Furnace type and atmosphere decide the alloy; >1000 °C or vacuum means Type N.
One Process, Many Furnace Types
Industrial heat treatment encompasses a wide range of furnace designs, each with distinct temperature, atmosphere and mechanical requirements for thermocouple wire. Choosing the wrong alloy type or wire diameter can result in premature sensor failure, inaccurate temperature readings, and costly furnace downtime.
Furnace Type vs. Recommended Wire
Box Furnaces (800–1200 °C, oxidising atmosphere)
The workhorse of the heat treatment industry. Batch loading and unloading subjects thermocouples to thermal cycling and occasional mechanical impact.
- Recommendation: Type K, φ0.5–1.0 mm, solid or stranded
- Atmosphere: Oxidising (air, combustion products) — Type K performs well
- Installation: Ceramic sheath recommended for mechanical protection
Vacuum Furnaces (500–1300 °C, low pressure)
Used for aerospace alloys and tool steels. The low-pressure environment demands wire with minimal outgassing and stable EMF in vacuum.
- Recommendation: Type N, φ0.25–0.5 mm bare wire
- Critical consideration: Type K must be avoided — chromium outgasses in vacuum and green rot develops rapidly
- Traceability: AMS 2750 requires batch-traceable wire for aerospace applications
Pit Furnaces (600–1000 °C, various atmospheres)
Vertical design for long shafts, tubes and coils. Thermocouples are lowered from above with long immersion lengths.
- Recommendation: Type K stranded, φ1.0–3.0 mm
- Why stranded: Vibration from charging cranes and thermal expansion of long wire runs. Stranded wire lasts 3–5× longer than solid in pit furnace installations
- Length consideration: Long runs (>10 m) need φ0.5 mm or thicker to avoid signal attenuation
Continuous Annealing Lines (400–1100 °C, controlled atmosphere)
Wire, strip and tube pass continuously through a heated zone. Temperature control must respond rapidly to line speed changes.
- Recommendation: Type K, φ0.1–0.5 mm bare wire
- Why thin: Fast thermal response tracks product temperature as line speed varies
- Replacement interval: Thin wire in continuous service — plan for replacement every 3–6 months
Ceramic Kilns (900–1300 °C, oxidising atmosphere)
Batch and tunnel kilns for tableware, technical ceramics and refractories. Oxidising atmosphere, high temperature.
- Recommendation: Type K or N, φ0.5–1.5 mm
- Above 1200 °C: Type N strongly preferred — Type K drift accelerates rapidly
- Sheath material: Ceramic (alumina) for temperatures above 1100 °C
Four Questions to Ask Before Ordering
- What is the maximum and typical operating temperature? If >1000 °C, consider Type N.
- What is the furnace atmosphere? Reducing/vacuum → Type N. Oxidising → Type K.
- How long is the wire run? >10 m → φ0.5 mm minimum.
- Is the thermocouple exposed to vibration or impact? Yes → stranded wire.
Specifying thermocouple wire for heat treatment? Tell us your furnace type, temperature and atmosphere — we recommend the right alloy, diameter and form. Contact engineering →
Frequently Asked Questions
Which thermocouple type suits a vacuum furnace?
Type N, φ0.25–0.5 mm bare wire. Type K must be avoided in vacuum: chromium outgasses and green rot develops rapidly. Aerospace work also needs batch-traceable wire to satisfy AMS 2750.
Why is stranded wire used in pit furnaces?
Because charging cranes and the thermal expansion of long immersion lengths create constant vibration, which work-hardens and fractures solid conductors. Stranded wire lasts 3–5× longer than solid in pit furnace installations.
What wire diameter is used on continuous annealing lines?
φ0.1–0.5 mm bare wire. The thin section gives the fast thermal response needed to track product temperature as line speed varies, at the cost of a planned replacement interval of 3–6 months.
Above what temperature should I switch from Type K to Type N in a kiln?
Above 1200 °C. Type K drift accelerates rapidly beyond that point, so Type N is strongly preferred, with an alumina ceramic sheath above 1100 °C.
Bottom line: Specify from four answers: the maximum and typical operating temperature (above 1000 °C points to Type N), the furnace atmosphere (reducing or vacuum points to Type N, oxidising to Type K), the wire run length (over 10 m needs φ0.5 mm or thicker), and whether the thermocouple sees vibration or impact (which calls for stranded rather than solid conductor).