In industrial instrumentation, especially during product selection and calibration, we often hear statements like:
“This pressure transmitter is Class 0.5, so the accuracy should be fine.”
or
“The measured value deviates by 0.2 kPa, so the instrument accuracy is poor.”
In fact, these statements often confuse accuracy and accuracy class, two closely related but fundamentally different concepts.
Understanding the difference is essential for proper instrument selection, calibration, and process control.
Whether you are selecting a pressure transmitter, flow meter, level transmitter, or temperature instrument, this distinction can directly affect measurement reliability and project performance.
What Is Accuracy?
Accuracy refers to how close the measured value is to the true value.
It describes the correctness of the measurement result.
For example, if the true pressure is 100 kPa:
- Measured value = 99.8 kPa → High accuracy
- Measured value = 92 kPa → Low accuracy
The smaller the measurement error, the higher the accuracy.
Common Formula
Example:
True value = 100 kPa
Measured value = 101 kPa
This means the measurement accuracy error is 1%.
What Is Accuracy Class?
Accuracy class is the maximum permissible error specified by the manufacturer or standard.
Typical classes include:
- 0.1
- 0.2
- 0.5
- 1.0
It is usually expressed as a percentage of full scale (FS).
This is a design specification, not the real-time measurement result.
Formula
Example:
A pressure transmitter:
- Range: 0–200 kPa
- Accuracy class: 0.5
Then:
This means under standard operating conditions, the measurement error should remain within:
±1 kPa
Key Difference Between Accuracy and Accuracy Class
| Item | Accuracy | Accuracy Class |
|---|---|---|
| Meaning | Closeness to true value | Maximum allowable error |
| Type | Actual result | Design specification |
| Unit | Absolute / relative error | % of full scale |
| Changes with measurement | Yes | No |
| Used for | Result evaluation | Instrument selection |
Common Mistake in Industrial Applications
Many customers believe:
Higher accuracy class means better actual measurement accuracy.
This is not always true.
For example:
A 0.2 class pressure transmitter with range 0–100 kPa
Actual working pressure = 5 kPa
Maximum allowable error:
Relative error under actual process:
This means the actual process accuracy becomes 4%, which may be unacceptable.
This is why proper range selection is often more important than simply choosing a higher accuracy class.
Practical Selection Advice for Flow Meters and Pressure Instruments
For best performance, the normal operating point should remain within:
30%–80% of full scale
This principle is especially important for:
- electromagnetic flow meters
- Coriolis mass flow meters
- pressure transmitters
- differential pressure transmitters
For example:
If your normal flow is 10 m³/h, selecting a 0–100 m³/h flow meter may result in poor low-end accuracy.
A 0–20 m³/h range would provide much better real-world measurement performance.
This is one of the most common mistakes during project bidding and procurement.
Final Thoughts
Accuracy tells you:
How correct the measurement result is
Accuracy class tells you:
What error limit the instrument is designed to meet
Understanding both parameters is critical for proper instrument sizing and long-term stable operation.
If you are not sure which accuracy class and measuring range is suitable for your application, feel free to contact us for professional technical support.
We can help recommend the right flow meter, pressure transmitter, or level instrument based on your process conditions.