Always route thermocouple and extension cable in separate conduit from power cables. Induced noise on the millivolt signal is amplified by the high-gain amplifier in the input card and produces temperature reading noise.

How to Choose a Thermocouple Type

Selecting the right thermocouple type involves five criteria evaluated in this order:

1. Process temperature range. Match the type to your process maximum with adequate margin. Do not use Type J above 700 °C, Type T above 300 °C, or base-metal types above 1,260 °C.

2. Process atmosphere. Type K and N suit oxidising (air) atmospheres. Type J suits reducing (low-oxygen) atmospheres. Noble-metal types require ceramic protection from any contaminating gas. Hydrogen, carbon, and metallic vapours degrade platinum sensors irreversibly.

3. Required accuracy. If ±1 °C matters, specify Class 1 or consider an RTD below 600 °C. For general process control where ±2.5 °C is acceptable, Class 2 is sufficient.

4. Physical constraints. Thermocouple response time depends on mass — bare junctions respond in milliseconds while heavy MIMS assemblies may take several seconds. For fast-cycling processes, choose a thin-sheath assembly.

5. Cost and replacement frequency. Base-metal types are low-cost and replaced frequently in consumable-sheath applications (e.g., molten metal). Noble-metal types represent a capital investment and require protection tubes.

Default recommendation: Use Type K Class 2 for general industrial processes from ambient to 1,000 °C in oxidising or inert atmospheres. Use Type J for legacy equipment with iron-constantan wiring already installed. Use Type T for food, pharmaceutical, and low-temperature HVAC. Use Type N when Type K shows calibration drift on high-temperature cycling. Use Type R or S for precision furnace control above 1,000 °C.

The Controls View: PLC Input Cards and Cold-Junction Compensation

This section is where most thermocouple guides stop short. Getting the sensor right is only half the problem — the input card configuration and cold-junction compensation determine whether the measurement is accurate end-to-end.

Matching the TC type on the input card

Every PLC thermocouple input module or temperature transmitter must be configured with the correct thermocouple type. The card applies a type-specific linearisation algorithm — a polynomial approximation of the IEC 60584 millivolt table — to convert the incoming voltage to a temperature reading. If the card is set to Type K but a Type J sensor is connected, the instrument reads approximately −3 °C to −20 °C low across the 200–600 °C range. The error is not constant, it is temperature-dependent, and it will not trip any diagnostic alarm.

On Allen-Bradley CompactLogix and ControlLogix platforms, the thermocouple type is set in the module's channel configuration within Studio 5000 Logix Designer. Set the Input Type parameter (e.g., "Thermocouple Type J", "Thermocouple Type K") per channel. On Siemens S7-1200 and S7-1500 with SM 1231 TC modules, the sensor type is configured in the module properties in TIA Portal under the relevant channel's Process Value Type. On Mitsubishi iQ-R with temperature input modules (R60TCTRT2TT2 or similar), the sensor type is set in the buffer memory parameters via GX Works3. Always verify this setting matches the physical sensor before commissioning.

Cold-junction compensation

A thermocouple measures the temperature difference between its hot junction and its cold junction — not the absolute process temperature. For the reading to be accurate, the instrument must know the temperature at the cold junction and add it to the measured differential.

Modern PLC input cards and temperature transmitters perform automatic cold-junction compensation (CJC) using a small thermistor or semiconductor temperature sensor built into the module's terminal block. This sensor continuously measures the terminal temperature. The module firmware adds the terminal temperature to the computed differential to give the absolute process temperature.

CJC works well under stable conditions. Three failure modes to be aware of:

• Thermal gradient across the terminal block. If one end of a multi-channel module is near a heat source, CJC accuracy degrades for channels at the hot end. Maintain even terminal temperature with ventilation and adequate panel spacing.
• Incorrect extension cable. If copper wire was used instead of proper thermocouple extension cable, the uncompensated transition junction temperature is not measured and the CJC correction is applied in the wrong place.
• Open thermocouple detection. Most TC input cards drive the input toward the maximum positive range (upscale burnout) or maximum negative range (downscale burnout) on an open-circuit fault, depending on the module's burn-out detection setting. Confirm which direction your card fails and set the PLC's high/low alarm limits accordingly so an open TC triggers an alarm rather than silently reading maximum scale.

Signal filtering and noise

Thermocouple signals are in the millivolt range and susceptible to electrical noise from VFDs, contactors, and high-current cables. Most input cards include a configurable digital filter — a first-order or averaging filter with adjustable time constant. A 500 ms to 2,000 ms filter setting is typical for process temperature measurement and eliminates most noise-induced jitter. For fast processes where temperature changes matter on a 100 ms timescale, reduce the filter time constant and route the extension cable more carefully.

For an overview of how thermocouples relate to the wider sensor portfolio used in industrial automation, see Types of Industrial Sensors.

Frequently Asked Questions

What are the thermocouple types?

The eight standard letter-designated thermocouple types are J, K, T, E, N, R, S, and B. Types J, K, T, E, and N are base-metal thermocouples using alloys such as iron, copper, nickel-chromium, and nickel-aluminium. Types R, S, and B are noble-metal thermocouples using platinum and platinum-rhodium alloys. Each type is defined by a specific alloy pair, an IEC 60584 millivolt output table, and a usable temperature range.

What is a Type K thermocouple?

A Type K thermocouple uses Chromel (nickel-chromium alloy, positive leg) and Alumel (nickel-aluminium alloy, negative leg). It covers −200 °C to +1,260 °C, produces approximately 41 µV/°C sensitivity at 600 °C, and is the most widely used thermocouple type in industrial automation. Its combination of broad range, oxidation resistance in air, and low cost makes it the default general-purpose choice for boilers, kilns, HVAC, and process heating.

Which thermocouple type has the widest temperature range?

Type B has the highest maximum temperature rating of any standard type — +1,700 °C continuous (and up to +1,820 °C in some special constructions). However, it has essentially zero output below 50 °C, making it unsuitable for low-temperature measurement. Type K and Type N cover the widest practical range for general industrial use: −200 °C to +1,260 °C.

How do you choose a thermocouple type?

Start with the process maximum temperature and eliminate types that cannot handle it. Then consider the atmosphere — Type K and N suit oxidising environments; Type J suits reducing atmospheres; noble-metal types require protection from contaminating gases. Next, check accuracy requirements: Class 2 (±2.5 °C) is standard; specify Class 1 if tighter tolerances are needed. Finally, confirm your PLC input card or transmitter supports the selected type before ordering sensors and extension cable.

Do all thermocouple types use the same extension cable?

No. Each type requires its own type-matched extension or compensating cable. Using generic copper wire instead introduces an uncompensated thermocouple junction at the cable transition point and creates a temperature-dependent measurement error. Extension cables are color-coded by type, but the color standard varies by region — always verify against IEC 60584-3 (Europe), ANSI/ASTM (USA), or JIS C1610 (Japan) as applicable to your installation.