Tag Archives: accuracy

Analog Output Accuracy: The Devil In the Details

DSEZINE415
Flowmeters typically contain multiple components that introduce error into the flow measurement system. A simple flow measurement system may be comprised of a primary flow element and a transmitter that processes signals from the primary flow element. Sometimes, the primary flow element and transmitter are physically integrated together as one piece, such as in potable water meters. More complicated flow measurement systems may include multiple components such as a flow computer or other electronic components that compensate for process pressure, process temperature, or other parameters.

It should not be forgotten that flow measurement systems are “systems” that measure flow. As an example, consider a hypothetical primary flow element that exhibits no error while the transmitter exhibits 5 percent accuracy. In this exaggerated example, the accuracy of the flow measurement system will be 5 percent. Assuming that the flow measurement error is that of the primary flow element only is an error of omission. Users should constantly be on guard to identify this type of error.

In most flowmeters, the primary flow element and transmitter are integrated electronically. For example, the wetted primary flow elements of Coriolis mass flowmeters, thermal flowmeters, and magnetic flowmeters are virtually useless without transmitters that contain their respective flow measurement algorithms and drivers. Therefore, flowmeter performance typically includes the combination of a primary flow element and a transmitter. Further, the performance of most flowmeters is predicated on the calibrated output that is usually the pulse/frequency output of the transmitter.

However, most process control applications of flowmeters involve the use of an analog output such as 4-20 mA to represent 0-100 percent of the desired flow rate. The analog signal is typically generated using circuits that convert the pulse/frequency signal (or its source) to an analog signal. This conversion introduces a measurement error that is constant throughout the signal range, so it can usually be expressed as a percent of full scale. The error introduced is typically between 0.03 and 0.10 percent of full scale depending upon the quality of the converter. To obtain the measurement accuracy of the analog output, this error is mathematically added to the accuracy of the flowmeter.

The analog output error may seem small, but at low flow rates, this error can become significant and actually dominate measurement accuracy. For example, consider a vortex shedding flowmeter that can operate from 10 to 100 units per minute with 0.75 percent of rate accuracy but has an analog output accuracy of 0.10 percent of full scale. At 10 units per minute, the pulse/frequency output has an accuracy of 0.75 percent of rate, whereas the analog output contributes an additional (0.1*100/10) or 1.00 percent rate error, so the measurement accuracy of the analog output is 1.75 percent of rate.

Most suppliers calibrate the pulse/frequency output. They typically state its accuracy as the performance of the flowmeter. The accuracy of the analog output conversion is often buried in the specifications in the fine print. Sometimes, it is not published and must be requested from the supplier. Sometimes the information is forthcoming, but often suppliers do not understand the question and try to state the analog output resolution (say 1 part in 4096, or 0.02 percent) as the analog output accuracy. After further investigation, many suppliers will admit that they do not know the analog output accuracy — even though most of their customers may use that output exclusively for their flow measurements. They also provide further enlightenment when they say that “no one ever asked for this before”.

The burden of obtaining the best flow measurement possible in a given application does not lie with the supplier — it lies with the user. Do not forget the fine points that may lurk in the details and the errors of omission that may be available for the asking.

This article originally appeared in Flow Control magazine (September 2004) at www.flowcontrolnetwork.com.

Level Gauge Performance (Part 3 of 3)

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To review — the performance of a level measurement system is quantified by means of its accuracy statements. The reader must understand not only which parameter is being described, but also the manner in which the statement is expressed. In level measurement, parameters are commonly described in terms of a(n):

absolute (fixed) distance error
percentage of the empty distance (farthest measurement in span)
percentage of the maximum sensor distance
percentage of measured distance
percentage of set span
percentage of maximum span

Note that other terminology may be used to express these concepts. Some variations actually used by suppliers include mm, cm and percentages of:

Span
Full span
Span in air
Rated span
Maximum span
Calibrated span
Maximum measured span
Maximum span of the sensor
Maximum measuring span
Span value
Range
Full range
Detected range
Measured range
Target range
Measuring range
Maximum range
Range distance
Maximum target range (in air)
Set measuring range
Range with no temperature gradient
Full scale
Maximum distance
Target distance
Measured distance
URL
Distance
Tank height
An undefined parameter (for example, 0.25%)

Many of the above terms do not have clear meanings. In addition, discussions with suppliers revealed different meanings for specifications that otherwise seemed to be clear and well defined. Regardless of the terminology used by the supplier, the reader is advised to confirm exactly what the meaning of the terms used in the specification in order to understand them correctly so as to correctly evaluate performance.

More importantly, the performance specifications may not describe performance. Consider some examples that were actually encountered.

Stated Accuracy  Meaning (after discussion with supplier)
0.25% Range                          0.25% of empty distance (farthest measurement)
1.2% of range                         1.2% of maximum sensor range
0.25% of measuring range       0.25% of maximum sensor range
0.25% of span                         0.25% of maximum sensor range
0.25%                                    0.25% of maximum sensor range
0.3%                                      0.3% of measured distance

These examples illustrate the difference between published specifications and their actual meaning. From the above data set, it would be conservative to assume that statements expressed as percentages are percentages of the maximum sensor range until they are confirmed otherwise by the supplier.

This article was excerpted from “The Consumer Guide to Non-Contact Level Gauges”

Level Gauge Performance (Part 2 of 3)

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A percentage of measured distance statement describes a parameter that is in error by a constant percentage of the actual distance measurement. In the measurement of a 1000 mm high vessel (100% level located 100 mm below the sensor) measured with an accuracy of 1 percent of measured distance, the absolute error can be calculated as:

An absolute (fixed) distance error statement describes an error that is constant. This error is independent of the calibration range and the actual level in the vessel. For example, the stated accuracy of a level measurement system in a 1000 mm high vessel (100% level located 100 mm below the sensor) might be ±10 mm. The absolute error at different levels is:

Level Absolute Error (1% of measured distance)
0% (empty) 1% of 1100 = 11.0mm
25% 1% of 850 = 8.5mm
50% 1% of 600 = 6.0mm
75% 1% of 350 = 3.5mm
100% (full) 1% of 100 = 1.0mm

A percentage of set span statement describes the error in terms of the full scale range. For example, the stated accuracy of a level measurement system in a 1000 mm high vessel (100% level located 100 mm below the sensor) might be ±1 percent of set span. The set span is 1100-100 or 1000mm, so the absolute error at different levels is:

Level Absolute Error (1% of set span)
0% (empty) 1% of 1000 = 10mm
25% 1% of 1000 = 10mm
50% 1% of 1000 = 10mm
75% 1% of 1000 = 10mm
100% (full) 1% of 1000 = 10mm

A percentage of maximum span statement describes the error in terms of the maximum sensor distance minus the blocking distance. For example, the stated accuracy of a level measurement system with a sensor that can measure from 400 mm to 8000 mm might be ±1 percent of the maximum span. The maximum span is 8000-400 or 7600 mm, so the absolute error at different levels is:

Level Absolute Error (1% of maximum span)
0% (empty) 1% of 7600 = 76mm
25% 1% of 7600 = 76mm
50% 1% of 7600 = 76mm
75% 1% of 7600 = 76mm
100% (full) 1% of 7600 = 76mm

In order to fairly compare performance, the same type of accuracy statement should be used for each level measurement system. For level measurement, the best measure of performance is usually the absolute (fixed) level error statement because it quantifies the amount of error expected to be present. Therefore, in most cases, statements should be expressed or converted to an absolute (fixed) level error statement before using the information for comparison purposes.

Note the significant variation in absolute errors associated with the different error statements above.

The preponderance of error statements used by suppliers will be discussed in Part 3.

Level Gauge Performance (Part 1 of 3)

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The purpose of installing a level measurement system is to measure level accurately in a reliable manner. Whereas issues dealing with physical properties, process parameters, electronic features, and interconnections are often considered extensively, the quantification of the expected measurement quality of the installed level measurement system can be virtually neglected. Often, relatively little emphasis is given as to how well the level measurement system will perform its intended purpose. Adding to the confusion are the differences in the manner in which performance is expressed and the incomplete nature of the available information. Nonetheless, the quality of level measurement should be a prime concern.

The performance of a level measurement system is quantified by means of its accuracy statements. The reader must understand not only which parameter is being described, but also the manner in which the statement is expressed. In level measurement, parameters are commonly described in terms of a:

• absolute (fixed) distance error
• percentage of the empty distance (farthest measurement in span)
• percentage of the maximum sensor distance
• percentage of measured distance
• percentage of set span
• percentage of maximum span

An absolute (fixed) distance error statement describes an error that is constant. This error is independent of the calibration range and the actual level in the vessel. For example, the stated accuracy of a level measurement system in a 1000 mm high vessel (100% level located 100 mm below the sensor) might be ±10 mm. The absolute error at different levels is:

Level Distance Absolute Error (10mm)
0% (empty) 1100mm 10mm
25% 850mm 10mm
50% 600mm 10mm
75% 350mm 10mm
100% (full) 100mm 10mm

A percentage of empty distance statement describes a parameter that is in error by a constant percentage of the farthest measurement distance in the span. In the measurement of 100-1100 mm high vessel (100% level located 100 mm below the sensor) measured with an accuracy of 1 percent of empty distance, the empty distance is 1100 mm, so the absolute error can be calculated as:

Level Absolute Error (1% of empty distance)
0% (empty) 1% of 1100 = 11mm
25% 1% of 1100 = 11mm
50% 1% of 1100 = 11mm
75% 1% of 1100 = 11mm
100% (full) 1% of 1100 = 11mm

The percentage of maximum sensor distance statement is similar to an absolute (fixed) distance error statement in that its absolute error is constant. However, the error is related to the maximum distance that can be measured by the sensor. For example, the stated accuracy of a level measurement system in a 1000 mm high vessel (100% level located 100 mm below the sensor) might be ±1 percent of maximum sensor distance of 6000 mm. The absolute error at different levels is:

Level Absolute Error (1% of maximum sensor distance)
0% (empty) 1% of 6000 = 60mm
25% 1% of 6000 = 60mm
50% 1% of 6000 = 60mm
75% 1% of 6000 = 60mm
100% (full) 1% of 6000 = 60mm

Note the significant variation in absolute errors associated with the different error statements above.

The remaining performance statements will be described in Part 2.

This article was excerpted from “The Consumer Guide to Non-Contact Level Gauges”

Can You Believe Performance Statements For Level Gauges?

DSEZINE415Many of my articles focus on performance statements for flowmeters. It is interesting how
another twist can come along just when the subject seems to be getting “old”. Whereas reference
flowmeter performance can generally be expressed as one of four parameters, reference level
gauge performance is generally expressed as one of six parameters. This was not so bad. Even
more complicating is that the (often well-defined) published specifications do not reflect what
the suppliers intend.

Research for “The Consumer Guide to Non-Contact Level Gauges” involved extracting
information from the level gauge specifications from about 60 suppliers. These suppliers used a
total of about 30 different terms to express performance. Let’s think about this for a minute — on
the average, there was one term used to express performance for every two suppliers. This would
seem to be far from consistent.

In the early going, the well-defined published specifications were tabulated and used for
calculations. As research progressed, conversations with suppliers made it apparent that the
published specifications could not be trusted — even if they were technically clear and well
understood. As a result, I had to contact the suppliers again to verify that their (supposedly) well-
defined published specifications reflected the supplier’s intentions.

For example, after speaking with the suppliers, 0.25% of measuring range, 0.25% of span, and
0.25% all meant 0.25% of maximum sensor range. Think about this for a minute. Measuring
range and span are well defined — or are they? The percentage is not well defined and begs the
question, “Percentage of what?”

On a positive note, despite the discrepancies noted in the published specifications, suppliers were
forthcoming in explaining their intentions. Wouldn’t it be nice if they would update/correct their
specifications to use standard terms that describe level gauge performance?

Originally published in David W Spitzer’s E-zine