The nameplate of a differential pressure (DP) transmitter may state an accuracy of ±0.075%, ±0.05%, or even ±0.04%. Many users assume this is the actual measurement accuracy they will achieve in the field. In reality, the final measurement error can be significantly higher due to factors such as turndown ratio, temperature changes, static pressure effects, and installation conditions.
What Does the Nameplate Accuracy Mean?
For a standard DP transmitter (without remote seals), the specified accuracy on the datasheet or nameplate is usually the reference accuracy under laboratory conditions.
For example:
- Sensor range: 0–100 kPa
- Reference accuracy: ±0.075% URL (Upper Range Limit)
The corresponding error is:
±100 kPa × 0.075% = ±0.075 kPa
This value is obtained under controlled test conditions and does not necessarily represent the accuracy achieved in actual process applications.
How Does Turndown Ratio Affect Accuracy?
Modern smart DP transmitters often support large turndown ratios, such as 10:1, 50:1, or even 100:1.
Assume:
- Sensor range: 0–100 kPa
- Configured operating range: 0–10 kPa
The turndown ratio is 10:1.
Although the transmitter is still specified as ±0.075% URL, the error is calculated based on the full sensor range:
±100 kPa × 0.075%
= ±0.075 kPa
When converted to the actual operating range:
±0.075 ÷ 10
= ±0.75% of calibrated span
In other words, a transmitter advertised as ±0.075% accuracy may effectively perform as ±0.75% of span when ranged down by 10:1.
This is one of the most common misunderstandings in transmitter selection.
Why Are Remote Seal DP Transmitters Less Accurate?
Remote seal differential pressure transmitters include additional components such as:
- Isolation diaphragms
- Capillary tubes
- Fill fluid
These components introduce additional measurement uncertainty.
Factors that can further affect performance include:
- Process temperature changes
- Ambient temperature fluctuations
- Capillary length
- Installation orientation
- Static pressure effects
As a result, remote seal systems typically require factory calibration as a complete assembly, and their overall accuracy is generally lower than that of a standard DP transmitter.
Depending on the application and configuration, the total system accuracy is often between ±0.1% and ±0.5%, and may be higher in demanding high-temperature or long-capillary installations.
Practical Selection Recommendations
Although many modern transmitters support turndown ratios of 50:1 or even 100:1, achieving the best measurement performance requires more than simply maximizing the rangeability.
For most industrial applications:
- Keep the normal operating point within 30%–80% of the calibrated range whenever possible.
- Avoid excessive ranging-down if high accuracy is required.
- Carefully evaluate temperature and static pressure effects, especially for remote seal installations.
A turndown ratio within 10:1 is generally recommended when measurement accuracy is critical.
Conclusion
The ±0.075% value shown on a transmitter nameplate does not automatically represent the actual field accuracy.
To determine the real measurement performance, engineers must consider:
- Turndown ratio
- Temperature effects
- Static pressure influence
- Remote seal configuration
- Installation conditions
Only after evaluating all these factors can the true accuracy of a differential pressure transmitter be assessed.