Selection Guides

PT100 Accuracy Classes Explained: AA, A, B and C per IEC 60751

2026-10-17

What PT100 tolerance classes AA, A, B and C really mean in °C at your working temperature — formulas, a worked tolerance table, validity ranges for wire-wound vs thin-film elements, and how to choose the right class.

When a datasheet says "PT100, Class A", it is making a precise promise defined by IEC 60751 — not a marketing label. The class tells you the maximum deviation between the sensor's indicated temperature and the true temperature, and that deviation grows with temperature. This guide shows how to turn a class into real °C numbers at your working point, and how to choose a class without overpaying for accuracy you cannot use.

The tolerance formulas

IEC 60751 defines four tolerance classes for platinum resistance thermometers. The permitted deviation depends on the measured temperature t (use the absolute value, without sign). The table below gives the formula for each class and the resulting tolerance at several temperatures:

Class	Formula	At 0 °C	At 100 °C	At 200 °C	At 500 °C
AA	±(0.10 + 0.0017·\|t\|)	±0.10 °C	±0.27 °C	±0.44 °C	—
A	±(0.15 + 0.002·\|t\|)	±0.15 °C	±0.35 °C	±0.55 °C	—
B	±(0.30 + 0.005·\|t\|)	±0.30 °C	±0.80 °C	±1.30 °C	±2.80 °C
C	±(0.60 + 0.01·\|t\|)	±0.60 °C	±1.60 °C	±2.60 °C	±5.60 °C

Two things stand out. First, the often-quoted "Class A = ±0.15 °C" is only true at 0 °C; at 200 °C the same sensor may deviate ±0.55 °C and still be fully in spec. Second, Class B — the industrial default — is roughly twice as loose as Class A across the range. Whether that matters depends entirely on your process window.

The validity range depends on the element construction

Each class is only valid over a limited temperature range, and the range differs between wire-wound and thin-film elements:

Class	Wire-wound element	Thin-film element
AA	−50 … +250 °C	0 … +150 °C
A	−100 … +450 °C	−30 … +300 °C
B	−196 … +600 °C	−50 … +500 °C
C	−196 … +600 °C	−50 … +600 °C

A thin-film Class A probe used at 350 °C is outside its validity range even if it still "works" — its deviation is no longer guaranteed by the standard. When you order, the class and the element type must be specified together.

You may also meet commercial designations such as "1/3 DIN" or "1/10 DIN". These are fractions of the Class B tolerance and are not IEC classes — check the formula behind the label before comparing quotes.

How to choose a class in practice

Class B is the right default for general industrial control: HVAC, machinery, tanks, pipelines. If your control tolerance is ±2 °C or wider, Class A buys you nothing.
Class A suits quality-critical points: food and pharmaceutical processes, energy metering, tight reactor control — anywhere a ±0.5 °C error at the working point has a cost.
Class AA is for reference and laboratory use, and for custody-transfer style measurements. Pair it with a 4-wire connection; with a 2-wire connection, lead resistance (about 2.6 °C of error per ohm on a Pt100) would destroy the AA accuracy before the signal reaches your instrument.
Class C appears in low-cost and high-volume products where a few degrees of error are acceptable.

One more trap: the class describes the sensing element, not the whole measurement chain. Transmitter error, lead effects, immersion depth and self-heating all add to the loop uncertainty. A Class A element behind a poorly immersed pocket still measures the pocket, not the process.

Specifying it correctly

A complete order specification looks like: Pt100, Class A, thin-film, 3-wire, −50…+300 °C. If any of the four items — class, element type, wiring, range — is missing, two suppliers can quote "the same" sensor with very different real performance.

Universal Ruike builds Pt100/RTD probes to order — element class, wiring, probe diameter, length and cable are all specified per application. Tell us your process temperature and the accuracy you actually need at that point, and we will match the class and construction instead of selling you the tightest label.