# Type K vs Type N — Which Alloy for High-Temperature Applications?

> Type N was developed to solve Type K's drift problems above 1000 °C. Here is the engineering basis for choosing between them.


> 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:

1. **Maximum temperature** — above 1000 °C, Type N.
2. **Atmosphere** — vacuum or reducing, Type N; clean and oxidising, Type K.
3. **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.