Thermocouples 400 Series – Maltec-T MT 401

Dpstar Group is the largest manufacturer of thermocouples based in Malaysia. We design and manufacture a complete line of high-quality standard and custom temperature sensors including thermocouples, RTD’s, thermowells, and float switches as well as a wide selection of temperature accessories. Through our engineering capabilities and 30 years of experience, our trademark brand Maltec-T is the only CE-certified manufacturer for thermocouples in Malaysia. With extensive experience, unique technology, and hard commitment, we become one of the best quality suppliers in temperature sensors field. We offer a full range of design services to provide you with the benefit of our knowledge and experience.

Thermocouples 400 Series

MT 401 complete with ceramic terminal block, constructed using ceramic insulators over 8/3.2 AWG thermocouple grade wire.

Working Principle of Thermocouples

Thermocouples are electrical devices used to measure temperature. Their accuracy, rapid reaction time, and ability to withstand heavy vibrations, high pressure, and extreme temperatures make them ideal for a wide range of applications.

To have a full understanding of how a thermocouple works, it’s important to know what a thermocouple is. A Thermocouple is a type of temperature sensor which is made up of two wires of dissimilar metals. It measures temperature using the Seebeck effect. The Seebeck effect can be elaborated as the generation of differential voltage due to the difference in electrical conductivity of two different materials. Named after French scientist, Thomas Johan Seebeck, who confirmed if two dissimilar metals are joined and heated, the difference in a temperature rise of these two metals gives rise to electromotive force (EMF). The same concept is reversed in the application of thermocouple.

As the electric current is passed through two welded dissimilar metals, the voltage difference occurs, which is reverse projected to calculate temperature difference. As, the electric current passes through a junction, due to limitations in conductivity and resistance of the metals, a rise in temperature takes place. Both the materials heat up at different temperatures and the difference in conductivity gives two different voltages for two different metals.

Thermocouple Types

There are many types of thermocouples, each with its own unique characteristics in terms of temperature range, durability, vibration resistance, chemical resistance, and application compatibility. Type J, K, T, & E are “Base Metal” thermocouples, the most common types of thermocouples. Type R, S, and B thermocouples are “Noble Metal” thermocouples, which are used in high-temperature applications.

Characteristics Of Thermocouples Types

Type B Thermocouples
Type B thermocouples can be used up to 1600°C with short term excursions up to 1800°C. They have a low electrical output, therefore are rarely used below 600°C. In fact the output is virtually negligible up to 50°C, therefore cold junction compensation is not usually required with this type.

Type E Thermocouples
Type E thermocouples are often referred to as Chromel-Constantan thermocouples. They are regarded as more stable than Type K, therefore often used where a higher degree of accuracy is required. Note – Constantan is Copper-Nickel.

Type J Thermocouples
Type J thermocouples degrade rapidly in oxidising atmospheres above 550°C. Their maximum continuous operating temperature is around 750°C though they can with stand short duration excursions to 1000°C. They are generally not used below ambient temperature due to condensation forming on the wires leading to rusting of the iron. Note – Constantan is Copper-Nickel.

Type K Thermocouples
Type K thermocouples are the most widely used thermocouples in the Oil & Gas, and refining industries due to their wide range and low cost. They are occasionally referred to as Chromel-Alumel thermocouples. Note that above about 750°C oxidation leads to drift and the need for recalibration.

Type N Thermocouples
Type N thermocouples can handle higher temperatures than type K, and offer better repeatability in the 300 to 500°C range. They offer many advantages over Type R & S at a tenth of the cost, therefore prove to be popular alternatives.

Type R Thermocouples
Type R thermocouples cover similar applications as Type S but offer improved stability and a marginal increase in range. Consequently, Type R tend to be used in preference to Type S.

Type S Thermocouples
Type S thermocouples can be continually used at temperatures up to 1450°C. They can with stand short duration excursions up to 1650°C. They need protection from high temperature atmospheres to prevent metallic vapour ingress to the tip resulting in reduction of emf generated. Protection commonly offered is a high purity recrystallised alumina sheath. For most industrial applications thermocouples are housed in a thermowell.

Type T Thermocouples
The Type T is a very stable thermocouple and is often used in extremely low temperature applications such as cryogenics, ultra low freezers and laboratory.

Thermocouple Color Codes

Each type of thermocouple has multiple colors associated with it. Standard grade and extension grade wire have their own respective colors, while the positive and negative leads also have associated colors.

The following chart includes thermocouple color codes by country:

Thermocouple Junction

Ungrounded
In this type, the junction is suspended within the sheath, surrounded by insulation, and not touching the edges. This helps protect the sensor and provides a steady and accurate temperature measurement reading. That, however, comes at the cost of response time as any temperature changes must permeate through to the enveloped junction.

Grounded
A grounded measurement junction is the union of two dissimilar metals that also touches the outside protective sheath. Any temperature changes will be picked up much quicker than an ungrounded probe with this direct physical connection. This rapid response comes with a risk though. By connecting the hot junction of the thermocouple to the protective sheath there is the potential for a ground loop to be created. This is another (undesirable) path for current to travel through and will interfere with the signals of your thermocouple. This interference will lead to inaccurate readings.

Exposed
Making most direct contact with the process, exposed measurement junctions are the quickest responding thermocouple junction type. These may have some sort of protective layer but ultimately the union of the two dissimilar metals is bare. The drawback to this type is the direct wear on the junction and either the acceptance of often replacing the instrument or restricting its use in harsh environments.

Applications

• Used as the temperature sensors in thermostats in offices, homes, offices & businesses.
• Used in industries for monitoring temperatures of metals in iron, aluminum, and metal.
• Used in the food industry for cryogenic and Low-temperature applications. Thermocouples are used as heat pumps for performing thermoelectric cooling.
• Used to test temperature in chemical plants, petroleum plants.
• Used in gas machines for detecting the pilot flame.

The advantages of Thermocouples

• High Accuracy
• It is Robust and can be used in environments like harsh as well as high vibration.
• Fast thermal reaction
• Wide operating temperature range
• Low cost and extremely consistent
• Self-powered (passive)
• Simple to use
• Interchangeable, easy connectivity
• Wide variety of thermocouple probes available
• Wide temperature ranges in many types
• Higher temperature capabilities than other sensors
• Not affected by resistance decreases or increases