Rangeability and turndown ratio are fundamental attributes of pressure transmitters. Simply put, rangeability refers to the ratio of maximum to minimum pressure that a pressure transmitter can accurately measure.Turndown ratio, often synonymous with rangeability, specifically denotes the maximum capacity a device has over its minimum measurable capability.
Are you wondering what the terms "rangeability" and "turndown ratio" in a pressure transmitter mean? Do these technical terms seem confusing and you're seeking clarity?
In this article, we will dive deep into the understanding of rangeability and turndown ratio in pressure transmitters in pressure transmitters. We will elaborate on what these terms mean, why they are important, and how they affect the operation of a pressure transmitter.
We'll also explore related concepts such as maximum and minimum pressure capabilities, the significance of measurement range, and how the concepts of rangeability and turndown ratio can be practically applied.
Rangeability, commonly referred to as turndown ratio, is a critical parameter in control systems, especially when dealing with process pressure transmitters. Whether it's the turndown ratio of a differential pressure transmitter, a gauge pressure transmitter or an absolute pressure transmitter, it's a simple formula that compares the maximum measurable range of a device to its minimum measurable range.
This ratio is often expressed by a number such as 3:1, 5:1, or even 100:1, indicating that the sensor is capable of accurately measuring pressures within that span. For example, in a transmitter with a 5:1 turndown ratio, if the maximum pressure (the high end) it can measure is 100 units, then it can accurately measure down to 20 units, which is its low end.
The range is the measurement limit and covers from the minimum pressure to the maximum pressure that the pressure transmitter cell can measure, for example from 0 to 100 bar. The maximum measurement span is 100 bar.
Upper Range Limit (URL) refers to the highest pressure that the transmitter was designed to measure, respected the cell upper range limit.
The Lower Range Limit (LRL) refers to the lowest pressure that the transmitter was designed to measure, respected the cell lower range limit.
The upper range value (URV) is the maximum pressure at which pressure transmitter is calibrated or set. It corresponds to the highest point on the output scale, such as the 20 mA point in a 4 to 20 mA output signal.
Lower Range Value (LRV) is the minimum pressure at which the pressure transmitter is calibrated. Its corresponds to the lowest point of the output scale, such as the 4mA point in a 4 to 20mA output signal.
Span (Calibrated span) is the working range that is equal to URV – LRV. This is the equivalent of the 4 to 20mA output signal.
Turndown (TD) or Rangeability of a pressure transmitter is calculated by dividing the maximum pressure the device can measure (Upper Range Limit, or URL) by the minimum pressure it can measure accurately (minimum calibrated span).
In mathematical terms:
Turndown Ratio = URL/ Calibrated Span.
For instance, let's assume that a given pressure transmitter has an Upper Range Limit (URL) of 100 bar and a calibrated span of 10 bar.
Using the formula, the turndown ratio would be 100 bar/ 10 bar= 10:1.
Turndown Ratio, a key aspect of any rangeable pressure measurement device like those with a 4 to 20mA output signal, outlines the extent of difference between the uppermost measurement range compared to the lowest feasible span of measurement output. Turndown enables a scaled range to be set within the measuring range. This ratio is fundamental in defining the functional bandwidth of sensing instruments and plays a pivotal role in their operational flexibility and accuracy.
The rangeability of a given pressure transmitter depends on several factors.
One of the most important is the maximum pressure the sensor can tolerate without being damaged or its performance diminishing, otherwise known as the upper range limit (URL). This is the maximum pressure capacity at which the device can operate safely.
Another influencing factor is the minimum pressure, the lower limit of the measuring range (URL ), which is the lowest pressure level the device can accurately measure.
These two factors depend on the characteristics of the transmitter's measuring cell and cannot be modified.
The final factor is the pressure transmitter's measuring range, which has a major influence on rangeability and turndown ratio. This measurable range is defined as the range between the upper value of the calibrated or set measuring range (URV) and the lower value of the calibrated or set measuring range.
Understanding rangeability and measurement dynamics is essential in many applications where pressure must be monitored and controlled. This is particularly important in fluid and gas control systems, where accurate measurement and maintenance of pressure levels are paramount to successful operation.
Here's everything else you need to know about rangeability and turndown ratio in a pressure transmitter, including how these attributes affect the accuracy and efficiency of a pressure measurement system.
Choosing a pressure transmitter with the correct turndown ratio can significantly impact the accuracy of the readings and, in turn, the overall system's performance. For instance, if your application requires a measurable and controllable range of pressure between 50 and 100 units, a transmitter with a turndown ratio of 5:1 and a maximum pressure rating of 500 units would not be suitable. Its minimum measurable pressure would be 100 units, which would not cover the lower end of the required range.
However, if the same application uses a transmitter with a 10:1 turndown ratio and a maximum pressure rating of 500 units, the minimum measurable pressure would be 50 units, perfectly covering the required range. This would result in more accurate readings and better control over the system.
Turndown ratio and rangeability also play an important role in calibration .
Calibration is the process of setting a transmitter's output to correspond accurately with the pressure being measured. A pressure transmitter with a greater span, or rangeability, can be calibrated more finely across a wider range of pressures, increasing its accuracy.
Calibration involves setting the high end (maximum pressure) and the low end (minimum pressure) of the transmitter.
The turndown ratio indicates the range within which the transmitter can be calibrated and still accurately measure pressure. This implies that a transmitter with a larger turndown ratio offers a wider range for calibration, thereby providing more flexibility in various applications.
An essential feature of pressure transmitters intelligent (SMART) is their readiness to be re-ranged without the need for calibration. Unlike analog instruments, which can only be re-ranged by re-calibration, the measuring range or span of digital sensors can be adjusted after manufacture at the factory, or on site, after initial installation.
In digital instruments, calibration and scaling are usually separate settings (i.e. it is possible to change the scale of an intelligent pressure transmitter without having to perform a complete recalibration).
So, if your process requirements change, you can adjust the pressure transmitter's span and zero point to meet the new requirements without having to replace the measuring device.
Scaling an instrument involves setting the upper and lower scale values so that it reacts with the desired sensitivity to variations in the pressure input.
For example, if a pressure transmitter has a turndown ratio of 10:1 and was initially calibrated at the factory for the maximum pressure range of 10 bar (0 bar = 4 mA output; 10 bar = 20 mA output), you can readjust it on site from 0 to 1 bar (0 bar = 4 mA; 1 bar = 20 mA) to accurately measure lower pressures.
Conversely, if a transmitter has a turndown ratio of 10:1 and was initially calibrated for the minimum pressure range of 1 bar (0 bar = 4 mA output; 1 bar = 20 mA output), you can readjust it up to 10 bar (0 bar = 4 mA; 10 bar = 20 mA) according to the new requirements.
This re-range ability, directly linked to the turndown ratio, can significantly improve the device's longevity and adaptability, making it a more valuable asset in any pressure monitoring or control system.
While a high turndown ratio might seem desirable for its flexibility, it's important to consider its limitations.
A very high turndown ratio might promise a wide operating range, but the accuracy of measurements can decrease at the very low end and very high end of the range.
In these scenarios, a pressure transmitter with a more modest turndown ratio, more closely matched to the actual operating conditions, may deliver more accurate results.
Understanding the rangeability and turndown ratio in a pressure transmitter is essential for the correct setup, accurate measurement, and effective control in various systemsinvolving gases or liquids. These parameters not only impact the device's accuracy but also influence the safety, efficiency, flexibility, and overall performance of your system.
Embrace the knowledge you have gained about rangeability and turndown ratio, and use it to optimize the performance of your pressure transmitters. Remember, selecting a transmitter with the right rangeability and turndown ratio for your specific application is a critical step towards achieving accurate pressure control and measurement. You're now equipped to make an informed decision. Keep up the good work, and keep exploring!
While a high turndown ratio offers flexibility in pressure measurement, accuracy can decrease at the very low end and very high end of the range. It's crucial to choose a transmitter with a turndown ratio that matches your system's specific pressure requirements. Consult a pressure transmitter manufacturer like Fuji Electric to help you determine what sensor fit the best for your application.
Yes, many smart pressure transmitters offer re-range capability, allowing you to adjust their calibrated measurement range after the initial setup. However, the extent of the adjustment depends on the device's turndown ratio.
No, the turndown ratio varies among pressure transmitters. It's an important parameter to consider when selecting a transmitter for your specific application. The Fuji Electric pressure transmitters FCX AIV offer turndown ratios of up to 100:1, making them particularly flexible in use.
Although measurement dynamics is an important parameter, its importance varies according to the application. In systems where pressure levels vary significantly, or where small fluctuations can have a major impact on performance, high measurement dynamics are generally beneficial. In systems where pressure levels are stable, lower measurement dynamics may suffice.
A high turndown ratio transmitter can handle a wider range of pressures, potentially reducing the need for multiple transmitters. However, the decision depends on various factors, including the system's specific pressure requirements and the accuracy needed at different pressure levels.
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