Thermal connectors, sometimes referred to as cold end terminations, are designed to connect temperature sensors to measuring instruments such as thermocouples or resistance temperature detectors (RTDs).
Thermocouple and RTD connectors are used in a wide variety of scientific and industrial systems such as aerospace, chemical, food and beverage, oil and gas, pharmaceutical, and nuclear power generation.
Two standard thermocouple connector sizes are available to meet the needs of specific instruments. Standard (large) connectors are 35 mm long, 25.4 mm wide, and 12.8 mm thick; their circular pins are approximately 15 mm long.
Miniature connectors are about half the size of standard units and have flat pins and sockets. Both sizes of pins and sockets are polarized to prevent mismating. Thermal connectors are available in several common pin configurations, including (Figure 1):
2-pin configurations are the most common and are used with single-circuit thermocouples or RTDs.
3-pin configurations that add a ground pin for a single-circuit, three-wire RTD sensor.
4-pin design for use with two thermocouple or RTD circuits or a single-circuit, four-wire RTD sensor.
What are the thermal connector contacts made of?
Thermocouples generate a voltage based on the temperature difference between two dissimilar metals joined together at a junction. To avoid introducing errors in temperature measurement, the contacts in a thermocouple connector are made of the same material as the thermocouple wires.
Figure 1. Example of 2-pin thermocouple connector and 3-pin RTD connector. (Photo: Technical Equipment Sales))
RTDs measure temperature changes based on changes in resistance in a platinum wire. Its connector uses copper contacts. RTDs can provide greater accuracy, especially over precise temperature ranges, while thermocouples can be used over wider temperature ranges.
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 thermocouple wiring and the temperature range it can withstand. Thermal connector contact materials are optimized for each type of thermocouple or RTD (Table 1).
Table 1.Various types of thermocouple and RTD connectors have positive and negative contacts made from specific materials to ensure accurate temperature measurements. (Table: Technical Equipment Sales)
What materials are used in the connector body?
Stainless steel is commonly used for RTD connector bodies due to its durability, corrosion resistance, and ability to withstand wide temperature ranges. Stainless steel is also suitable for industrial applications where many RTDs are used.
Thermoplastic is the most commonly used material for thermocouple connectors. It offers 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 can withstand temperatures from 350 to 425 °C, depending on the formulation. Ceramics can be used in high-temperature applications at 650 °C or higher, but they are fragile and the most expensive (Figure 2).
Figure 2. The thermocouple connector body is made of materials designed for specific temperature ranges. (Photo: Evolution Sensors and Controls)
Color Coding
Many applications include multiple types of thermocouples. To ensure proper connection and use of the correct thermocouple, the leads are color coded. In the United States, color coding is defined in ANSI/ISA MC96.1.
The different insulation colors simplify the identification of thermocouple types and ensure that the correct device is used for each application. The colors also indicate the polarity of the leads to ensure proper insertion into the thermal connector.
Thermocouples “age” due to high temperatures and environmental conditions, which causes them to become less accurate over time. 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 a variety of industrial and scientific applications. Well-designed thermal connectors help ensure the accuracy of these measurements and can expedite the replacement of worn thermocouples and RTDs. due to the wide variety of thermocouple designs, leads are color coded to expedite the identification of devices and their polarity.