Thermocouples: Essential Concepts and Information

Principle

Thermocouples use the Thermoelectric Effect (Seebeck Effect) to measure temperature. When a temperature difference exists along a metal, a small electrical current is generated that corresponds to this temperature variation. The strength of this current differs depending on the metal.

In a thermocouple, a circuit is formed by two dissimilar metals (e.g., Chromel and Alumel in Type K thermocouples). The voltage difference between the “hot junction” (where the metals meet) and the “cold junction” (the opposite end) accurately reflects the temperature difference. Cold junction compensation is then applied using a resistive temperature sensor integrated into your converter, allowing precise temperature calculation at the hot junction.

Advantages of Thermocouples

Wide Temperature Range: Suitable for various applications.
Cost-Effective Sensors: Less expensive compared to other types.
Durability: Resistant to vibration and damage.
Quick Response: Provides fast readings.
Simple Construction: Easy to build and implement.

Disadvantages of Thermocouples

Lower Accuracy: Less precise than RTDs.
Higher Converter Costs: More expensive than other sensors.
Limited Extension Options: Cannot use regular electrical cables for extension.

Thermocouple Styles and Types

Thermocouples come in various types, each identified by a letter and colour code. While the letter codes are globally consistent, colour standards can vary by region. In Australia, the American ANSI standard is used, though you may encounter Japanese or German systems in imported equipment. The codes, temperature ranges, and materials are detailed on the following page.

Key Considerations for Thermocouples

Pay close attention to sensors with black sheathing containing red and white wires. This could be:

ANSI Type J Sensor
JIS Type R Sensor
2-Wire RTD

Though Type J is common in Australia and the US, verifying with a mustimeter might be necessary in some cases.

Extending Thermocouples

While RTDs can be extended with regular electrical cables, thermocouples require specific extension cables to avoid creating extra cold junctions. For instance, Type K thermocouples use inexpensive thermocouple wire, whereas Type R thermocouples may use a cost-effective alternative with a similar thermal response.

Trivia: The Opposite Effect

According to the laws of conservation of energy, the Peltier Effect is the opposite of the Thermoelectric Effect. While thermocouples generate current from a temperature difference, the Peltier Effect creates a temperature difference by inducing a current. Though less efficient, this effect is useful in small-scale applications like portable coolers, computer cooling, and other compact devices.

Need a specific thermocouple standard drawing? We’ve got you covered. Contact us today, and let’s find the perfect match for your requirements!

Constructing a Contact Temperature Sensor

Designing the Sensor

Custom-designed sensors are often necessary, as off-the-shelf RTDs may not meet specific requirements. While there are common designs, most temperature sensors are tailored for particular applications. Sensors typically consist of three main components:

1. The Tip:

Houses the junction or RTD bulb and endures the harshest conditions. Attachment methods to the monitored item also need consideration.

2. The Middle:

This section may include various lengths of tube, wire, or conduit. Options such as bulkhead fittings, brackets, and protective materials (e.g., PVC, PFA, fiberglass braid, stainless-steel armour) provide customisation for heat, chemical, and impact resistance.