Identifying a thermocouple and a resistance temperature detector (RTD) typically involves examining their physical characteristics, color codes, and wiring configurations. Here's how you can identify each type:

1. Physical Characteristics:

   • Thermocouple: Thermocouples are typically made of metal wires twisted or welded together at one end to form the sensing junction. The wires may be enclosed in a protective sheath made of metal or ceramic material. Thermocouples are often slender and may have exposed junctions in some designs.
   • RTD: Resistance temperature detectors (RTDs) consist of a length of wire wound around a ceramic or glass core. The wire is typically made of a pure metal such as platinum, nickel, or copper. RTDs may be housed in protective sheaths made of metal or other materials. They often have a more robust and solid construction compared to thermocouples.

2. Color Codes:

   • Thermocouple: Thermocouples are often identified by color codes on the wires or connectors, which indicate the types of metal used in the thermocouple pair. Common thermocouple types have specific color combinations, such as Type K (chromel-alumel) being yellow(+) and red(-), Type J (iron-constantan) being white(+) and red(-), etc.
   • RTD: RTDs typically use color codes to identify the type of wire used and its temperature coefficient characteristics. For example, a platinum RTD may have a color code of white for positive and red for negative wires.

3. Wiring Configurations:

   • Thermocouple: Thermocouples are often connected to instrumentation using special connectors designed for thermocouple wires. The connectors are specific to the thermocouple type to ensure proper polarity and accuracy. Additionally, thermocouples are often used in pairs, with one wire acting as the positive lead and the other as the negative lead.
   • RTD: RTDs are usually connected using three or four-wire configurations to compensate for lead resistance and ensure accurate temperature measurements. The wires may be connected to screw terminals or plugs on the instrument or transmitter. RTDs also typically have color-coded wires, with one wire designated as the positive lead.

4. Measurement Principle:

   • Thermocouple: Thermocouples generate a voltage signal proportional to the temperature difference between the sensing junction and the reference junction. The voltage signal is interpreted by instrumentation to determine the temperature.
   • RTD: RTDs change resistance with temperature according to a known resistance-temperature relationship. The resistance change is measured using a constant current source, and the temperature is calculated using the RTD's resistance value and a conversion equation.

By examining these characteristics, including physical appearance, color codes, wiring configurations, and measurement principles, you can accurately identify whether a sensor is a thermocouple or an RTD.