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:
- Maximum temperature — above 1000 °C, Type N; below, Type K is usually sufficient.
- Atmosphere — vacuum or reducing, Type N; clean and oxidising, Type K.
- 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.
Need help deciding? Contact our engineering team with your operating temperature and atmosphere — we reply with a data-backed recommendation within one business day.
Frequently Asked Questions
Above what temperature does Type K start to suffer green rot?
Green rot affects Type K between roughly 800 °C and 1050 °C in reducing or low-oxygen atmospheres, where chromium in the Chromel positive leg oxidises selectively instead of forming a protective oxide scale.
How much does Type N reduce EMF drift compared with Type K?
At 1000 °C over 500 hours, Type K typically drifts −2 to −5 °C while Type N stays within ±1 °C — roughly a five-fold improvement, because the 1.4% silicon in Nicrosil forms a protective SiO₂ sub-layer.
Is Type N more expensive than Type K?
Yes. Type N wire carries a 1.2× to 1.4× cost multiplier over Type K. For a new installation above 800 °C the premium is normally recovered the first time an unplanned furnace shutdown is avoided.
Can Type N be substituted into an existing Type K installation?
Not directly. If the instrument, cold-junction compensation and connectors are calibrated for Type K, substituting Type N makes the reading wrong. For retrofit work we supply Type K to Class 1 tolerance with full batch traceability instead.
Bottom line: Below 1000 °C in clean oxidising air Type K remains the default. Above 1000 °C, in vacuum or reducing atmospheres, or wherever long-interval drift costs more than the wire premium, specify Type N.