Measuring temperature inside a vacuum chamber requires a thermocouple feedthrough that preserves the measurement junction's accuracy across the vacuum boundary. The two most requested types are K and C — and they are not interchangeable.
Type K: the General-Purpose Standard
Type K (NiCr-NiSi / Chromel-Alumel) is the most common thermocouple in industry:
- Range approximately -200 °C to 1200 °C, sensitivity ~41 µV/°C.
- Low cost, widely available, compatible with virtually every controller and recorder.
- Susceptible to drift and oxidation above ~800 °C in air; in vacuum the picture changes because there is no oxygen to oxidize the alloy — but outgassing and alloy evaporation must still be managed.
Type C: for the Extreme End
Type C (W5%Re / W26%Re, tungsten-rhenium) is a refractory thermocouple for very high temperatures:
- Range approximately 0 °C to 2320 °C.
- Used in vacuum furnaces, crystal growth, thermal processing and high-temperature materials research where K and R types would melt or drift.
- Brittle wire — mechanical handling and routing need care; the thermocouple junction is typically made in-situ by the user.
- Requires specialized readout support; many industrial controllers now include Type C tables.
Through-the-Wall Considerations
Whichever type you choose, the feedthrough itself must not corrupt the measurement:
- True thermocouple feedthroughs use the thermocouple alloys themselves as the pins (positive and negative legs), so the feedthrough introduces no dissimilar-metal junction — the only junction is your measuring junction.
- For power-plus-sense applications, combined feedthroughs integrate thermocouple pins (Chromel/Alumel for K, or W-Re for C) alongside power conductors in one ceramic seal.
- Ceramic sealing gives excellent insulation at high temperatures and keeps leakage currents out of the measurement circuit.
Quick Comparison
| Property | Type K | Type C |
|---|---|---|
| Range | -200 to 1200 °C | 0 to 2320 °C |
| Sensitivity | ~41 µV/°C | ~20 µV/°C |
| Wire | NiCr / NiSi | W-Re |
| Cost | Low | High |
| Typical vacuum use | Process chambers, R&D | Furnaces, crystal growth, extreme processes |
HermiCore supplies both Type C and Type K thermocouple feedthroughs, 1 to 4 pairs, with miniature connectors on the atmosphere side. If your process runs hot, tell us the expected temperature profile and we will help you select the right couple and seal design.
