When selecting an RTD (Resistance Temperature Detector), one of the most important specifications is its accuracy class. These classes are defined in the international standard IEC 60751, which specifies performance requirements for industrial platinum RTDs.
Understanding these accuracy classes helps engineers choose the appropriate sensor for process control, laboratory measurement, or general industrial monitoring.
What Is IEC 60751?
IEC 60751 is the international standard that defines:
Resistance–temperature relationship for platinum RTDs
Tolerance (accuracy) classes
Temperature coefficients
Construction requirements
Test methods
It applies primarily to platinum RTDs such as Pt100 and Pt1000 sensors.
What Does “Accuracy Class” Mean?
An accuracy class defines the maximum permissible deviation between the sensor’s measured temperature and the true temperature, within a specified range.
The deviation is expressed as a formula combining:
A fixed error component
A temperature-dependent error component
This reflects the fact that RTD accuracy changes slightly as temperature increases or decreases.
IEC 60751 Accuracy Classes Overview
The most common classes are:
Class AA
Class A
Class B
Class C
(Some manufacturers may also reference 1/3 DIN, 1/5 DIN, etc., which are tighter tolerances derived from Class B.)
Accuracy Class Formulas (Platinum RTDs)
According to IEC 60751, the maximum permissible deviation (ΔT) is defined as:
Class AA: ±(0.1 + 0.0017 |t|) °C
Class A: ±(0.15 + 0.002 |t|) °C
Class B: ±(0.3 + 0.005 |t|) °C
Class C: ±(0.6 + 0.01 |t|) °C
Where:
t = temperature in °C
|t| = absolute value of temperature
This means allowable error increases slightly as temperature moves away from 0 °C.
Example Accuracy Comparison
At 100 °C:
Class AA: ±(0.1 + 0.0017×100) = ±0.27 °C
Class A: ±(0.15 + 0.002×100) = ±0.35 °C
Class B: ±(0.3 + 0.005×100) = ±0.8 °C
At 0 °C:
Class AA: ±0.1 °C
Class A: ±0.15 °C
Class B: ±0.3 °C
This demonstrates why higher-class sensors are preferred in precision applications.
Temperature Ranges for Each Class
Typical applicable ranges (may vary by manufacturer):
Class AA: Limited range (often narrower than A or B)
Class A: Moderate industrial temperature range
Class B: Wide industrial range
Class C: Basic industrial use
Higher accuracy classes often have more restricted operating ranges.
What Is “1/3 DIN” or “1/10 DIN”?
These are tighter tolerance versions derived from Class B.
For example:
1/3 DIN = one-third of Class B tolerance
1/5 DIN = one-fifth of Class B tolerance
These are commonly used in laboratory or calibration applications but are not separately defined as primary classes in IEC 60751.
What Accuracy Class Should You Choose?
Class AA
Best for:
Calibration laboratories
High-precision research
Pharmaceutical production
Reference measurements
Class A
Best for:
Industrial control systems
Process automation
HVAC systems
Quality monitoring
Class B
Best for:
General industrial use
Machinery monitoring
Non-critical temperature measurement
Class C
Best for:
Basic monitoring applications
Cost-sensitive installations
Important: System Accuracy vs Sensor Accuracy
RTD class tolerance only defines the sensor element accuracy. Overall system accuracy also depends on:
Transmitter accuracy
Lead wire configuration (2-wire, 3-wire, 4-wire)
Installation quality
Immersion depth
Calibration practices
A Class AA sensor in a poorly designed system will not achieve Class AA performance.
Does Higher Accuracy Mean Longer Lifespan?
Not necessarily.
Accuracy class defines initial tolerance, not long-term stability. All RTDs may experience drift over time due to:
Thermal cycling
Mechanical stress
Contamination
High-temperature exposure
Regular calibration is recommended for critical processes.
Key Takeaways
IEC 60751 defines platinum RTD accuracy classes.
Class AA provides the tightest tolerance; Class C the widest.
Tolerance increases slightly with temperature.
Application requirements should determine class selection.
System design is as important as sensor class.
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