# Selecting Thermocouple Wire for Industrial Heat Treatment Furnaces

> Box, vacuum, pit and continuous furnaces each impose different requirements on thermocouple alloy conductors. A practical selection guide for heat treatment professionals.


> 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

1. **What is the maximum and typical operating temperature?** If >1000 °C, consider Type N.
2. **What is the furnace atmosphere?** Reducing/vacuum → Type N. Oxidising → Type K.
3. **How long is the wire run?** >10 m → φ0.5 mm minimum.
4. **Is the thermocouple exposed to vibration or impact?** Yes → stranded wire.



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> **Specifying thermocouple wire for heat treatment?** Tell us your furnace type, temperature and atmosphere — we recommend the right alloy, diameter and form. [Contact engineering →](/contact/)