Vacuum Furnace Thermocouples (VFTs) must be able to accurately sense the temperature inside of a furnace while simultaneously preventing any outside atmosphere from entering. This requires a sealed construction that also provides a means to penetrate a furnace wall through an additional seal. VFTs are typically used for temperature control of the furnace and/or as a High Limit sensor.
Temperature control is a means of cycling the heating elements or burners on and off to maintain a given temperature, called the setpoint. Used with some sort of PLC or other electronic hardware, this circuit is critical for the furnace to follow a certain temperature ramp profile and to then maintain dwell or soak temperature within very tight tolerances. High Limit sensors are used in a secondary safety circuit that shuts off electricity or gas flow if the temperature exceeds a certain value before damage to equipment can occur or when the furnace operation is unsafe.
CEL’s Vacuum Furnace Thermocouples are specifically engineered for optimal performance and life in industrial vacuum furnaces. The foundation of the design is the thermocouple element. VFTs are available in a variety of calibrations/gauges and can be supplied in either a single element or dual element configuration. Dual element thermocouples allow the end user to minimize the number of ports in the vacuum furnace. Elements are available in Type “B”, “C”, “R”, and “S”.
The defining characteristic of a VFT is the vacuum-tight cold-end of the assembly. The hermetic metal sleeve and sealed end construction along with the alumina protection tube maintain 3 separate atmospheres: vacuum within the furnace, isolated atmosphere within the thermocouple and the outside atmosphere. The vacuum within the furnace is maintained by the alumina protection tube with a back-up seal provided by the cold-end seal in the event of a break or leakage. A loss of vacuum would result in a potential loss of product being treated and a definite loss of time. This dual-redundant seal design avoids a single point of failure and allows the process to continue to completion.
The end seal can be constructed in multiple ways which are ultimately driven by the application and external design constraints. The simplest method is an epoxy seal. The most reliable is a Wire Saver Gland (WSG). CEL has an exclusive WSG design integrated with the sleeve to provide a 1×10-5 mmHg seal in a compact package. Alternative designs can reach upwards of 3 inches long. Even if the space is available, that results in 3 additional inches of unnecessary element length. With precious metal prices at an all time high, the WSG from CEL can provide a significant cost reduction. The combination of the alumina protection tube on one end and the end seal on the other provides a controlled environment for the thermocouple element to reside. This environment prevents contaminants from reaching the element and may also consist of an inert fill gas like Argon. In the case of Type “C” thermocouples, this fill gas is necessary to prevent oxidation of the element at elevated temperatures.
CEL builds each VFT to order and modularity is fundamental to the design to provide the maximum flexibility in lengths of the protection tube and sheath, the cold-end termination, and the process fitting to provide a sealed interface to your furnace. With so many points of customization, costly furnace reconfigurations are avoided without compromising the measurement accuracy or location. Typically, the process fitting consists of a compression fitting that can be located anywhere along the length of the sheath that mates to either an industry-standard pipe thread or a KF-style vacuum flange. Both adjustable and re-adjustable styles are available in a range of materials, sizes, and features.
The most common termination is a standard-size thermocouple plug. CEL’s plugs and jacks are manufactured using thermoset plastic to ensure long life, low-outgassing, and performance at temperatures of up to 400°F. The plugs are color-coded in compliance with ANSI standards and all the internal components are compensated according to the calibration. (Type “B” plugs are an exception as they are not compensated.)
CEL supplies VFTs with Special Limits of Error elements as a standard offering and will provide Lot or Individual Certification upon request.
Fully sealed high temp thermocouple. Used in applications where outside pressure must be completely sealed out.