Pressure Transmitter Accuracy Specifications
Pressure transmitters play a crucial role in the process industry, offering precise measurements for effective control.
However, the advertised accuracy specification of these transmitters may not always reveal the complete picture. It often focuses on only a subset of accuracy components, leaving out other factors that can impact the overall accuracy in practical applications.
In this blog, we will learn the accuracy specifications of popular pressure transmitters, shedding light on various accuracy components that influence their performance. We explore the effects of factors such as re-ranging, ambient temperature, mounting position, static pressure, long-term drift, vibration, power supply, and more.
By examining these components and providing illustrative examples, we aim to enhance your understanding of pressure transmitter accuracy and empower you to make informed decisions when selecting the right instrument for your specific needs.
Background
We often encounter misleading accuracy claims in pressure transmitter specifications, where manufacturers highlight a single accuracy number without providing a comprehensive view of all the relevant components. This can create confusion and misrepresent the actual practical accuracy you can expect in your application.
It seems that the pursuit of superior accuracy figures has led to this practice, with some manufacturers emphasizing limited accuracy figures on brochures and websites while burying the full specifications in the user manual. In reality, a pressure transmitter’s accuracy is influenced by multiple components that must be taken into account to assess the total accuracy accurately.
In this blog, we aim to shed light on the crucial factors you should consider by reviewing the specifications of popular pressure transmitters. We will provide insights into the important accuracy components and present typical numbers associated with each partial accuracy component. It’s important to note that our intention is not to undermine or devalue any specific transmitter.
Since the accuracy of your calibration equipment is impacted by the transmitter’s accuracy, we often receive questions from customers in this regard. While the calibrator should be more accurate than the transmitter, the ideal accuracy ratio between the two is subjective. However, documenting the total uncertainty of the calibration is essential.
When selecting a process transmitter, it’s vital to base your tolerance criteria on the specific requirements of your process rather than relying solely on the specifications of the installed transmitter. Now, let’s delve into the detailed examination of these accuracy components and gain a comprehensive understanding of their significance.
Pressure transmitter accuracy components
“Reference accuracy”
It is common to come across a separate “limited” accuracy statement prominently featured on the cover of brochures or websites. This statement, often referred to as “reference accuracy” or something similar, only includes a few accuracy components, such as linearity, hysteresis, and repeatability.
However, it’s important to note that this “best-case accuracy” does not encompass all the practical accuracy components that should be considered, including factors like mounting position and ambient temperature.
Therefore, it would be incorrect to assume that this specification reflects the accuracy you can expect in real-world conditions when the transmitter is installed in your process.
In some cases, this “best-case accuracy” may be as low as 0.04% or even 0.025% of the range, particularly for highly accurate pressure ranges and transmitters.
Different pressure ranges
It is important to recognize that the best (reference) accuracy specified for a pressure transmitter may only be applicable to certain pressure ranges and may vary depending on the type of pressure being measured (absolute or gauge).
For example, while the best accuracy for specific ranges can be as low as 0.04% of the range, other ranges available for the same transmitter model may have a higher accuracy, such as 0.1%. Accuracy specifications can vary significantly for different pressure ranges offered by the transmitter.
Therefore, it is crucial to be aware of the accuracy values specific to the exact pressure ranges and models that you are utilizing in your application. This will ensure that you have accurate and reliable measurements for your intended pressure range.
Re-ranging
HART (smart) transmitters offer the flexibility to be re-ranged over a wide turndown ratio, often reaching up to 100:1 or even more. Accuracy specifications for these transmitters are typically provided for the full range or with a limited turndown ratio.
It is important to note that if you re-range a HART transmitter to a smaller range than the maximum range, it can potentially impact the accuracy of the measurements. In such cases, it is advisable to investigate and determine the extent of error introduced by the reduced range to ensure accurate readings.
By understanding the potential accuracy implications of re-ranging a HART transmitter to a smaller range, you can make informed decisions and take appropriate measures to account for any additional errors that may arise.
Ambient temperature effect
Pressure transmitters, like other measurement devices, are often exposed to varying environmental conditions and the temperature of the pressure media can fluctuate significantly during operation.
One important aspect to consider is the temperature coefficient, which represents an accuracy component that is influenced by the environmental temperature. Although the specification of temperature dependency can sometimes be challenging to comprehend, it is crucial to understand its implications and seek clarification from the supplier if necessary.
Different transmitters exhibit varying degrees of temperature dependency, ranging from around 0.01% of the range to as high as 0.5% of the range. In some cases, certain models may even specify a temperature effect exceeding 1% of the range. If your process involves significant temperature variations, it is essential to account for the temperature dependency of the pressure transmitter.
By considering this factor, you can ensure accurate measurements and mitigate any potential inaccuracies caused by temperature fluctuations.
Static (line) pressure effect
When using differential pressure transmitters under static line pressure conditions, where both inputs have a certain pressure, it’s important to consider the effect of the common-mode static line pressure on the transmitter output. While an ideal differential transmitter would measure only the difference between the inputs, in reality, the static pressure does have some impact on the output.
If both inputs are open to atmospheric pressure, the differential pressure is naturally zero. Similarly, if the same pressure is applied to both inputs, the differential pressure remains zero. However, the static pressure does influence the transmitter output to some extent.
The line pressure effect on the transmitter output typically ranges from 0.025% to 0.4% of the range, depending on the specific transmitter model. This effect is commonly observed in the zero point of the transmitter, while the span is relatively less affected.
During calibration, it is recommended to test the influence of line pressure by applying the same pressure (low and high) to both inputs and noting the change in the zero point. It’s worth noting that line pressure may also have some impact on the span, which adds complexity to handling and calibration. In such cases, a differential pressure standard is required for accurate calibration.
Long term stability
Over time, all measurement devices, including pressure transmitters, experience a gradual decrease in accuracy. This is referred to as stability. Different pressure transmitters have varying stability specifications, typically ranging from 1-year to 10-year stability.
For example, a transmitter with a reference accuracy of 0.04% of the range may have a 1-year stability specification of 0.2% of the range. Other models might have a similar 0.2% of the range stability specification that is valid for 5 or even 10 years. Some exceptional transmitters boast a remarkable 1-year stability as low as 0.01% of the range.
Also please consider the long-term stability effect, particularly in relation to the frequency of recalibrations.
The transmitter may drift by the specified stability amount before the next scheduled recalibration, which may require adjustment or trim to maintain accuracy. Therefore, considering the long-term stability is crucial in ensuring the reliable performance of pressure transmitters over extended periods.
Mounting position (orientation) effect
The mounting position of a pressure transmitter can have an impact on its accuracy. Most pressure transmitters come with a specification for the mounting position, which should be taken into consideration.
In general, changes in the orientation of the transmitter can affect the zero value while leaving the span accuracy unaffected. During normal usage, the orientation of the transmitter remains unchanged. However, it becomes important to consider the orientation if the transmitter is calibrated in a workshop and then installed in the process, or if the transmitter is removed from the process for recalibration.
If a transmitter features a remote seal, the location of the capillary tubes can significantly impact the zero value. Again, this is not something that changes during regular usage but may affect the calibration if the transmitter is removed from its installed location.
By considering the mounting position and taking appropriate measures during calibration and installation, the accuracy of the pressure transmitter can be maintained in line with the specified requirements.
Vibration effect
The effect of vibration on pressure transmitters is a consideration that is particularly relevant when the transmitter is installed in a vibrating environment.
The specified vibration effect on accuracy is typically relatively small and may be indicated as “less than 0.1% of range.” This means that the accuracy of the transmitter is expected to be minimally impacted by vibration within the specified range. It is important to assess the vibration conditions at the installation site and compare them with the specified vibration tolerance of the transmitter.
If the vibration levels exceed the specified limits, additional measures may be necessary to mitigate the potential impact on accuracy, such as using vibration isolation methods or selecting a transmitter specifically designed for high vibration environments.
By considering the vibration specifications and ensuring proper installation in accordance with the manufacturer’s guidelines, the accuracy of the pressure transmitter can be maintained even in the presence of vibrations.
Power supply effect
In order for a 2-wire pressure transmitter to function, it requires an external power supply, usually provided by a 24 VDC source. Pressure transmitters are designed to operate within a wide range of supply voltages, often ranging from 10 VDC and upwards.
It’s worth noting that changes in the supply voltage during operation can have a slight impact on the accuracy of the transmitter. However, the effect of power supply voltage on accuracy is typically minimal and can be specified as “smaller than 0.01% of span per 1 Volt change,” for example.
Under normal circumstances, with a reliable and stable power supply, this is generally not a concern and should not significantly affect the accuracy of the pressure transmitter.
Total accuracy specification
Some pressure transmitters provide a “total accuracy” specification that takes into account multiple accuracy components, including the reference accuracy, ambient temperature effect, and static/line pressure effect.
This total accuracy value offers a more user-friendly representation of the actual accuracy that can be expected from the transmitter.
For example, if the reference accuracy is 0.04% of range, the total accuracy specification may be 0.14% of range when considering the temperature and line pressure effects. This means that the reference accuracy is multiplied by a factor of 3 to 4 when including these additional factors.
Another model may have a reference accuracy of 0.075% of range, which increases to 0.2% of range when the temperature effect is included, and further rises to 0.3% of range when considering the static pressure effects. While the “total” accuracy specification provides a more realistic estimation of practical accuracy, it’s important to note that it may still not include all accuracy components.
However, it serves as a useful indicator for understanding the overall accuracy of the pressure transmitter in real-world conditions.
Contamination in usage
In industrial processes where pressure transmitters are used, there is a significant risk of the transmitter’s diaphragm getting contaminated by the pressure media or dirt particles.
This contamination can have a substantial impact on the accuracy of the transmitter’s measurements. While the extent of contamination cannot be precisely specified, it poses a considerable risk during regular usage. This risk becomes even more critical if the recalibration period is extended to several years.
Therefore, it is important to consider this factor in the overall risk analysis, in addition to the transmitter’s long-term drift specification. In cases where the transmitter becomes heavily contaminated and starts providing significantly inaccurate readings, it is typically noticeable in the measurement results.
However, if the contamination is minimal and the measurements are only slightly affected, it can be challenging to detect during normal usage. Therefore, appropriate measures should be taken to mitigate the risk of contamination and ensure regular maintenance and cleaning of the transmitter to maintain its accuracy.
Best-case and worst-case examples
When considering the total accuracy of a pressure transmitter, it is important to combine all the individual accuracy components listed above. The commonly used method is the “root sum of the squares” (RSS), which takes into account the statistical likelihood of multiple components aligning in the same direction simultaneously.
Simply adding all the components together as a straight sum would represent a worst-case scenario, which is statistically unlikely in practice. For a best-case summary of the transmitter’s accuracy, one should consider the smallest accuracy components while disregarding those that may not be relevant or significant.
On the other hand, for a worst-case scenario, all accuracy components should be considered at their maximum values, assuming they are all present and affecting the accuracy simultaneously. By applying these approaches, a more comprehensive understanding of the expected accuracy in practical usage can be obtained.
Best-case accuracy
To get the best-case accuracy, the following assumptions were used:
- Pick the best reference accuracy
- Choose the most accurate model and range
- Never do re-ranging as it will not affect accuracy
- Use the transmitter in a limited temperature range, close to ambient temperature (smallest temperature effect)
- Assume no static/line pressure effect (for gauge measurement)
- Assume no vibration effect
- Assume a good power supply with no effect on accuracy
- Always Include a one-year drift
Remember to consider your specific accuracy requirements and application considerations when selecting a pressure transmitter. After analyzing the specifications of various transmitters, it becomes apparent that the smallest combined accuracy achievable is approximately 0.15% of the range.
However, for the majority of other models, the best-case accuracy tends to be around twice that value, reaching approximately 0.3% of the range. It’s worth noting that there are also several models available with even larger best-case accuracy values.
Worst-case accuracy
To find the worst-case accuracy, the following assumptions were used:
- Assume for potential re-ranging.
- Utilize the range with a larger temperature effect.
- Consider the impact of static/line pressure.
- Factor in a small vibration effect.
- Account for a small power supply effect.
- Include a one-year drift in the accuracy assessment.
Upon considering the worst-case accuracy specifications, it becomes evident that the combined accuracy ranges from approximately 1% to 1.5% of the range for the most accurate transmitters. However, it is important to note that certain models may exceed this range and exhibit higher levels of inaccuracy.
Summary
As said before, it is crucial to examine the accuracy specifications of modern pressure transmitters, taking into account all the various components that impact accuracy. It is common to overlook these details and solely focus on the “reference accuracy” presented in marketing materials and other promotional content.
The objective of this post is to heighten your awareness of the multiple factors that contribute to the overall accuracy achievable in real-world applications.
This principle applies not only to pressure transmitters but to all types of measurement equipment. It is always advisable to carefully review all specifications, including the fine print and accompanying footnotes.
Below more articles provide valuable insights and information to further enhance your knowledge:
1) What is a smart transmitter?
2) Pressure Sensor Specifications
3) The Importance of Using Pressure Gauges with Pressure Transmitters
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