Temperature sensor types and temperature ranges

A Pt100 measures the resistance of a platinum element and is the accuracy choice up to about 600 °C. A thermocouple measures the voltage between two different metals and reaches up to 1,700 °C depending on the type. IEC 60751 defines Pt100 accuracy classes; IEC 60584 defines thermocouple tolerances.

Thermocouples. In the KP programme, type K is recommended up to 1,150 °C and type N up to 1,250 °C, type S and R up to 1,600 °C, and type B up to 1,700 °C. Below 600 °C, a Pt100 usually gives better accuracy and stability.

A Pt100 whose deviation stays within ±(0.15 + 0.002 · t) °C, valid from −100 to +450 °C to IEC 60751. At 100 °C that is ±0.35 °C. Class B allows ±(0.30 + 0.005 · t) °C over a wider range.

The number states the nominal resistance at 0 °C: a Pt100 element measures 100 Ω, a Pt500 500 Ω and a Pt1000 1,000 Ω. All three follow the same IEC 60751 characteristic and tolerance classes, so accuracy is identical class for class. The higher base resistance of a Pt1000 makes lead-wire resistance far less significant and allows a lower measuring current, which reduces self-heating — an advantage in 2-wire connections and compact instruments.

In class 1, ±1.5 °C or ±0.004 · t, whichever is higher — so ±1.5 °C up to 375 °C and ±4.0 °C at 1,000 °C, the upper end of the tolerance class. In class 2, ±2.5 °C or ±0.0075 · t.

Both are nickel-based thermocouples with nearly identical standard ranges. Type N is a refinement of type K: its Nicrosil/Nisil alloys form a protective silicon-oxide layer, so it resists the drift and the selective chromium oxidation ("green rot") that affect type K in oxygen-starved combustion atmospheres. That is why type N carries the higher recommended maximum in the KP programme — 1,250 °C against 1,150 °C — and why it is preferred for long service in furnaces and kilns, while type K remains the versatile general-purpose choice.

Type J gives a slightly stronger signal per degree than type K and remains common in older plants and machinery. Its iron conductor limits it to 700 °C and to dry conditions — so for new installations at higher temperatures, or where long-term stability matters, type K or type N is the usual choice.

All three are platinum-rhodium thermocouples for the highest temperatures. Type R (13 % rhodium) gives a slightly higher output than type S (10 %); type S is the classic reference type. Type B, with rhodium in both conductors, reaches 1,700 °C — and because it produces almost no signal below about 50 °C, cold-junction errors barely affect it.

When the process window is narrow, or when the measurement documents a critical step. A 1/10 DIN Pt100 stays within ±0.03 °C at 0 °C — ten times tighter than Class B. For most duty, Class B is the practical standard; specify a tighter class where the process demands the accuracy.

It is how far the standard's reference tables go. Type K is defined up to 1,372 °C, but 1,150 °C is the recommended maximum for continuous service in the KP programme — the span between the two is a reserve for peak conditions rather than a working range.