Author Archives: David W Spitzer

Analog Output Accuracy: The Devil In the Details

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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.

Relationship Advice: Making the Most of Vendor Partnerships

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Getting the most out of vendors is a challenge facing all who are active. There are techniques that you can use whether you are trying to get the best service at the local print shop or detailed information on a flowmeter. You will certainly be able to use your own experience to add to the tactics presented here.

Before you even think about calling the vendor, you need to determine what it is that needs to be done. For example, if a flow measurement is not stable, you may want to observe the effect(s) of putting the control loop in manual.  This ensures that the valve operates smoothly. You may also want to observe other measurements (such as pressures, temperatures, and flows) that affect the flow through the flowmeter. Vendors are trained to do certain things well, but troubleshooting your proprietary process or addressing internal politics is usually not among them. Vendors should know about the equipment that they sell, but even the best will stumble when asked to solve problems outside of their areas of expertise. Further, they potentially risk legal consequences if something should go terribly wrong.

Who should you call once you have determined that contact with the vendor is needed? Try to determine who has the information or who controls the resources that you need and call that person. If you do not know who this is, try to determine who would know who that person is and their contact information. This may be your local representative, but it could be your supervisor or a technician who previously needed similar information from the vendor. For example, significant delays can be avoided by directly contacting the factory person who schedules field technical service instead of leaving a message for your local representative who is only in the office for two hours on Friday and will return the call during the next week.

Understand what the vendor does and does not do. As previously mentioned, vendors should know about the equipment that they sell. Do not expect them to solve all of your problems. They may know a lot about widgets and have extensive related experience, but they sell products and they do not work for you. If you force vendors into the position of solving your problems, they may (reluctantly) do so, but their solution will usually entail the minimum of their effort and the minimum of their cost to successfully sell their product. For example, an instructor related a story about a contactor that had a 10 horsepower motor for installation on a fan that only needed a 3 horsepower motor. The instructor (who taught motor efficiency) knew that the energy consumption of the 10 horsepower motor would be higher and made him install the 3 horsepower motor. Installing the 10 horsepower motor was the easy way out for the contractor because he could “unload” a motor that he would probably not be able to use — and he was not paying the electric bill.

Be sure that you completely and honestly communicate with vendors in a clear straightforward and respectful manner. Vendors are indispensable in solving certain problems, so not acting in this manner today may return to haunt you tomorrow. Given the comments above, there is nothing wrong with respectfully asking a vendor for referrals to find people who might be able to help solve the problem at hand.

As a last resort, carry a stick. Sometimes vendors (like all of us) need a little push. Over the years, I reluctantly called a few supervisors and contacted the factory for information that was not forthcoming locally. There is always the threat of curtailing future sales, but with certain products, this approach can be a double-edged sword.

In summary, vendors are people who should be contacted and used to perform work that supplements your work within their areas of expertise. You and the vendor should work together respectfully, and the vendor should not be asked (or forced) to perform your work.

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

Multi-Phase Flow Measurement: Considering All the Options to Meet All the Requirements

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Some flow measurement applications just stick in your craw and may even keep you up at night. Losing sleep may be an exaggeration, but some applications are ‘tough’. The problem is that these applications usually involve the simultaneous solution of many flow problems, each of which may have a feasible solution when taken individually.

For example, consider the problem of multi-phase flow where individual liquid and vapor flow measurements are needed. Measuring the liquid flow rate and gas flow rates individually might be straightforward, but measuring them in the same stream will create havoc in most flowmeters. Not surprisingly, solutions to this problem include separating the vapor from the liquid and measuring each stream individually. Another approach is to install a flowmeter that can handle this service, such as a Coriolis mass flowmeter with software designed for two-phase flow or a correlation flowmeter that can measure fluid velocity and void fraction.

And what if the stream contains two liquid components? Again separating the components and measuring each stream may be an option, but it is often not practical. In some applications, the densities of the liquids may differ enough such that the flow rate of the individual components can be inferred from measurements of the total flow and liquid density, predicated on a known relationship between density and composition. Another option is to measure the total flow and use an analyzer measurement to infer the flow rates of the individual components.

And what if the stream contains liquids and solids? Separation may be an option that the process will not tolerate.

And what if the stream contains a liquid, vapor and solids? Now the problem is more complicated and the phases may have to be separated in order to make the individual measurements.

And what if the temperature is 500 degC? And what if the pressure is 500 bar? And what if 200:1 turndown is needed? And what if there is only 1 diameter of straight run to install the flowmeter? And what if the pipe is located 10 meters above grade with no access platform? And what if the ambient temperature is 80 degC? And what if…?

The list can go on and on. In most cases, meeting one of these requirements is possible. However designing a flow measurement system to measure under a combination of these conditions is difficult and may require a somewhat unlimited budget. Compromising on the requirements and relaxing budgetary constraints may bring solutions into focus, but the long and short of it is that there just may not be a feasible flow measurement system available.

You just may have to measure what you can feasibly measure as a surrogate for the desired measurement. For example, measuring the individual liquid, vapor and solid flow rates in a stream may not be feasible. However, useful information might be obtained by separating the vapor flow from the other phases, and measuring the liquid/solid flow and the vapor flow. This may not be the ideal, but these measurements could provide useful information about the process at reasonable cost.

It should be understood that in some applications, there just may not be any flow measurement system available at any price that meets all of the various requirements.

Originally published in Flow Control magazine (July 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”

Flowmeter Turndown: Don’t Let the Numbers Fool You

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Did you ever wonder what it would be like to be a billionaire? If nothing else, it sounds like an exciting lifestyle. But how would you like to be a billionaire stranded on a desert island? In this situation, all of the money in the world will not help you survive. You may be wondering what this has to do with flow measurement. Simply put, it has a lot to do with turndown.

Turndown is the ratio of the maximum flow rate that a flowmeter can measure within a stated performance to the minimum flow rate that the flowmeter can measure within a stated performance. Without getting into the subtleties of performance statements, this ratio can often be large. How many times have you heard vendor claims of “up to 40:1” or “up to 100:1”? The key operative words here are “up to”. Interpretation of these statements means that you may achieve as high as 40:1 or 100:1, but the flowmeter could provide 4:1 or 10:1 (or less) and still be within the limits of the claim.

For example, magnetic flowmeters can typically measure the flow of a liquid traveling at a velocity of 30 feet per second. In slurry service, velocities above about 15 feet per second are sometimes recommended to prevent solids from accumulating. However, at these velocities, energy costs can increase and the pipes generally wear more rapidly. Therefore, in typical process applications, liquid velocities of 6 to 8 feet per second are more common.

In another example, it is common for ultrasonic flowmeters to accurately measure velocities from approximately 1 to 40 feet per second. This means that such a flowmeter can have a turndown of “up to 40:1”. This may sound great, but in a typical application, only about 6:1 or 8:1 turndown would be achieved because the full scale flow is typically 6 to 8 feet per second. In short, the stated maximum turndown is based upon a range of flow rates (8 to 40 feet per second) that will not be encountered.

Conversely, piping systems for abrasive liquids are often designed to operate at much lower velocities to reduce abrasion. Maximum flow rates in these applications can be as low as 2 feet per second (or lower). It the above ultrasonic flowmeter applied to this application, the flowmeter would operate accurately over a turndown of only 2:1. This turndown is far from the 40:1 turndown implied by its “up to 40:1” specification.

When selecting a flowmeter, be sure that you define the turndown that you need, determine the turndown that the flowmeter will provide based upon the actual flow rates or velocities encountered, and then make your decision. One more thing — be sure to stay away from desert islands where you might get stranded.

Originally published in Flow Control magazine.

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