The J Type Thermocouple

The Type J Thermocouple

The J type thermocouple is the cost-effective, versatile and reliable solution for a wide range of temperature measurement applications.

Type J Thermocouple: quick overview

  • Composition: iron (Fe) (+) and constantan (Cu-Ni) (-)
  • Overall temperature range: -210 °C to 1200 °C (-346 °F to 2192 °F)
  • Operating temperature (class 1): -40°C to 750°C (-40 °F to 1382 °F)
  • Cable in thermocouple quality: -210 °C to 750 °C (-346 °F to 1382 °F)
  • Extension cable: 0 °C to 200 °C (32 °F to 392 °F)
  • Characteristics: low cost – remarkable sensitivity
  • Recommended use: inert, vacuum and reducing atmospheres
  • Not recommended use: oxidizing atmospheres at high temperatures
  • Limit tolerance in class 1: ±1.5 °C or 0.40% (59 °F or 0.40%)
  • Limit tolerance in class 2: ±2.5 °C or 0.75% (77 °F or 0.75%)


The type-J thermocouple is a temperature measurement instrument. It is a widely used and versatile solution for general-purpose applications and is extensively employed in various sectors, including industry, science, food, and medicine.

The J-type thermocouple belongs to the category of base metal thermocouples, as it is composed of common materials. In fact, its structure includes a positive conductor made of iron wire (99.5% iron, with traces of manganese and copper) and a negative conductor composed of a constantan alloy wire (a combination of 55% copper, 45% nickel, and traces of cobalt, iron, and manganese).


Technical features

Its usefulness is limited by a relatively narrow temperature range and a shorter lifespan at elevated temperatures.

The positive conductor, being made of iron, has a Curie point at 770 °C (1418 °F), defining the temperature range of type J (in Class 1) between -40 °C and 750 °C (-40 °F to 1382 °F).

The type-J thermocouple is particularly suitable for use in vacuum, inert, and reducing environments, while its use in oxidizing atmospheres above 550 °C (1022 °F) is not recommended. This is because iron undergoes irreversible molecular changes, permanently compromising its standard output voltage in relation to temperature, with no recovery during cooling.

The maximum continuous operating temperature is approximately 800 °C (1472 °F), although temperatures up to 1000 °C (1832 °F) can be managed for short periods.

The sensitivity of type J is around 50 microvolts per degree C.

From an economic and reliability standpoint, the type-J thermocouple is comparable to the type K thermocouple.


For which application is a J-type thermocouple suitable?

The J-type thermocouple is widely used in various sectors, including industrial, scientific, food and medical, thanks to its remarkable versatility.

This device is suitable for a wide range of applications, including high temperature environments and atmospheres characterized by vacuum, oxidation (air), reduction or inertia (inert gases).

In the industrial context, the type-J thermocouple proves particularly useful in monitoring temperatures during manufacturing processes such as the production of plastic materials and resins.

Additionally, it also proves effective in environments involving poorly reactive inert materials, such as sand, cement or substances containing asbestos.

Type J thermocouples are available in different types and constructions, to suit a wide range of applications.

  • Mineral insulated type J thermocouples: rugged sensors, ideal for most applications. They offer a wide selection of terminations, such as transition ferrules, cables, connectors, heads, etc.
  • Tapered tip type J thermocouples: fast response thermocouples for most applications.
  • Type J thermocouples for intensive use: for industrial applications such as furnaces, dryers, ovens, boilers, flues, etc.
  • Type J thermocouples for general use: a wide range of thermocouples to adapt to a variety of applications. Portable, surface, bayonet, bolt, adhesive, etc. series.
  • ATEX/IECEx approved Type J thermocouples: wide range of thermocouples with various terminations such as transition sockets, connection heads, etc.

Tips and recommendations

Due to the high oxidation of the type J positive part (iron) of the wire at temperatures above 540 °C (1000 °F), it is advisable to use wires with a larger diameter to counterbalance this phenomenon.

Additionally, the iron wire may experience rust and brittleness at temperatures below zero. Without adequate protection, an atmosphere containing ammonia, nitrogen, and hydrogen can corrode it.

As a result, the use of stainless-steel metal sheaths with MgO insulation is recommended, representing a preferable choice over a simple thermocouple construction.

The recommended maximum operating temperature is 760 °C (1400 °F). Avoid using type J in sulfurous atmospheres exceeding 540 °C (1004 °F).

Furthermore, it is not recommended for use at temperatures above 760 °C (1400 °F), as a sudden magnetic transformation would cause permanent calibration issues.

To prevent confusion, it is important to note that type J wire can be mistakenly exchanged with normal red and white copper wire. In case of uncertainty, verification with a magnet is advised: iron wire is strongly magnetic, unlike copper wire.


Why choose a J-type thermocouple?

  • Cost-effectiveness: among the different types of thermocouples, type J stands out as the most convenient solution from an economic point of view.
  • Constant output: It provides a consistent output of 1 mV per 18 °C (64,4 °F), ensuring stable and reliable measurements.
  • Wide range of use: protected by adequate mineral insulation and appropriate external sheathing, type J can be used in a wide temperature range, precisely from 0 °C to 816 °C (32 °F to 1500 °F).
  • Wear resistance: it is not subject to wear in the temperature range between 371 °C and 538°C (700 °F to 1000 °F), ensuring reliable durability over time.
  • Industrial versatility: due to its versatility, type J can be used in various industrial applications, making it suitable for a wide range of thermal measurement needs.

In summary, the J-type thermocouple presents itself as a convenient, stable, durable and versatile solution to meet the various thermal measurement needs in many industries.


MgO mineral oxide insulation

In type J thermocouples, MgO mineral oxide insulation stands out as the primary choice due to its numerous advantageous characteristics.

Among these, the fast response, small size, wide temperature range, durability, precision, resistance to thermal shock and vibration make it an ideal choice for virtually all laboratory or industrial process applications.

Regarding initial calibration tolerances, MgO insulation offers excellent performance over the temperature range of 0 °C to 750°C (32 °F to 1382 °F).

Its typical tolerance is in the Standard class, which is at +2.2 °C (35.96 °F) or +0.75%, making it a perfect fit for type J thermocouples, ensuring precise and reliable measurements.


Difference between Type-J thermocouple and Type-K thermocouple

The Type-K thermocouple and the Type-J thermocouple share many similarities, both being characterized by versatility, reliability, and cost-effectiveness.
However, their applications do not overlap as often as it might appear, as they have some substantial differences.

The Type K thermocouple is recommended for high temperature uses, corrosive environments and situations where the cable is exposed to oxidation, including humid environments and environments immersed in water.
The Type K is the most widespread in the production and industrial sector thanks to its reliability and cost-effectiveness.

The J-Type thermocouple is the best choice at lower temperatures, with a range starting from -40°C, making it ideal also for vacuum applications.

For all other situations that do not specifically fall within these criteria, either the K-Type or J-Type thermocouple can be used, provided the appropriate temperature range is met.

In short, the substantial differences between J-type thermocouple and K-type thermocouple are:

Temperature range:
– Type J: usually used in a range from approximately -210 °C to +750 °C (-346 °F to 1382 °F).
– Type K: wider operating range, from approximately -270°C to +1300° C (-454 to 2372 °F).

Ambient temperature sensitivity:
– Type J thermocouple has higher sensitivity than type K at room temperatures.

Favorite applications:
– Type J: suitable for vacuum applications and reducing atmospheres. Good for moderate temperatures.
– Type K: widely used in a variety of applications, including higher temperatures and diverse atmospheric conditions.

Thermal response:
– The J-type thermocouple has a faster response than the K-type.

Oxidation sensitivity:
– Type K thermocouple is more resistant to oxidation compared to the Type J.

Environmental compatibility:
– Type J: suitable for applications where the cable is subject to oxidation.
– Type K: used in corrosive environments and high-temperature applications.

Thermo-electromotive force:
– In the J-type thermocouple the thermo-electromotive force is 20% greater than in the K-type thermocouple.


Advantages of Type J Thermocouples

  • They are the most economical thermocouples.
  • Effective and reliable.
  • Compact size.
  • Suitable for reducing atmospheres.
  • They are versatile and have widespread use in various industries.
  • Fast measurement times.
  • Can be used for high-temperature measurements.
  • They have good corrosion resistance, making them suitable for measurements in electric furnaces.

Disadvantages of Type J thermocouples

  • Its applicability is limited to temperatures not exceeding 750 °C (1382 °F).
  • They contain an iron wire as a conductor, making them susceptible to rust formation in humid environments. The presence of rust can lead to inaccurate readings or, in the worst case, cause an open circuit.
  • They are prone to oxidation, making them less than ideal for humid environments or low temperatures.
  • Usage at temperatures above 760 °C (1400 °F) induces rapid magnetic alteration, resulting in irreversible recalibrations.