Exploring the Functionality of Thermal Connectors

A thermal connector, sometimes called a cold-end termination, is designed to connect temperature sensors to measuring instruments, like thermocouples or resistance temperature detectors (RTDs).

Thermocouples and RTD connectors are used in a wide range of scientific and industrial systems in aerospace, chemical, food and beverage, oil and gas, pharmaceutical, and nuclear power generation.

Two standard thermocouple connector sizes are available to fit the needs of specific instruments. Standard (large) connectors measure 35 mm long, 25.4 mm wide, and 12.8 mm thick; their round pins are approximately 15mm long.

Miniature connectors are about half the size of standard units and have flat pins and sockets. The pins and sockets on both sizes are polarized to prevent miss mating. Thermal connectors are available in several common pin configurations, including (Figure 1):

2-pin configurations are the most common and are used with a single-circuit thermocouple or RTD.
3-pin configurations that add a ground pin for single-circuit, three-wire RTD sensors.
4-pin designs are used for two thermocouple or RTD circuits or a single-circuit, four-wire RTD sensor.

Figure 1. Examples of a two-pin thermocouple connector and a three-pin RTD connector. (Images: Technical Equipment Sales)

What are thermal connector contacts made with?

A thermocouple generates a voltage based on the temperature difference between two dissimilar metals joined together at a junction. To avoid introducing errors in the temperature measurement, the contacts in a thermocouple connector are made from the same material as the thermocouple wires.

An RTD measures temperature changes based on the resistance change in a platinum wire. Its connectors use copper contacts. RTDs can offer higher accuracy, especially in precise temperature ranges, while thermocouples are available for a wider range of temperatures.

The American National Standards Institute (ANSI) and the International Organization for Standardization (ISA) have defined a series of thermocouple codes (letters and colors) that indicate the type of metal used in the thermocouple’s wiring and the temperature range it can withstand. Thermal connectors have contact materials optimized for each type of thermocouple or RTD (Table 1).

Table 1. The various types of thermocouples and RTD connectors have positive and negative contacts made with specific materials to ensure the accuracy of temperature measurements. (Table: Technical Equipment Sales)

What materials are used in the connector bodies?

Stainless steel is often used for RTD connector bodies due to its durability, corrosion resistance, and ability to withstand a wide range of temperatures. Stainless steel is also suited for industrial applications where many RTDs are employed.

Thermoplastic is the most common material used for thermocouple connectors. It provides a good combination of mechanical strength, temperature resistance, and cost and can be used in applications from about -40 to 200 °C.

Glass-filled thermosets are less durable but, depending on the formulation, can withstand temperatures from 350 to 425 °C. Ceramics can be used in high-temperature applications to 650 °C or higher, but they are fragile and the most expensive (Figure 2).

Figure 2. Thermocouple connector bodies are made with materials designed for specific temperature ranges. (Image: Evolution Sensors and Controls)

Color-coding

Many applications include multiple types of thermocouples. To ensure correct connections and the use of the proper thermocouple, the lead wires are color-coded. In the U.S., the color coding is defined in ANSI/ISA MC96.1.

Different insulation colors simplify the identification of thermocouple types, ensuring that the correct device is used in each application. The colors also indicate the polarity of the leads to ensure proper insertion into the thermal connector.

Thermocouples are subject to “aging,” which causes them to become less accurate over time due to high-temperature operation and environmental conditions. In a standard environment, thermocouples may need to be replaced every 1 to 3 years. Color coding speeds up the replacement process.

Summary

Accurate temperature measurements are important in various industrial and scientific applications. Properly designed thermal connectors help ensure the accuracy of those measurements and can speed up the replacement of worn-out thermocouples and RTDs. Because of the wide variety of thermocouple designs, the leads are color-coded to speed the identification of devices and their polarity.