Diagram of a Thermocouple

Diagram of a Thermocouple — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable diagram of thermocouple: download the diagram as SVG or open it and print to paper or PDF.

A technical reference for thermocouple diagrams explaining the Seebeck effect, type designations, temperature ranges, cold-junction compensation, and correct connection methods for temperature measurement circuits.

A thermocouple is a temperature sensor consisting of two dissimilar metal conductors joined at one end (the hot junction or measuring junction). When a temperature difference exists between the hot junction and the other ends of the conductors (the cold junction or reference junction), a small electromotive force (EMF) is generated. This phenomenon is the Seebeck effect, discovered by Thomas Seebeck in 1821: thermoelectric EMF is proportional to the temperature difference between the two junctions, with the constant of proportionality (the Seebeck coefficient) dependent on the specific metal pair.

The diagram of a thermocouple circuit shows the hot junction, the two dissimilar conductors running to the cold junction, and the measuring instrument (millivoltmeter or thermocouple input device). Critically, any junctions formed between the thermocouple conductors and the measuring instrument's copper terminals also generate thermoelectric EMFs. Cold-junction compensation (CJC) is the process of measuring the temperature at the cold junction (the instrument terminals) and electronically or mathematically correcting the total measured EMF to yield the true hot junction temperature referenced to 0 °C.

Thermocouple types are standardised by IEC 60584 and designated by letter codes. Type K (chromel-alumel) is the most widely used, covering approximately –200 °C to +1260 °C with a Seebeck coefficient of approximately 41 µV/°C. Type J (iron-constantan) covers –210 °C to +760 °C and is common in older industrial equipment. Type T (copper-constantan) covers –250 °C to +350 °C and is particularly suited to cryogenic and refrigeration applications due to its accuracy at low temperatures. Type E (chromel-constantan) covers –200 °C to +900 °C and has the highest Seebeck coefficient of the common types (approximately 68 µV/°C), making it the most sensitive.

Extension cables and compensating cables must match the thermocouple type precisely. Using standard copper cable between the thermocouple and the instrument introduces additional junctions of unknown Seebeck coefficient, producing measurement errors. Type K extension cable uses chromel and alumel conductors throughout its length.

How to wire diagram of thermocouple

Select the appropriate thermocouple type Match the thermocouple type to the temperature range and environment. Type K is the general-purpose choice for most industrial applications (–200 °C to +1260 °C). Type T is preferred for refrigeration and cryogenics. Type J suits existing industrial installations. For oxidising high-temperature environments above 1000 °C, consider type N, R, or S.
Route thermocouple cable from hot junction to instrument Use thermocouple extension cable or compensating cable of the correct type designation throughout the entire run from the hot junction to the instrument input terminals. Keep thermocouple cables separated from power cables to avoid electromagnetic interference inducing noise on the low-level millivolt signal.
Make clean, secure hot junction connections For welded junctions (most reliable): the two thermocouple wires are welded together at the measuring tip using TIG welding, resistance welding, or a thermocouple welder. For twisted junctions (temporary or experimental): twist the two wires together tightly and ensure only the junction point makes thermal contact with the measurement object. Avoid mechanically stressed or corroded junctions.
Connect to the measuring instrument using correct polarity Identify the positive (+) and negative (–) thermocouple conductors by colour code or material identification. Connect the positive conductor to the instrument's positive (T+) input and the negative conductor to the negative (T–) input. Reversing polarity inverts the temperature reading and may confuse CJC circuitry.
Configure the instrument for the correct thermocouple type Set the thermocouple input type on the measuring instrument to match the thermocouple installed (K, J, T, E, N, R, S, or B). Incorrect type selection causes the instrument to apply the wrong EMF-to-temperature conversion table, producing systematic temperature errors.
Verify cold-junction compensation is active Confirm the instrument's CJC function is enabled and that the CJC sensor (typically an on-board thermistor or semiconductor sensor at the instrument terminals) is functioning correctly. A quick verification: if the instrument is at a known ambient temperature and a short circuit is placed across the thermocouple input (simulating 0 °C differential), the instrument should read approximately ambient temperature via CJC, not 0 °C.
Verify accuracy with a reference thermometer Place the hot junction alongside a calibrated reference thermometer in a stable, controlled temperature. Compare readings. If discrepancy exceeds the expected measurement uncertainty, investigate polarity, extension cable type, CJC function, and instrument calibration status.

Specifications

Type K thermocouple metals	Positive: Chromel (Ni-Cr alloy); Negative: Alumel (Ni-Al alloy)
Type K temperature range (IEC 60584)	–200 °C to +1260 °C
Type K Seebeck coefficient (approximate, at 25 °C)	Approximately 41 µV/°C
Type J temperature range	–210 °C to +760 °C
Type T temperature range	–250 °C to +350 °C
Type E temperature range	–200 °C to +900 °C
Type E Seebeck coefficient (approximate)	Approximately 68 µV/°C (highest of the common base-metal types)
Standard governing thermocouple calibration	IEC 60584-1, ANSI/ASTM E230

Safety warnings

Thermocouple probes in high-temperature or high-pressure processes can store significant thermal energy and are subject to process pressure. Follow appropriate process isolation and depressurisation procedures before extracting or replacing thermocouple probes from process vessels.
Thermocouple extension cables routed in the vicinity of high-voltage power cables may pick up electromagnetic interference, causing erratic or offset temperature readings. Use screened extension cable and maintain physical separation from power cables. Ground the screen at one end only.
In explosive atmospheres (ATEX/IECEx zones), thermocouple installations must use certified intrinsically safe barriers or Ex-rated instruments appropriate for the zone classification. Standard thermocouple instruments are not inherently safe for use in classified hazardous areas.
Thermocouples exposed to reducing atmospheres, sulphur-containing gases, or vibration may exhibit accelerated drift or mechanical failure. Select sheath materials and thermocouple types appropriate to the process environment, and establish a calibration verification interval based on process criticality.

Tools needed

Digital multimeter (millivolt DC range for thermocouple EMF measurement)
Thermocouple calibrator or reference source (for instrument verification)
Thermocouple welder (for custom junction fabrication)
Insulated wire stripper suitable for thermocouple wire gauge
Reference thermometer or thermocouple reference standard

Common mistakes

Using standard copper extension cable instead of type-matched thermocouple extension cable, introducing unknown junction EMFs and systematic temperature measurement errors.
Reversing the positive and negative thermocouple conductors at the instrument, causing inverted temperature readings (apparent temperature decreases as the actual temperature increases).
Setting the measuring instrument to the wrong thermocouple type (e.g. selecting Type J when a Type K thermocouple is installed), causing systematic errors of many degrees across the measurement range.
Creating multiple unintentional measurement junctions by using different metals in a splice or junction box along the thermocouple extension circuit.
Ignoring CJC—assuming the instrument automatically references to 0 °C without enabling or verifying cold-junction compensation, leading to temperature readings offset by approximately the ambient temperature.

Troubleshooting

Instrument reads approximately ambient temperature regardless of hot junction temperature: Cause: The thermocouple leads are short-circuited at or before the cold junction, or the thermocouple extension cable is incorrectly wired so both instrument inputs are connected to the same conductor. The instrument is measuring only the CJC ambient reference with no hot junction signal. Fix: Disconnect the thermocouple at the instrument terminals. Measure resistance between the two terminals—a near-zero reading indicates a short. Trace the circuit back to locate the short circuit in the cable, connectors, or thermocouple head.
Temperature reading is significantly too high or too low by a consistent offset: Cause: The instrument thermocouple type setting does not match the installed thermocouple type, or extension cable of the wrong type has been used over part of the circuit, introducing a systematic EMF error. Fix: Verify the instrument type setting matches the thermocouple installed. Check all extension cable markings along the full circuit run. Perform a spot-check calibration by immersing the hot junction in an ice bath (0 °C) and verifying the instrument reads 0 °C (with CJC active) or the ambient temperature equivalent.
Temperature reading is noisy or fluctuating randomly: Cause: Electromagnetic interference is being picked up by unscreened thermocouple cable routed near power cables, variable-frequency drives, or other high-frequency sources. Alternatively, there is a loose or intermittent connection at a terminal block or connector. Fix: Replace unscreened cable with screened thermocouple extension cable, ground the screen at the instrument end only. Check and re-torque all terminal connections in the thermocouple circuit. Route cable away from electromagnetic interference sources.

Frequently asked questions

What is the Seebeck effect and how does it apply to thermocouples?

The Seebeck effect describes the generation of an electromotive force (voltage) when two dissimilar metals are joined and their junctions are held at different temperatures. The EMF is proportional to the temperature difference between the hot junction (measuring point) and the cold junction (reference point). A thermocouple exploits this effect to convert temperature into a measurable electrical signal.

What is cold-junction compensation in a thermocouple circuit?

Cold-junction compensation accounts for the thermoelectric EMF generated at the cold junction (the terminals where the thermocouple wires connect to the measuring instrument). Because the instrument terminal temperature is not at 0 °C (the calibration reference), an additional EMF error exists. CJC measures the terminal temperature with a separate sensor and adds or subtracts the equivalent correction to the measured EMF, yielding the true temperature referenced to 0 °C.

What are the most common thermocouple types and their temperature ranges?

Type K (chromel-alumel): –200 °C to +1260 °C; Type J (iron-constantan): –210 °C to +760 °C; Type T (copper-constantan): –250 °C to +350 °C; Type E (chromel-constantan): –200 °C to +900 °C; Type N (nicrosil-nisil): –200 °C to +1300 °C. Types R, S (platinum-rhodium alloys) and B cover temperatures up to approximately 1700–1820 °C for high-temperature industrial applications.

Can I use standard copper wire to extend a thermocouple circuit?

No. Standard copper wire introduces additional metal junctions with different Seebeck coefficients at every transition point, generating spurious EMFs that cause measurement errors. Thermocouple extension or compensating cables made from the same metals as the thermocouple (or specifically matched alloys) must be used throughout the circuit from the hot junction to the measuring instrument.

How do I identify the positive and negative conductors of a thermocouple?

Thermocouple conductors are identified by colour coding standardised by IEC 60584-3. For type K under IEC: the positive conductor (chromel) is green and the overall cable is green. Under ANSI/ASTM E230: the positive conductor (chromel) is yellow, overall cable is yellow. Always verify against the applicable regional standard and the product documentation, as colour codes differ between standards.