Skip to Content

How can you tell the difference between J and K type thermocouples?

Thermocouples are temperature sensing devices that are made by joining two dissimilar metals together. The junction between the two metals produces a small voltage that varies with temperature. This voltage can be measured and interpreted to determine the temperature.

The two most common types of thermocouples are J and K types. Both use different combinations of metal alloys and have different temperature ranges and characteristics. However, telling the difference between J and K type thermocouples can be challenging, especially if the wires are unmarked.

In this article, we will provide an overview of J and K type thermocouples and explain the key differences between them. We will also provide tips on how to visually identify unmarked J and K thermocouple wires.

Overview of J Type Thermocouples

J type thermocouples are composed of iron (Fe) and constantan (Cu-Ni) conductors. The combination of these two alloys produces a thermoelectric voltage that varies predictably with changes in temperature.

Some key facts about J type thermocouples:

– Temperature range: -210 to 1200°C (-346 to 2192°F)
– Upper continuous temperature limit: 750°C (1382°F)
– Standard tolerance: ±2.2°C or ±0.75%
– Color code: Black (+) and White (-)
– Common uses: Low temperature measurements in inert environments

J types have excellent thermocouple stability and a high output (around 50 μV/°C). They are suitable for measurements in vacuum, inert and oxidizing environments. However, they have a lower temperature range compared to other thermocouple types.

Overview of K Type Thermocouples

K type thermocouples are composed of nickel-chromium (NiCr) and nickel-aluminium (NiAl) alloys. The combination produces a thermoelectric voltage suited for measurements over a wide temperature range.

Key facts about K type thermocouples:

– Temperature range: -270 to 1372°C (-454 to 2501°F)
– Upper continuous temperature limit: 1260°C (2300°F)
– Standard tolerance: ±2.2°C or ±0.75%
– Color code: Yellow (+) and Red (-)
– Common uses: General purpose measurements from cryogenic to high temperatures

The nickel alloy conductors give K types excellent corrosion resistance. They have a lower thermal EMF output than J types (around 40 μV/°C) but can withstand higher temperatures. K types are the most commonly used general purpose thermocouple.

Key Differences Between J and K Type Thermocouples

While J and K thermocouples are similar temperature sensors, there are some key differences between them:

Difference J Type K Type
Temperature range -210 to 1200°C
(-346 to 2192°F)
-270 to 1372°C
(-454 to 2501°F)
Max continuous temperature 750°C (1382°F) 1260°C (2300°F)
Thermoelectric output ~50 μV/°C ~40 μV/°C
Conductor metals Iron + Constantan
(Fe + Cu-Ni)
Chromel + Alumel
(NiCr + NiAl)
Color code Black (+) / White (-) Yellow (+) / Red (-)
Common uses Low temperature measurements General purpose wide range measurements

As shown in the table, the main differences are:

– **Temperature range** – K types have a wider temperature range, especially at higher temperatures. J types are better for cryogenic or lower temperature applications.

– **Max temperature** – The upper limit for continuous operation is higher for K types at 1260°C vs. 750°C for J types.

– **Output** – J types have a slightly higher thermoelectric output, approximately 50 μV/°C vs. 40 μV/°C for K types. This can improve measurement resolution.

– **Conductor metals** – J types use iron and constantan while K types use nickel-chromium and nickel-aluminium alloys. This gives K types better corrosion resistance.

– **Color codes** – J types follow black (+) and white (-) polarity while K types are yellow (+) and red (-). This allows quick visual identification.

– **Common uses** – The higher temperature range makes K types suitable for general applications. J types are typically used for more precise, low temperature measurements.

So in summary, K types have a wider temperature measurement range while J types provide better performance and accuracy at lower temperatures. The different conductor metals also make them suitable for different environments.

Identifying Unmarked J and K Thermocouple Wires

Thermocouple wires are often marked or color coded for easy identification. However, you may encounter unmarked wires, especially on older thermocouples.

Here are some tips for visually identifying unmarked J and K thermocouple wires:

– **Use a magnet** – A magnet will be attracted to J type wires due to the iron conductor. There will be no magnetic attraction on K type wires.

– **Look for oxidation** – J types oxidize faster, so the iron wire may appear rusty or corroded. K types are more oxidation resistant.

– **Check wire thickness** – K type wires are often thicker than J types due to the higher temperature capacity. Thicker gauges like 20 AWG may indicate K type.

– **Inspect connectors** – J types commonly use black and white mini-plug connectors. K types often use yellow and red connectors following the color code.

– **Consider the application** – If used at high temperatures above 750°C, they are most likely K type as this exceeds the J type limit. Lower temperature applications make J type more likely.

– **Measure resistance** – The different conductor metals in J and K types result in different electrical resistance values. Measuring the resistance across the two wires can help identify the type.

– **Test with ice water** – Placing the thermocouple junction in an ice bath provides a known 0°C reference point. The voltage output indicates if it is J or K type based on the Seebeck coefficient.

Using a combination of these visual inspection and measurement techniques, you can often determine the thermocouple type. When in doubt, consulting wiring diagrams, installation records, or OEM datasheets can also help identify mystery thermocouples.

Using Extension Grade Cables

When thermocouples wires are extended or repaired, it is important to use properly matched extension grade thermocouple wire.

Using regular copper wire will introduce measurement errors due to Seebeck effects at the dissimilar metal junctions. Always use wire that matches the original thermocouple type, following the correct polarity.

Here are some tips on using extension wires:

– Clearly label or tag wires to avoid confusion
– Solder all connections to ensure a solid thermoelectric circuit
– Use twist-on thermocouple connectors when possible for easy repairs
– Route extension cables away from heat sources or electrical noise
– Choose the right gauge wire to minimize resistance – 20 to 24 AWG
– Keep connections tight and moisture-free

Following proper wiring practices will ensure accurate and reliable temperature measurements. Consult thermocouple reference tables when making connections or junctions between dissimilar metals.

Proper Grounding Techniques

Proper grounding is essential for accurate thermocouple measurements and noise reduction. Here are some recommended grounding techniques:

– **Use grounded thermocouple connectors** – Many mini-plug and twist connectors have a ground tab or screw. Connect this to the measurement system ground.

– **Connect shields** – For shielded thermocouple cable, connect the shield to ground at the measurement end only. Avoid ground loops.

– **Use grounded junction blocks** – Junction blocks allow multiple thermocouples to connect to a common ground point.

– **Follow ground wiring practices** – Use dedicated ground wires, avoid sharing ground connections with other devices, and connect to a single point ground.

– **Use isolators when necessary** – Isolators allow the thermocouple ground reference to float for systems without a ground.

Proper grounding provides a return path for electrical noise and minimizes voltage interference on the thermocouple circuit. Always consult thermocouple and measurement device manuals for the proper grounding scheme.

Cold Junction Compensation

Thermocouples measure the temperature difference between the hot junction (sensing end) and the cold junction (terminal end). To determine absolute temperature, the cold junction temperature must also be measured.

This is known as cold junction compensation (CJC) and there are several methods:

CJC Method Description
Internal thermistor A thermistor inside the terminal block measures the reference temperature.
External CJC sensor A dedicated temperature sensor provides the cold junction temperature.
Manual entry The known cold junction temperature is manually entered into the measurement system.
Ice bath The terminal block is placed in an ice bath for a fixed 0°C reference.
Software compensation Software algorithms estimate cold junction temperature and compensate values.

The best method depends on the application, but built-in thermistors or external CJC sensors provide the most accuracy. Properly compensating for cold junction temperatures ensures the highest thermocouple measurement and calibration accuracy.

Thermocouple Use for Calibration

Thermocouples are commonly used as temperature standards for calibration due to their predictable thermoelectric properties. Here are some guidelines for calibration use:

– Select the thermocouple type suited to the calibration temperature range. K types work for wide calibration ranges.

– Use tight tolerance thermocouples, Class 1 for highest accuracy.

– Record the Seebeck coefficient or voltage-temperature relationship for the thermocouple type.

– Place the thermocouple junction in close proximity to the sensor under calibration.

– Allow time for thermal equilibrium to ensure an equalized temperature.

– Measure the thermocouple voltage and correlate it to temperature using reference tables.

– Verify calibration equipment like ice baths, furnaces, and thermowells meet specifications.

– Send thermocouples to an accredited lab for periodic recalibration.

With proper practices, thermocouples provide an inexpensive yet highly accurate temperature source for field and laboratory calibration of other devices and sensors.


While J and K are the two most common thermocouple types, they have key differences in their temperature range, material composition, color codes and applications.

K type thermocouples are suitable for wide ranging general temperature measurements up to 1372°C. J types provide better accuracy and precision at lower temperatures below 750°C.

Unmarked thermocouple wires can be identified through techniques like magnet testing, visual inspection of connectors, and checking electrical resistance. Proper installation procedures, junction connections and cold junction compensation also ensure accurate measurements.

With their predictable voltage-temperature relationship, thermocouples serve as an accurate calibration source for other temperature sensors and devices. Understanding the differences and capabilities of J vs K types allows selection of the optimal thermocouple for a given application.