How do you interpret accuracy grades? A-grade, B-grade, and AA-grade: a practical comparison of their differences.
When purchasing Temperature Sensors, many people focus solely on the term "PT100" but overlook the core indicator that determines measurement reliability: the accuracy class. If the accuracy class is not selected correctly, even the most expensive sensor may provide you with a "beautiful but erroneous" temperature reading. Today, using real-world data, we will clearly explain the differences between AA, A, and B classes.
- What exactly is the accuracy class?
The accuracy class, simply put, refers to the maximum allowable deviation between the temperature value measured by the sensor and the true temperature. The smaller the deviation, the higher the accuracy. This parameter is not arbitrarily specified by manufacturers; rather, it is defined in strict accordance with the international standard IEC 60751 (which corresponds to the Chinese national standard GB/T 30121-2013).
In actual procurement processes, the most common mistake is to simply write "High-precision PT100" without specifying the accuracy class. As a result, upon delivery, it may be discovered that the measurement error is excessive; upon tracing back, it becomes clear that the supplier has defaulted to providing Class B—while your manufacturing process clearly requires Class A.
- AA Grade vs. A Grade vs. B Grade: Tolerance Formula versus Measured Data
According to the IEC 60751 standard, the tolerance formulas for the three grades are as follows (where |t| denotes the absolute value of the measured temperature, in °C):
| accuracy class | Allowance formula (°C) | ±0°C tolerance | 100 °C tolerance | 200 °C tolerance |
| AA Rating | ±(0.1 + 0.0017|t|) | ±0.10℃ | ±0.27℃ | ±0.44℃ |
| Grade A | ±(0.15 + 0.002|t|) | ±0.15℃ | ±0.35℃ | ±0.55℃ |
| B-class | ±(0.3 + 0.005|t|) | ±0.30℃ | ±0.80℃ | ±1.30℃ |
| Class C | ±(0.6 + 0.01|t|) | ±0.60℃ | ±1.60℃ | ±2.60℃ |
Key finding: The higher the temperature, the greater the gap between the grades. At 0 °C, the difference between Grade A and Grade B is 0.15 °C; at 200 °C, this gap widens to 0.75 °C; and at 400 °C, it exceeds 1.35 °C. For precision temperature-controlled processes, such deviations are sufficient to result in non-conforming products.
- Applicable Temperature Ranges and Use Cases for Each Grade
| accuracy class | Applicable temperature range | Typical Application Scenarios | Cost Reference |
| AA Rating | -50℃ ~ +250℃ | Laboratory metrology, standard transfer, semiconductor wafers | highest |
| Grade A | -100℃ ~ +450℃ | Pharmaceutical sterilization, chemical reaction vessels, precision heat treatment | secondary |
| B-class | -196℃ ~ +600℃ | HVAC systems, boiler feed water systems, and general industrial monitoring systems | lowest |
- Practical Selection Guide: 3 Principles to Avoid Common Pitfalls
Principle 1: Determining grades based on process tolerance limits
Adhere to the "1/3 rule" —the sensor accuracy should be at least 3 times higher than the system's allowable error. For example, if the process requires temperature control within ±0.5°C, the sensor accuracy must reach at least the ±0.15°C level, which corresponds to selecting Grade A. If only Grade B (±0.3°C) is selected, the combined effect of installation errors will render the system completely unable to meet the required specifications.
Principle 2: High-temperature scenarios must be upgraded in tier.
The temperature coefficient term in the permissible error formula increases as the temperature rises. For Class B, the error is only ±0.3°C at 0°C, but it can increase to ±1.3°C at 200°C. If your process temperature exceeds 200°C, Class B may not be sufficient; it is recommended to select at least Class A.
Principle 3: The wiring system must match the precision grade.
This is one of the most easily overlooked points: if AA-grade sensors are configured using a two-wire connection, the errors introduced by lead resistance may directly negate their accuracy advantages. The empirical rule states: A-grade or higher-grade sensors must be equipped with either a three-wire or four-wire configuration; a two-wire configuration is only suitable for B-grade or lower-grade sensors in short-distance applications.
- 3 common misconceptions—how many have you encountered?
Misconception 1: "PT100 means high accuracy"—Incorrect. PT100 is merely a sensor model; the accuracy class is what determines measurement accuracy. For the same Pt100 Sensor, the error between Class B and Class AA at 100 °C differs by a factor of 3.
Misconception 2: "The higher the precision, the better" —this is not necessarily true. In applications such as HVAC systems and boiler feed water systems, Class B performance is entirely sufficient; blindly opting for Class AA will only increase costs, and Class AA is not applicable in high-temperature operating ranges.
Misconception 3: "Just checking the nameplate is sufficient" —this is insufficient. The specifications on the nameplate indicate the factory-verified accuracy; however, sensors may experience drift during actual operation. It is recommended to calibrate critical processes once per year to verify whether the actual accuracy remains within the specified tolerance range.
- When selecting equipment, don't overthink it—leave professional matters to the professionals.
Weilian Fengran Sensing Technology specializes in the research, development, and manufacturing of platinum resistance thermocouple (RTD) Temperature Sensors, offering AA-grade, A-grade, and B-grade PT100/PT1000 products across all accuracy classes, with customized selection solutions available based on specific operating conditions.
Not sure which grade to choose? Let us know your process temperature and accuracy requirements, and we'll help you make the right choice in just one step.