Type N is immune to green rot; Type K drifts −2 to −5 °C at 1000 °C.
The green-rot problem
Type K thermocouples use Chromel (NiCr 90/10) for the positive leg. At temperatures between 800 °C and 1050 °C in low-oxygen (reducing) atmospheres, the chromium in the positive leg oxidises preferentially. This produces a greenish scale on the wire surface — hence “green rot” — and causes a large negative EMF drift (typically −2 to −5 °C after 500 hours at 1000 °C).
Type N was developed at NPL (UK) and NIST (USA) specifically to solve this. The positive leg, Nicrosil (NiCrSi 84.4/14.2/1.4), contains silicon which forms a protective SiO₂ sub-layer that suppresses chromium oxidation. The result: Type N drift is typically less than 1 °C after 500 hours at 1000 °C — a 5× improvement over Type K.
| 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 + Mn/Si) | Nisil (NiSiMg 95.4/4.4/0.15) |
| Max temperature | 1370 °C | 1300 °C |
| Drift at 1000 °C / 500h | −2 to −5 °C | < ±1 °C |
| Green rot susceptibility | Yes — above 800 °C in low O₂ | No — SiO₂ protection layer |
| EMF at 1000 °C | 41.276 mV | 37.326 mV |
| Sensitivity at 500 °C | ≈42 µV/°C | ≈38 µV/°C |
| Cost factor | 1.0 (baseline) | 1.2–1.4× |
| Magnetic? | Alumel: slightly magnetic | Both legs non-magnetic |
| Standard | IEC 60584-1 / ASTM E230 | IEC 60584-1 / ASTM E230 |
Recommendation
Three checks decide it:
- Maximum temperature — above 1000 °C, Type N.
- Atmosphere — vacuum or reducing, Type N; clean and oxidising, Type K.
- Existing instrumentation — if the instrument and connectors are already configured for Type K, confirm the cost of changing the whole loop before specifying Type N.
Choose Type K if: Your application operates in clean, oxidising atmospheres below 1000 °C, you have existing Type K instrumentation, and cost is the primary concern. Type K remains the industry standard with the widest availability of connectors, instruments and reference data.
Choose Type N if: Your application operates above 1000 °C, involves vacuum or reducing atmospheres, or requires minimal drift over long service intervals. Type N is increasingly specified for aerospace (AMS 2750), power generation and semiconductor manufacturing where sensor replacement is costly.
Frequently Asked Questions
Which is more stable over time above 1000 °C, Type K or Type N?
Type N. Type K drifts −2 to −5 °C after 500 hours at 1000 °C in reducing atmospheres, while Type N stays within ±1 °C — roughly a five-fold improvement.
What makes Type N resistant to green rot?
The silicon in Nicrosil (NiCrSi 84.4/14.2/1.4) forms a protective SiO₂ sub-layer at temperature, suppressing the oxidation of chromium that causes green rot in the Chromel leg of Type K.
What is the EMF difference between Type K and Type N at 1000 °C?
Type K produces 41.276 mV and Type N produces 37.326 mV at 1000 °C. The curves are not interchangeable, so instrumentation must be configured for the type actually installed.
How much more does Type N cost?
Type N carries a 1.2–1.4× cost factor against Type K. It is specified for aerospace (AMS 2750), power generation and semiconductor work where sensor replacement is expensive.
Bottom line: Type K and Type N are not interchangeable at 1000 °C — 41.276 mV against 37.326 mV, and only Type N resists green rot. Stay with Type K below 1000 °C in clean oxidising air; move to Type N above 1000 °C, in vacuum or reducing atmospheres.