Thermocouple

A thermocouple is a device used for temperature measurement. They are preferred due to their simple and practical use, suitability for harsh conditions, and cost advantage compared to sensors.

Origin

The working principle of the thermocouple, known as the Seebeck effect, is named after Thomas Johann Seebeck, who in 1821 observed a slight deflection of a compass needle while conducting experiments with conductive wires. In the same year, Humphrey Davy demonstrated the relationship between electrical resistance and temperature. Five years later, Becquerel produced a platinum-platinum device. However, the first true thermocouple device as understood today was developed by Leopoldo Nobili in 1829.

Working Principle

The term thermocouple refers not to the device where the temperature reading is displayed, but to the cable used in the measurement. The cable contains two wires that are electrically insulated from each other. Due to the temperature being measured, electron movement occurs, resulting in a potential difference (voltage difference). This phenomenon is explained by the Seebeck effect.

The Seebeck effect describes how electrons increase their energy when one point of a wire is heated and move toward the cooler region with lower energy. The two wires are in full contact at the junction point, where no potential difference exists. However, because the number of electrons transferred from the hot region to the cold region differs between the two wires, a potential difference arises. Since the circuit is not closed, no current flows and electrons cannot migrate to the other wire.

What determines the number of displaced electrons? This number depends on the temperature difference and the material properties of the wires. Therefore, if both wires are made of the same material, the number of moving electrons will be identical, resulting in no voltage difference and making temperature measurement impossible. When the wire materials are different, a difference in electron count occurs. However, to ensure this difference is significant and easily measurable, specific pairs of materials are used together. Each voltage difference corresponds to a specific temperature value, enabling accurate temperature measurement.

The measuring device may be a specialized instrument designed solely for temperature measurement or a multimeter equipped with a temperature sensor. The critical factor is having a known reference temperature at the measurement point, also called the cold junction. In older applications, an ice-water mixture was used for the cold junction, allowing users to know the temperature was precisely 0 °C. Today, the reference value is measured using a temperature sensor inside the measuring device itself. This allows temperature measurement even when the hot and cold junctions are in the same environment and no temperature difference exists between them.

Types

• K-Type Thermocouple (Nickel-Chromium / Nickel-Alumel): The most common model. It is inexpensive and reliable, with a wide operating temperature range (-270 °C to 1260 °C).
• J-Type Thermocouple (Iron/Constantan): Has a slightly narrower operating temperature range than the K-type. Due to iron’s oxidation issues, it is not long-lasting, especially at high temperatures. It is similar to the K-type in terms of cost and reliability.
• T-Type Thermocouple (Copper/Constantan): Provides very stable measurements and is especially used for extremely low-temperature applications (cryogenics) (-270 °C to 370 °C).
• E-Type Thermocouple (Nickel-Chromium/Constantan): Produces stronger signals compared to K- and J-types, resulting in lower error rates and higher accuracy. Its operating temperature range is -270 °C to 870 °C.
• N-Type Thermocouple (Nicrosil/Nisil): Has an operating temperature range and accuracy similar to the T-type.
• S-Type Thermocouple (Platinum Rhodium - 10% / Platinum): Has a high maximum operating temperature limit (-50 °C to 1480 °C). It is commonly used in biotechnology and pharmaceutical applications. It is highly reliable with excellent accuracy and can even be preferred for low-temperature measurements in some cases.
• R-Type Thermocouple (Platinum Rhodium - 13% / Platinum): Preferred for high-temperature applications (-50 °C to 1480 °C). It is more expensive than the S-type due to its higher rhodium content. Its application area is similar to that of the S-type.
• B-Type Thermocouple (Platinum Rhodium - 30% / Platinum Rhodium - 6%): Preferred for extremely high-temperature applications (0 °C to 1700 °C). It is highly accurate and stable at elevated temperatures.

Considerations During Use

In some thermocouples, the measurement point is welded into a spherical shape (bead type) to ensure and maintain full contact. When measuring on a hard surface, the bead structure may prevent perfect thermal contact, leading to inaccurate temperature readings. The contact surface area should be increased. To achieve this, thermal paste should be used to fill any gaps, and/or the probe should be wrapped with a conductive material such as aluminum or copper foil.

• Natural convection tests conducted under ambient conditions should be isolated from external air flow as much as possible.

• Ensure the correct thermocouple type (e.g., K-type) is selected on the device and verify its calibration.

• The surface being measured and the probe must be clean.

• Measurements should be kept as brief as possible to minimize heat sink effects (to avoid excessive cooling due to increased surface area).

• The measurement should be repeated several times.

• Where possible, additional measurements should be taken at nearby but different locations.