This technical description details a custom-engineered thermocouple cable designed for precise temperature measurement in demanding environments, specifically within gas turbine wheelspace monitoring applications. This cable solution is tailored to meet stringent performance and reliability requirements, often associated with aerospace-grade applications.
Thermocouple Construction and Properties:
The cable incorporates a Type N thermocouple, utilizing Nicrosil and Nisil conductors. These alloys are chosen for their high-temperature stability and accuracy, particularly in oxidizing environments. The thermocouple conductors are 16 AWG, constructed with a 7/24 stranding configuration (7 strands, each comprised of 24 individual wires). This stranded construction enhances the cable’s flexibility and resistance to fatigue, crucial for applications involving vibration and thermal cycling. The conductors each have a diameter of 0.060 inches. The thermocouple is constructed with “Special Limits” calibration, providing enhanced accuracy over a specified temperature range.
Insulation and Shielding:
The thermocouple conductors are individually insulated with orange and red Perfluoroalkoxy Alkane (PFA) insulation. PFA is a high-performance fluoropolymer known for its excellent electrical insulation properties, chemical inertness, and wide operating temperature range.1 The color coding of the insulation facilitates easy identification and polarity verification during installation. The insulated conductors are twisted together to minimize electromagnetic interference (EMI) and improve signal integrity. A 20 AWG, 19/32 stranded drain wire (19 strands each comprised of 32 individual wires) is included to provide a path for grounding and further enhance EMI protection. A nickel-plated copper braid shield is applied over the twisted pair and drain wire to provide robust EMI shielding. This shielding is essential for maintaining signal integrity in electrically noisy environments, such as those found near gas turbines.
Outer Jacket and Finished Dimensions:
The cable is jacketed with a brown PFA jacket, conforming to the ANSI color code for Type N thermocouples. This jacket provides additional protection against abrasion, chemicals, and high temperatures. The nominal finished diameter of the cable is 0.215 inches. This precise diameter is critical for ensuring proper fit and integration within the gas turbine monitoring system.
Application and Performance:
This high-temperature thermocouple cable is designed for use in gas turbine wheelspace monitoring applications. The Type N thermocouple, with its “Special Limits” calibration, provides accurate and reliable temperature measurements in the high-temperature environment of the turbine. The stranded conductors and PFA insulation ensure flexibility and durability, while the twisted pair, drain wire, and braid shield minimize EMI and maintain signal integrity. The cable’s construction and materials are selected to meet the stringent performance and reliability requirements often associated with aerospace-grade applications.
Technical Considerations:
- Temperature Rating: The PFA insulation and jacket determine the maximum operating temperature of the cable.
- Accuracy and Stability: The Type N thermocouple and “Special Limits” calibration ensure accurate and stable temperature measurements.
- EMI/RFI Shielding: The twisted pair, drain wire, and braid shield provide robust EMI/RFI protection.
- Flex Life: The stranded conductors enhance the cable’s flex life, making it suitable for applications involving vibration and thermal cycling.
- Chemical Resistance: The PFA insulation and jacket provide excellent resistance to chemicals and fluids commonly found in gas turbine environments.
- Dimensional Tolerances: Precise dimensional tolerances are essential for ensuring proper fit and integration within the monitoring system.
- Voltage Rating: The insulation thickness and material determine the voltage rating of the cable.
- Calibration: The “Special Limits” calibration should be traceable to national standards.
“All problems exist in the absence of a good conversation.” – Thomas Leonard