Pressure Measurement

Factors that Influence the Accuracy of your Pressure Transmitter

Pressure transmitters are essential for accurate measurements in the context of industrial processes. However, a number of factors may have an impact on achieving and maintaining this level of accuracy. 

We will examine the main factors influencing pressure transmitter precision in this extensive guide, along with strategies for reducing these variables for accurate and consistent readings.

  • Prior to exploring the variables that impact accuracy, let us quickly review the fundamental components of pressure transmitters. 
  • In order to help produce high-quality goods and reduce downtime, these devices are made to measure pressure levels in manufacturing processes. 
  • For pressure transmitters to be effective in a variety of applications, their accuracy is essential.
Factors that Influence the Accuracy of your Pressure Transmitter 1
  • The temperature of the environment and the fluid being measured are critical factors influencing pressure transmitter accuracy. 
  • Transmitters come with specific temperature tolerances, and extreme conditions can lead to variations in accuracy.
  • It is essential to choose a transmitter with suitable temperature ranges for your application. 
  • Some transmitters are temperature-compensated during manufacture to counteract temperature variations, ensuring accuracy despite environmental fluctuations. 
Factors that Influence the Accuracy of your Pressure Transmitter 2

Temperature-compensated transmitters address this challenge by incorporating additional components and techniques during manufacturing. These can include:

  • Temperature sensors: These sensors constantly monitor the transmitter’s internal temperature.
  • Signal conditioning circuits: These circuits analyze the temperature readings and adjust the output signal of the primary sensor to compensate for the thermal effects. The adjusted output signal provides more accurate measurements over a wider temperature range.
  • Special materials: Some manufacturers use materials with minimal thermal expansion coefficients for critical components.
  • Understanding static pressure is essential for differential pressure transmitters. Static pressure, also known as line pressure, refers to the pressure in a pipe or process. 
  • The differential pressure transmitter measures zero pressure when both pressure taps are at atmospheric pressure. The measured pressure is zero in theory if both inputs are subjected to the same higher pressure. In practice, static pressure affects differential pressure transmitter accuracy.
  • Understand the impact: Static pressure (line pressure) can affect accuracy and repeatability by shifting the zero point and altering the span of measured differential pressure.
  • Calibration : Regularly calibrating the transmitter in-field helps account for static pressure, ensuring accurate readings.
  • Consider compensation: Choose transmitters with built-in static pressure compensation for automatic adjustments and consistent measurements.

Click here to refer to the Instrument Calibration Procedure of Various Temperature , Pressure , Level, Flow, Analytical Instruments and Control Valves.

  • Scale Impact: The accuracy of pressure transmitters is strongly influenced by the particular measuring scale that is utilized.
  • Definition of Scaling Dynamics: The terms “scaling dynamics” here refer to the parameters that determine the lowest changeable measuring range of the instrument.
  • Dynamic Range Significance: In order to ensure reliable readings across a wide range of pressures, it is essential to use a pressure transmitter with a high dynamic range.  It ensures that the transmitter remains precise across a wide range of pressure levels, making it suitable for various industrial applications.
  • Long-Term Stability Challenge: Measuring instruments, such as pressure transmitters, experience accuracy degradation over time.
  • Zero Point Drift: The zero point of the transmitter can shift, adversely affecting measurement accuracy.
  • Regular Calibration: To mitigate this issue, it is advisable to engage a measuring device calibration company for periodic calibration to maintain accurate and reliable measurements.
  • Stability Duration Variation: Pressure transmitters exhibit different stability specifications, ranging from 1 year to 5 or 10 years, or even longer.RA transmitter with 0.04% range accuracy might have a 1-year stability of 0.2% of the range.
  • Extended Stability: Some transmitters maintain a 0.2% of range stability specification over an extended period, such as 5 or 10 years, providing users with long-term measurement reliability.

Click here to determine the calibration Error Calculations.

  • The mounting location of a pressure transmitter can affect its accuracy even after it has been factory calibrated. This is especially important for diaphragm seal types because of the oil volume in the measuring cell. 
  • This movement happens because the zero point, not the scale range, is slightly impacted by the weight of the oil. 
  • It is essential to use field calibration to fix this. The majority of transmitters come with a manifold and adjustment screw for manual zeroing. Modifications can be performed on “smart” HART® protocol transmitters using PC software or portable instruments.
  • Usually, changing the transmitter’s orientation only affects the zero point and not the span. This orientation doesn’t change when things are running smoothly. When calibrating in a workshop prior to installation or taking the transmitter out for recalibration, take orientation into consideration.
  • The zero point of remote seal transmitters is greatly influenced by the placement of their capillary tubes, which makes them extremely sensitive to this factor. As previously mentioned, this is normally corrected during installation, although moving the transmitter may affect recalibration.
  • Industrial equipment vibrations can cause pressure transmitter measuring cells to become unstable, which could reduce the device’s accuracy and service life.
  • The effect of vibration is specified for several pressure transmitters.
  • Obviously, this only needs to be taken into account if the transmitter is mounted in a vibrating environment.
  • For example, the vibration effect on accuracy can be described as “less than 0.1% of range,” which is typically quite minimal.
  • Voltage Supply Range: Pressure transmitters are typically supplied with 24 V DC, but they can often operate within a broader voltage range. Many transmitters can function within a wide supply voltage range, sometimes as low as 10 VDC.
  • Potential Issues: Incorrect or fluctuating voltage may impact measurement accuracy and could potentially damage the transmitter with overvoltage.
  • Recommendation: To ensure measurement accuracy, it’s advised to use an isolated, stabilized, and regulated power supply.
  • 2-Wire Transmitter Power: 2-wire transmitters require an external power supply, typically a 24 VDC supply.
  • Accuracy Impact: Though the effect of supply voltage changes during operation is generally small, it’s recommended to maintain stability for accuracy.
  • Specification Example: The impact of power supply voltage is often specified as “smaller than 0.01% of span per 1 Volt change.”
  • Practical Consideration: With a reliable power supply, these voltage-related issues are typically not a concern in regular operation.
  • In any circumstance, accuracy must be the primary priority when choosing a pressure transmitter supplier. 
  • A wide measuring dynamic, high long-term stability, on-site commissioning, calibration services, and the capacity to tolerate extreme vibrations are some of the features that are in demand. 

Error is defined by the concept of sensor accuracy, and the two terms are essentially synonymous:  The difference between the pressure the sensor indicates and the actual pressure, represented as a percentage of the sensor’s full scale, is the sensor’s accuracy.  When a pressure sensor, for instance, with a 100 psi full scale range, shows a pressure of 76 psi when the actual pressure is 75 psi, the error is 1 psi. This can be expressed as a percentage and divided by the entire scale to determine the accuracy (or inaccuracy) of the sensor: 1%.  

To make an informed decision, it’s essential to understand the accuracy components listed in pressure transmitter specifications. 

While selecting a provider for your pressure transmitter:

  • Demand accurate measurements in any circumstance.
  • A wide 1:100 measurement dynamic between 1 mbar and 500 bar is required.
  • High long-term stability is required.
  • Make a request for on-site calibration and commissioning services.
  • Request pressure transmitters designed to endure the strongest tremors.
  • Make sure that your transmitters have a broad power range.

Manufacturers may provide a “reference accuracy” that includes linearity, hysteresis, and repeatability. However, this may not represent the total accuracy in practical applications.

Let’s break down the various accuracy components found in pressure transmitter specifications:

  • Establishes baseline accuracy under reference conditions.
  • Includes linearity, hysteresis, and repeatability.
Factors that Influence the Accuracy of your Pressure Transmitter 3
  • Accuracy may vary for different pressure ranges.
  • Consider variations in accuracy for absolute and gauge pressure measurements.
  • HART transmitters can be re-ranged with a wide turndown ratio.
  • Re-ranging may affect accuracy, especially when reducing the measurement range.
Factors that Influence the Accuracy of your Pressure Transmitter 4
  • Temperature coefficient influences accuracy.
  • Specifications may vary from 0.01% to 0.5% of range.
  • Static pressure influences zero and span.
  • Line pressure effect may range from 0.025% to 0.4% of range.
Factors that Influence the Accuracy of your Pressure Transmitter 5
  • Specifies accuracy degradation over time.
  • Ranges from 0.01% to 0.2% of range over one to five years.
  • Mounting position affects zero point.
  • Calibration adjustments may be needed based on orientation changes.
  • Vibrations impact accuracy.
  • Specifications may state that the influence is “less than 0.1% of the range”.
Factors that Influence the Accuracy of your Pressure Transmitter 6
  • Voltage changes can influence accuracy.
  • Typically defined as “less than 0.01% of span change per 1 Volt change.”
  • Combines reference accuracy, ambient temperature, and static pressure effects.
  • Provides a more realistic picture of expected accuracy in practical applications.
  • Contamination can significantly impact accuracy.
  • Consider the risk of membrane contamination in real-world applications.

It is important to take into account a variety of aspects and components that contribute to the final measurement precision when assessing the overall accuracy of pressure transmitters. The “root sum of the squares” (RSS) method is frequently used for combining different uncertainty components. This statistical method avoids a hopeful worst-case situation of all components matching in one direction.

To determine the best-case accuracy, we make specific assumptions aimed at optimizing the precision of pressure transmitters:

  • Select the Best Reference Accuracy:  Choose the reference accuracy with the highest level of precision among the options that are available.
  • Choose the Most Accurate Model and Range: Determine and use the pressure transmitter model and range that provides the most accuracy.
  • Avoid Re-ranging: Avoid changing the measuring range because it can introduce inaccuracies.
  • Limit Temperature Range: Use the transmitter within a specific temperature range, preferably near to ambient. To reduce variations, select the least available temperature effect.
  • Assume No Static/Line Pressure Effect: If sensor measurements are dominant, ignore static or line pressure effects.
  • Neglect Vibration Effect: Assume that vibrations have no impact, simplifying the procedure by eliminating this potential source of error.
  • Ensure a Stable Power Supply: Assume a stable and reliable power supply with no negative influence on measurement accuracy.
  • Consider One-Year Drift: Factor in the expected drift over a one-year period to account for long-term stability.

Following a comparison of specifications from several transmitters, the most optimistic scenario produces a combined accuracy of around 0.15% of the measuring range. This best-case accuracy can reach roughly 0.3% of the range in some cases, underlining the variation among different models.

In contrast, the worst-case accuracy scenario assumes the maximum influence of all accuracy components at the same time:

  • Opt for Maximum Reference Accuracy: Select the reference accuracy that has the highest potential variance.
  • Select Model and Range with Maximum Inaccuracy: Identify the pressure transmitter model and range associated with the greatest possible errors.
  • Allow Re-ranging Impact: Recognize the potential negative impacts of re-ranging on accuracy.
  • Expand Temperature Range: Consider the impact of a broader temperature range, incorporating the largest available temperature effect.
  • Account for Static/Line Pressure Effect:  Identify and account for the impact of static or line pressure on measurements, especially in gauge pressure circumstances.
  • Consider Vibration Effects: Account for potential inaccuracies introduced by vibrations, assuming the maximum vibration effect.
  • Consider Unstable Power Supply: Acknowledge the impact of an unstable power supply, allowing for voltage fluctuations that could affect accuracy.
  • Include One-Year Drift: Incorporate the potential maximum drift over a one-year period, accounting for the least favorable long-term stability.

Depending on the transmitter model and specifications, the overall accuracy may be 1% to 1.5% of the measuring range in worst-case scenarios. It’s important to remember that statistical uncertainty components make worst-case scenarios unlikely.

Click here for Calculator for Instrument Accuracy: Calculating Accuracy in Engineering Units

The accuracy of pressure transmitters is crucial for precise process monitoring, preventing hazards, and safeguarding industrial systems. Below is a simplified guide to calculating accuracy, highlighting key considerations and parameters.

Transmitter accuracy assesses error during pressure measurement at zero and variable points. Graphically, the zero point displays a linear graph, while real-time measurement results in a slightly curved graph. The average deflection between these graphs determines accuracy, influenced by various factors.

Two errors contribute to transmitter accuracy:

  • Primary error affecting accuracy.
  • Calculated using methods such as End Point or Five Point (BFSL Method).
  • Fluctuation in pressure readings at the same point.
  • Calculated at 5 calibration points of the BFSL process.
  • Crucial factors with minimal impact include:
  • Difference between zero span readings.
  • Difference between zero span and full-scale span.
  • Measures differences in multiple readings at the same pressure.
  • The overall accuracy is the sum of non-linearity, hysteresis, offset error, span tolerance, and non-repeatability.
  • Full-Scale Output is the maximum output signal that the pressure transmitter can produce. 
  • This is often specified by the manufacturer and can be found in the transmitter’s technical specifications.
  • Use a reference standard or another accurate pressure measurement device to determine the actual pressure applied to the pressure transmitter.
  • Record the output signal of the pressure transmitter. This is the electrical signal (e.g., mA-current) that corresponds to the pressure it is measuring.
  • Calculate the difference between the measured output and the actual pressure.
  • Use the formula mentioned earlier to calculate the accuracy as a percentage of Full-Scale Output(FSO).
  • A lower percentage indicates better accuracy. For example, an accuracy of 1%FSO means that the transmitter’s output may deviate by up to 1% of the full-scale output.
  • Always refer to manufacturer specifications for accuracy details under specific conditions. Calibration ensures ongoing accurate measurements. Source transmitters from reliable suppliers for optimal industrial process control.
  • IEC 61511: Focuses on safety instrumented systems for the process industry. Addresses reliability and performance, important for safety-critical applications.
  • NIST Technical Note 1297: Guidelines for evaluating and expressing measurement uncertainties. Helpful when considering accuracy specifications and uncertainty in measurements.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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