# Type K vs Type N — Why Type N Outperforms Type K Above 1000 °C

> The engineering basis for choosing between Type K and Type N thermocouple wire: green rot, oxidation resistance, EMF drift and cost trade-offs.


> Type N resists green rot; Type K drifts −2 to −5 °C after 500 h at 1000 °C.

## The Problem with Type K Above 1000 °C

Type K (**Chromel**/**Alumel**) is the most widely used base-metal thermocouple in industrial service. It is reliable, well-documented, and cost-effective for general industrial use up to approximately 1000 °C. But above this temperature, particularly in reducing or low-oxygen atmospheres, Type K develops a well-known failure mechanism: **green rot**.

Green rot is the selective oxidation of chromium in the Chromel (NiCr 90/10) positive leg. At 800–1050 °C in atmospheres with insufficient oxygen, the chromium preferentially oxidises, leaving behind a nickel-rich matrix with a greenish surface scale. This changes the Seebeck coefficient of the wire, producing a **large negative EMF drift** — typically −2 to −5 °C after 500 hours at 1000 °C.

## Enter Type N

Type N (**Nicrosil**/**Nisil**) was developed jointly by NPL (UK) and NIST (USA) specifically to solve this problem. The positive leg, Nicrosil (NiCrSi 84.4/14.2/1.4), contains **1.4% silicon** which forms a protective SiO₂ sub-layer at high temperature. This oxide layer suppresses chromium oxidation, making Type N effectively immune to green rot.

## Head-to-Head Comparison

| Property | Type K | Type N |
|----------|--------|--------|
| Positive leg | Chromel (NiCr 90/10) | Nicrosil (NiCrSi 84.4/14.2/1.4) |
| Negative leg | Alumel (NiAl 95/5) | Nisil (NiSiMg 95.4/4.4/0.15) |
| Max. temperature | 1370 °C | 1300 °C |
| Drift at 1000 °C / 500 h | −2 to −5 °C | < ±1 °C |
| Green rot | Susceptible | Immune |
| Cost multiplier | 1.0× (baseline) | 1.2–1.4× |

## When to Choose Type K

Type K remains the right choice when your application operates in **clean, oxidising atmospheres below 1000 °C**, you have existing Type K instrumentation and connectors, and budget is the primary constraint. It has the widest availability of matching connectors, transmitters and reference data.

## When to Choose Type N

Choose Type N when your application involves **temperatures above 1000 °C**, vacuum or reducing atmospheres, or when **minimal sensor drift over long service intervals** is critical. Type N is increasingly specified for aerospace (AMS 2750 pyrometry), gas turbine exhaust monitoring, and semiconductor manufacturing — applications where the cost of sensor replacement far exceeds the incremental cost of Type N wire.

## Our Recommendation

Work through three checks in order:

1. **Maximum temperature** — above 1000 °C, Type N; below, Type K is usually sufficient.
2. **Atmosphere** — vacuum or reducing, Type N; clean and oxidising, Type K.
3. **Instrumentation** — if the instrument, connectors and cold-junction compensation are already calibrated for Type K, stay with Type K unless you are replacing the whole loop.

If you are specifying wire for a **new installation** above 800 °C, we recommend Type N. The 20–40% cost premium is recovered the first time you avoid an unplanned furnace shutdown due to sensor drift. For **retrofit applications** where the instrument is calibrated for Type K, we supply Type K with Class 1 tolerance and full batch traceability.



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> **Need help deciding?** Contact our engineering team with your operating temperature and atmosphere — we reply with a data-backed recommendation within one business day.