Summer 2015

Pressure Change Measurement Leak Testing Errors by: Jeff M. Pryor and William C. Walker Article on page 8

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Pressure Change Measurement Leak Testing Errors

By: Jeff M. Pryor and William C. Walker................................ 8

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President’s Message .........................6 AGS Sustaining Members..................6 AGS 2015 Conference ................12/13 AGS Publication Order Form............19

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2014/15 Board of Directors

The Enclosure Editors

Rodney B. Smith - [email protected] Scott Hinds - [email protected]

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American Glovebox Society The Enclosure is published for the benefit of the glovebox engineering profession serving nuclear, biomed, pharmaceutical, semi-conductor, aerospace technology, and other industries. Circulation: Our continually increasing circulation is now at 3,100 copies. We have subscribers throughout the United States, Canada and around the world. Readership: Glovebox designers, users, engineers, fabricators, manufacturers, suppliers, contractors, libraries, and universities. Advertising: Deadlines for display ads are: March 1st & September 1st. All rights reserved: Material may be reproduced or republished with credit to source and author (when indicated). Submittals: All letters, articles and photographs are welcome and should be directed to the editor’s attention. Material will not be returned unless accompanied by a self-addressed, stamped envelope. Disclaimer: The information contained herein is correct to the best of our knowledge. Recommendations and suggestions are made without guarantee or representation as to the result since conditions of use are beyond our control. The contents are intended as a source of information. The Enclosure is published by: American Glovebox Society 526 So. E Street Santa Rosa, CA 95404 Tel.: (800) 530-1022 or (707) 527-0444 Fax: (707) 578-4406 E-mail: [email protected] Website: www.gloveboxsociety.org Postmaster: Send address changes to: American Glovebox Society 526 So. E Street Santa Rosa, CA 95404

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President Scott Hinds Western Refining Bus: (505) 500-7776 [email protected]

Carl Fink CTL Corp. Bus: (860) 651-9173 [email protected]

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Lyle Freeman Premier Technology, Inc. Bus: (208) 785-2274 [email protected] Tony Heinz Leak Test Specialists Phone: (407) 737-6415 [email protected]

Greg Wunderlich URS (303) 843-3135 [email protected]

Rick Hinckley Los Alamos National Laboratory Phone: (505) 667-9931 [email protected]

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Bradley Hodges B&W Y-12 Bus: (865) 576-5850 [email protected]

Liaisons Ike Dimayuga, Canadian Liaison Atomic Energy of Canada, Ltd. Bus: (613) 584-8811 [email protected] Martyn Page, UK Liaison AWE, plc Bus: 44 1189850633 [email protected]

Russ Krainiak Integrated Containment System Bus: (252) 946-0166 [email protected] John Newman Springs Fab Advanced Technology Group Bus. (303) 438-1570 [email protected] Gary Partington Walker Barrier Systems Bus. (608) 562-7761 [email protected] Ron Smith Bus: (803) 613-969 [email protected] James Spolyar Aseptic Barrier Systems LLC [email protected]

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President’s Message By: Scott Hinds

“Listen to the record!”

“Come to the Conference!”

Only people who listen to my style of rock and roll from the late ‘70’s would know where that phrase came from. On the back of the first Boston album (yes - vinyl), the verbiage that described the members and recording style of the band Boston ended each paragraph with “listen to the record!” in an enthusiastic request to hear the music. Well, in borrowing that style from one of my favorite bands, I apply it to my favorite society and say “Come to the Conference!”

Donna S. Heidel is the Technical Director for Industrial Hygiene for the Bureau Veritas North America and she is our 2015 AGS Keynote Speaker. Ms. Heidel leads the development of industrial hygiene services to support highly effective management of occupational health risks associated with emerging technologies. She will address the society on the challenges associated with applying glovebox solutions to reduce worker risks while maintaining quality levels in hazardous drug and nanoparticle applications.

The American Glovebox Society’s Officers, Board of Directors and Management team has assembled a comprehensive educational and technical program which is slated to fill three full days of glovebox discussion focused on our theme of Equipment Integration. Our Fundamentals and Focused Topic training will provide all-inclusive skill sets to the beginner to complex, across-the-board subject matter discussions to the most experienced glovebox professional. At press time, all of our technical session presentations have been filled with presenters from across the globe to challenge the way we think. Our case study and panel discussions will provide topical debate issues to keep us current with our industry.

“Come to the Conference!” A mainstay for the success of our conferences, our exhibitors and vendors will flood the main exhibition hall with the latest technology available to our industry. From windows and whole scale glovebox designs to exotic instrumentation and safety applications, our vendors will dazzle, delight and demonstrate all there is to know about the world of gloveboxes, containments and enclosures.

AGS Sustaining Members Byers Precision Fabricators, Inc. PO Box 5127 Hendersonville, NC 28793 Bus: (828) 693-4088 [email protected]

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KoreaKiyon Co., Ltd. Seoul, Korea [email protected]

Scheematic Engineering Industries Plot No. 37A Phase IV, IDA Jeedimetla, Hyderabad, INDIA Bus: +91 9581111446 [email protected]

Marks Brothers, Inc. 12265 SE 282nd Avenue Boring, OR 97009 Bus: (503) 663-0211 [email protected]

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“Come to the Conference!” And of course – San Diego!! Our venue could not be a better family friendly setting. The restaurants, the Zoo, the botanical parks, the beaches, the sunshine, the resort, the shopping, the Gaslamp District, the music, the amusement parks, did I mention the beaches??

“Come to the Conference!” Finally, the year has gone by too fast and I do greatly appreciate everyone’s support in helping me be your American Glovebox Society’s president for 2015. A long time ago, I joined the AGS as a standards committee member with Beth Sliski as the AGS-G001 chairperson. For me, that time was 1989. G001 was just many words in a computer. The society was young and we had goals of many more documents to develop. As a societal body, we have accomplished a great deal in the 27 years since I joined the AGS and the nearly 30 years since the AGS has been in existence. Our influence in the world of containment science and engineering still grows with each day. My perspective is that we are a society and membership that truly enjoys what we do. We are an impassioned and involved group of like minded individuals that relish the challenge of balancing work, family, friends and ambitions with our own time constraints. Although we juggle many balls in the air at one time, please take the occasion to enjoy a moment with your family, phone your mother, and take a day off to soak it all in. I look forward to meeting old and new friends this July and passing the torch to another year.

“Come to the Conference!” Regards, Scott Hinds P.E. AGS President 2014-2015

July 27-29, 2015 Town and Country Resort and Conference Center - San Diego, CA

Keynote Speaker: Donna S. Heidel Biosketch

Donna S. Heidel is the Technical Director for Industrial Hygiene for Bureau Veritas North America where she leads the development of industrial hygiene services to support the effective management of occupational health risks associated with emerging technologies. Prior to her employment with Bureau Veritas she coordinated the Prevention through Design program at the National Institute for Occupational Safety and Health, including the occupational health and safety management systems for safely synthesizing manufactured nanoparticles and commercializing nano-enabled products. Ms. Heidel also has experience in the pharmaceutical industry, including 15 years at Johnson & Johnson, as the World Wide Director of Industrial Hygiene. While at J&J, she supported the development and implementation of engineering containment and control systems for high-potency drugs. She is certified by the American Board of Industrial Hygiene (CIH), and is an AIHA fellow. She has received the AIHA President’s Award in 2011 and in 2013. She also serves on the AIHA Board of Directors. She is the past chair of the AIHA Control Banding Working Group.

Abstract

The guidelines developed by the American Glovebox Society for design, fabrication and testing of glovebox isolators have been successfully implemented by the pharmaceutical industry not only to control worker exposure to hazardous drugs but also to provide contaminant control for sterile products. Innovative designs, including flexible walls, materials transfer systems, and clean-in place capabilities have supported the need for occupational exposure control while meeting the specific process requirements for pharmaceutical dosage form manufacture. The successful application of this technology by the pharmaceutical industry is now being applied to safely synthesize manufactured nanoparticles and nanoenabled products. The occupational health challenges associated with hazardous drugs and manufactured nanoparticles and the application of glove box solutions to significantly reduce worker exposure risks will be discussed.

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Related Recent Publications

Heidel, D; Segrave, A; Baker, J. Occupational Safety and Health Management Systems for the Safe Commercialization of Nano-enabled Products Nanotechnology 2013: Bio Sensors, Instruments, Medical, Environment and Energy, Chapter 5: Environmental Health & Safety, Nanotech 2013 Vol. 3. Geraci, C; Heidel, D; Sayes, C; Hodson, L.; Schulte, P; Eastlake, A; Brenner, S. Safe Nano Design: Molecule to Manufacturing to Market; Responsible Practices for Safe Nano Design. Journal of Nanoparticle Research. 2014. Heidel, D., Ripple, S. Closing the Exposure Gap, Occupational Exposure Bands, ERAM, and Prevention through Design. The Synergist (2012), Volume 23, Number 4. Prevention through Design Plan for the National Initiative; DHHS (NIOSH) Publication No. 2011–121Heidel, D; Murray, K; Gutmann, S. Chapter 9: Identify Impacts. AIHA Value Strategy Manual. pp. 73-79. American Industrial Hygiene Association, 2010. ❖

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www.leaktestingspec.com 7

Pressure Change Measurement

Leak Testing Errors by: Jeff M. Pryor and William C. Walker Author’s Note A statement of leakage rate can only be considered complete when it contains a leakage rate combined with a test temperature and pressure. Typical for the leak test industry in general, the formula contained in this article is given with an output of atmospheric cc/sec. (referred to as SCC/S in the article). Additionally, the formulas given are not corrected to standard temperature since the definition of standard temperature varies widely across the industry. While the principals of the article hold true for all testing, the math may vary slightly depending on the desired output units.

Reprint Permission From Materials Evaluation, Vol. 72, No. 5. Reprinted with permission of the American Society for Nondestructive Testing, Inc.

Introduction

A

pressure change test is a common leak testing technique used in construction and nondestructive testing (NDT). The test is known for being fast, simple and easy to apply. While this technique may be quick to conduct and require simple instrumentation, the engineering behind this type of test is more complex than is apparent on the surface. A pressure change measurement test (PCMT) is an evaluation of the leak-tightness of a closed system. A PCMT happens when a system is driven into a state of pressure differential (by being placed either under an elevated system pressure or under a relative vacuum), then closed and monitored over a period of time for change in pressure. The resulting change in the internal pressure can then be related to a leakage rate using the ideal gas law formula. While a PCMT should be a relatively simple test to physically perform, the calculation of a leakage rate from the resulting pressure decay data requires careful consideration of all measurement errors. The American Society of Mechanical Engineers Boiler and Pressure Vessel Code (BPVC) requires that the resolution, repeatability and accuracy of the instruments used be compatible with the specific test system; however, it offers no guidance on how this requirement is to be met (ASME, 2011). Uncompensated measurement errors can mask the true leakage rate, leading to incorrect results. Therefore, they are of the utmost importance in this type of test. This paper describes the basic concepts associated with a PCMT, the types of measurement errors involved with the test, and the governing equations incorporating the measurement errors into the leakage rate calculation. The mathematical principles discussed here apply to ideal gases such as air or other monoatomic or diatomic gasses; however, these same principals can be applied to polyatomic gasses or liquid flow rate with altered formula specific to those types of tests using the same methodology. The methodologies of measurement discussed here are readily applicable to any specification set and to all PCMTs, which is particularly valuable since most construction specifications have equivalent language requiring the test engineer to evaluate leakage rates consistent with the BPVC. The leakage rate is typically correlated to standard conditions (a standard atmosphere at sea level at standard temperature with a standardized volume and so on) and produces output such as

standard cubic centimeters per second, standard cubic feet per minute and many more. The determination of a leakage rate is primarily based on a change in pressure within a closed system. Accordingly, system pressure must be monitored since this is the basis for the test. Although a change in system pressure is the critical measurement for detection of a system leak, there are other physical attributes beyond leakage in a closed system that impact the change in system pressure. These attributes include system temperature, ambient pressure and system volume. A change in system temperature will result in a proportional change in system pressure. Ergo, temperature monitoring is necessary during testing since any observed change in system pressure will need to be compensated for all observed changes in temperature. Changes in ambient (that is, barometric) pressure can also influence the assessed leakage rate. A PCMT depends on a pressure differential from the inside of the system to the outside of the system in absolute terms. Consequently, atmospheric pressure changes affect these types of tests since they change the absolute pressure differential of the system under test. Finally, system volume must be known so that the calculation can be performed. The calculation of a leakage rate from a closed system is derived from a modified form of the ideal gas law, expressed as follows: (1)

Q=

where

( P ×V) ( StdAtm × t )

Q = leakage rate (at standard conditions), rP = relative change in system internal pressure (Pinitial – Pfinal), StdAtm = ambient (standard) pressure, V = internal volume, t = time (duration of the PCMT). In accordance with the gas law formula, the units are in absolute Continued on page 10

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Continued from page 8

Pressure Change Measurement Leak Testing Errors

pressures and absolute temperatures in the units. The selection of units must be consistent and compatible throughout the entire computation set from start to finish. An expanded form of the leakage rate formula, which includes the effect of barometric pressure, can be expressed as: (2) where B = barometric pressure, T = temperature. The subscript i indicates the initial instrument reading, and subscript f indicates the final instrument reading. This expanded form of the ideal gas law (Equation 2) allows for the compensation of pressure changes due to variations in barometric pressure and temperature. An examination of this expanded formula highlights the previously discussed need to monitor system temperature and ambient pressure during the test. The failure to compensate for barometric pressure when using gage pressure to express pressure as absolute is a common error in these tests. The change in barometric pressure on short duration tests (for example, a few minutes) is generally negligible.

The calculation of a leakage rate from a closed system is derived from a modified form of the ideal gas law

An assessment of the accuracy of the instrumentation is critical with respect to quantifying the propagation of uncertainty associated with the calculated leakage rate. The addition of a statement of measurement uncertainty to a test result indicates that not only was the calculated leakage rate acceptable (less than the maximum allowed leakage rate), but that the calculated leakage rate is statistically significant (the inherent variability in the instrumentation measurements did not mask the true value). The following paragraphs discuss these instrumentation attributes in detail.

System Resolution System resolution is the smallest measurement that can be seen with a specific test and setup. This calculation must be taken in the context of the test system, not just the individual instrument’s increment. One must take into account the complete system, under test conditions, and then use this system information to find out what a particular instrument’s effects are on the test. It is only through this process that the system resolution of a particular instrument can be defined. Resolution must be defined for each instrument used in the testing system. The test resolution as a whole cannot be greater, or more sensitive, than the least sensitive instrument. Using the standard PCMT formula (Equation 3) and some mathematical manipulation, substitution of each instrument’s resolution into the actual test conditions produces the following formulae for evaluating component resolutions: initial leakage rate formula (3)

However, on longer duration tests, a change in barometric pressure can significantly affect test results. The lack of barometric compensation can mask unacceptable leaks, causing the acceptance of otherwise rejectable items. By initially choosing to use an absolute pressure gage, the barometric compensation can be removed from the equation (this is the preferred technique). Similarly, a lack of accurate temperature measurement monitoring over longer test times can result in erroneous calculations and test outcomes. An often-overlooked issue regarding the evaluation of a PCMT leakage rate is accounting for performance attributes of the instrumentation as used in the test system. These attributes include: l Resolution of the test instrumentation (not just the gage resolution), l Test sensitivity, or repeatability, of the instrumentation,

pressure resolution (4) temperature resolution (5) time resolution (6)

l Accuracy of the instrumentation, referred to as measurement errors or uncertainties. The resolution and repeatability characteristics of the instrumentation must be compatible with the test conditions. An assessment of instrumentation resolution and sensitivity demonstrates that the calculated leakage rate results obtained are possible under the system configuration by specifying the minimum detectable leak rate achievable with the instrumentation used in the test.

where Q = measured leakage rate, Gres = pressure gage resolution (smallest pressure increment measured during test), Pi = initial absolute pressure, Pf = final absolute pressure, Continued on page 14

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Pressure Change Measurement Leak Testing Errors StdAtm = standard atmospheric pressure (used for correlation to standard conditions), Ti = initial absolute temperature, Tf = final absolute temperature, Tres = temperature gage resolution (smallest temperature increment measured during test), t = total time elapsed during test, tres = time resolution (time increment measured; for example, 0.1 s for stopwatch = 0.0167 min), tunit = time unit (time increment of output; for example, 1 minfor ft3), V = volume. The PCMT resolution is the maximum of the evaluated instrument resolutions considering pressure, temperature and time (Equations 4, 5 and 6). Using this set of instruments and under these specific test conditions, this is the smallest leak that could possibly be seen under the most ideal of conditions (that is, a detectability limit). Although the system volume must be determined with an accuracy and precision required for the test, volume resolution is ignored since the volume is static during the test. As a note of caution: volume does have a measurement uncertainty associated with it, which should not be ignored by the test engineer. Volume measurement uncertainty must be considered in context with the application involved. The uncertainty in the volume measurement is included in the total propagated uncertainty for the leakage rate. As the test volume decreases, the significance of the volume upon the test and its uncertainty increase.

Resolution Example In this example, only the pressure resolution will be evaluated. The system undergoing the test has an acceptance criteria of 6.5 × 10–2 Std L/min (2.3 × 10–3 SCFM). The test volume is determined to be 1360 L (48 ft3), and test pressure is planned to be established at +102 mm (4 in.) water column. Finally, the test duration is planned to be one hour. The pressure gage used for the test had subdivisions of 704 mm (27.7 in.) water column (6895 Pa [1 psi]). The evaluated resolution of the pressure instrumentation is as follows: = (Gres × V) / (StdAtm × rt) = (704 mm water column × 1360 L) / (10340 mm water column × 60 min)

System Repeatability Although closely related, there is a difference between test resolution and repeatability. System repeatability or sensitivity is defined as the smallest test increment that is considered repeatable. Just because it is theoretically possible to see a leak as small as the resolution does not mean that it would be expected to read this every time, which speaks to the repeatability requirement. A prudent test engineer would require that the test sensitivity be between two to ten times the maximum test resolution to ensure repeatability. If the test sensitivity were greater (larger) than the smallest allowable leak, then the test, as configured, would be incapable of delivering the desired results. It would therefore be unacceptable for use. In the case of inadequate sensitivity or resolution, either different instruments or another test configuration would be needed. While more accurate gages are often the best long-term solution, this pricy alternative is not the only solution. Changes in any of the test variables will affect the test sensitivity.

Repeatability Example Again, evaluating only at the pressure component, a PCMT test to be performed on a system where the acceptance criteria is 9 ×10–2 Std L/min (3.2 × 10–3 SCFM). The test volume is determined to be 2832 L (100 ft3) and testing will be conducted at a pressure of +102 mm (4 in.) water column for one hour using a pressure gage that had subdivisions of 12.7 mm (0.5 in.) water column. The following pressure resolution result is obtained: = (Gres × V) / (StdAtm × rt) = (12.7 mm water column × 2832 L) / (10340 mm water column × 60 min) = 5.8 × 10–2 Std L/min or = (0.5 in. water column × 100 ft3) / (407 in. water column × 60 min) = 2.0 × 10–3 SCFM The repeatability (also known as sensitivity) is defined as 2X minimum resolution. Therefore, this pressure gage has a minimum sensitivity of 0.12 Std L/min (4 × 10–3 SCFM). This pressure gage does have the required resolution (less than the acceptance criteria); however, it is incapable of repeatedly seeing the required leakage rate in this test system (the sensitivity is greater than the acceptance criteria). Therefore, this gage is not compatible with this test. See the PCMT decision tree (Figure 1) for a graphic example of how system resolution and repeatability affect testing.

= 1.5 Std L/min

System Accuracy

or

Accuracy in the world of metrology (the science of measurement) is the degree to which a measured value agrees with the actual, or true, value. There are uncertainties associated with any and all measurements. An estimate of the magnitude and statistical confidence of these uncertainties forms a statement of uncertainty for the original measurement. A measurement without a statement of uncertainty is incomplete. In order to know what uncertainty is, one must first understand the definition.

= (27.7 in. water column × 48 ft3) / (407 in. water column × 60 min) = 5.4 × 10–2 SCFM This gage is evaluated to be incapable of evaluating a leakage rate equal to or less than the acceptance criteria leakage rate. Therefore, this pressure gage is not compatible with the test as planned.

Continued on page 16

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Continued from page 14

Pressure Change Measurement Leak Testing Errors

“The objective of a measurement is to determine the value of the measurand, that is, the particular quantity to be measured” (ISO, 1995). A measurement depends on the method of measurement and the measurement procedure. “In general, the result of a measurement is only an approximation or estimate of the value of the measurand and thus is complete only when accompanied by a statement of uncertainty of that estimate” (ISO, 1995).

To fully understand what an uncertainty calculation involves, an examination of the parts of the measurement system and how these affect the test outcome is in order. A measurement system is made of separate components that make up the total system, for example, pressure, temperature and so on. Each system component must be evaluated separately. The principal of uncertainty applies separately to the individual measurement instrument (if more than one instrument makes up that component), the system component and the measurement system as a total. See the PCMT decision tree (Figure 1) for a graphic example of how combining separate pressure instruments effects the total pressure uncertainty. Each instrument component has a minimum increment to which it can be read. This is called resolution uncertainty or simply resolution. Note that this is the same component resolution used in the calculation for system resolution. If using an analog gage with a reading increment of 5 pressure units (for example, 0, 5, 10…), it would be impossible to read a 0.1 unit change. An argument could be made that 2.5 unit increments or maybe even 1 unit increments could be read; however, only the 5 pressure unit increments can be read with true repeatability and certainty in this case. Reading between the divisions of an analog gage is beyond the design of the instrument and adds additional un-quantifiable uncertainties. This minimum measurement resolution is of extreme importance when selecting the appropriate gage for a specific test.

Each instrument component also has some uncertainty involved Figure 1. Pressure change measurement test (PCMT) decision tree. RSS = root sum square. with accuracy called measurement uncertainty. This is different than component resolution; The three elements that make up a measurement uncertainty measurement uncertainty has to do with just how accurate are resolution, repeatability and accuracy. the specific component actually measures the value. Most The total uncertainty of the system as a total is the most complex manufacturers give a specification on accuracy of instrument of the three BPVC required elements to calculate. Uncertainty of as percent of full scale. For instance if a 2000 kPa (290 psi) measurement is often compensated for during the test to ensure gage had an accuracy of ±3% full scale, then the measurement the most reliable test results. Uncertainty becomes more critical as could be otherwise stated as ±60 kPa (9 psi). This means that a the test results require more accuracy and precision. Depending reading of 1000 kPa (145 psi) could represent an actual pressure on the specific objectives of the test, uncertainty could be left of somewhere between 940 and 1060 kPa (136 and 154 psi), out of the calculation set if the test engineer determined these provided no other uncertainties are entered into the equation. calculations are insignificant in the particular test, that is, very During the gage selection process, getting a highly accurate gage small in comparison to the expected results. It is important to note compatible with the leak test is of paramount importance. that resolution, repeatability and uncertainty calculations must be Continued on next page performed for each different testing scenario.

16

Continued from previous page Each measurement component has some uncertainty involved with how close the instrument has been calibrated to known standards, called traceability uncertainty. Remembering that no measurements are perfect, traceability has to do with the calibration lab and the transfer errors in measuring the calibration standards. Each time a calibration standard is measured and the value is transferred to another standard, there is some increase in uncertainty. This uncertainty value must be supplied by the calibration lab for each measurement component in the test system.

Each instrument component also has some uncertainty involved with accuracy called measurement uncertainty. Each of the three separate uncertainty elements plays a role in the total uncertainties of the instrument or component. Total instrument uncertainty accounts for being able to quantify what a particular instrument can in truth measure and how accurately, not just what the instrument displays as a reading. The three uncertainty elements must be combined to get a total instrument uncertainty that will be of further use. Combining the uncertainty elements into a component total requires a somewhat different approach. For example, measurement accuracy from the manufacturer is stated as ± percent error. It is not known if the uncertainty is positive or negative or where measurement error is within the range stated. If the tolerance is ±10 units, an assumption cannot simply be made to add ten units to the readings; the measurement could be off only five units. To further complicate this issue, if there were two components, one reading could be positive and the other negative, in effect canceling each other out. Likewise, if both were positive then they would add error to each other. It would not be correct to simply sum all the errors or to take any other simplistic approach. It is possible, however, to base the calculations on what is known as statistical probabilities. The International Organization for Standardization has published a text that is most useful for this calculation, the Guide To The Expression Of Uncertainty In Measurement (GUM) (ISO, 1995). The GUM states that a valid prediction can be made based on the standard deviation distribution curve (see Figure 2). Combining the uncertainty elements of components may be made by the root sum square (RSS) technique to obtain a total component uncertainty value. Further, the combination of the total component uncertainties may be performed using the RSS

technique to find the total system uncertainty. With this information, there is an actual calculation or real number that can be used for the determination of the uncertainties. One note of caution: when computing uncertainties, ensure that the units used in the calculation are consistent with the final leakage rate calculation, using the technique given here, or there will be a mixing of terms, giving incorrect results. Presently, the most common pressure gage used is a handheld digital gage with a pressure transducer. The principle of uncertainty applies to both the sending-receiving unit (handheld unit) and to the pressure transducer itself. To find the measurement uncertainty for the entire pressure component, each of the separate instrument uncertainty elements are evaluated. In this example for pressure, combining the total uncertainties for the sending-receiving unit with the uncertainties for the transducer will yield the total pressure component uncertainty. One recommendation on this subject is to calibrate the receiving unit and the transducer as a single unit so that there are not two sets of uncertainties to deal with. Additionally, this gives greater accuracy. See the PCMT decision tree (Figure 1) for a graphic example of combining uncertainties (the add as needed box).

Figure 2. Standard deviation distribution curve. To calculate the total instrument uncertainties, one technique is to use Table 1 for the total instrument uncertainties calculation. Note: in this application, for each of the uncertainty components (accuracy, traceability and resolution), half of the value is used in the RSS since the units given are assumed to be Kp = 2 (value of the coverage factor Kp that produces an interval with a confidence level, p, assuming a normal distribution) to convert to Kp = 1. The accuracy and traceability terms are gaussian distributions. For the term of resolution, a bounded distribution is assumed (also known as uniform or rectangular), which is further compensated by dividing by the square root of three to offset this bounding before squaring. Now that an examination of the individual instrument component uncertainties has been made, an examination of the total instrument system Continued on next page

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Pressure Change Measurement Leak Testing Errors

must be performed using the weighting process. For instance, one unit of pressure change may have more effect on the resulting leakage rate than of one unit of temperature change. Performing the weighting of these factors involves a bit of differential calculus, so it will not be discussed in this paper; however, the formula used for the most common PCMT is supplied in the following section without any justification or discussion. The technique used is fully discussed in the GUM (ISO, 1995). If used, these formulas should be verified and modified, as needed, to fit the specific test needs. The formula supplied should serve as a sound basis for the construction of a specific weighting formula set and help with establishing the correct technique.

Weighted Uncertainties Beginning with Equation 3: (7) where (8) and

LR = Q + UQ

(15) where

LR = reported leakage rate, Q = calculated leakage rate, UQ = total combined uncertainty. This technique includes the measured leakage rate factoring in the measurement errors in the instrument system as is required in PCMT. This technique also shows a quantifiable way of meeting compliance with the BPVC statement, “The gage(s) used shall have an accuracy, resolution, and repeatability compatible with the acceptance criteria” (ASME, 2011). The reported leakage rate would represent the largest leak that could be present with a better than a percentage level of confidence. The percentage level of confidence of calculated leakage rate would depend on the accuracy associated with the instrumentation used in the test. Provided standard uncertainty values are utilized, then the level of confidence for the reported leakage rate will be 1-sigma (that is, a 68.3% confidence band). If an expanded level of confidence is required (for example, 2-sigma or 3-sigma), then the reported leakage rate would be expressed as: LR = Q + kUQ

(16) where

k = coverage factor. (9)

(10

(11)

(12)

Accordingly, for a 2-sigma level of confidence (that is, 95.5% confidence band), the value of k is 2. For a 3-sigma level of confidence (that is, 99.7% confidence band), the value of k is 3.

Uncertainty Example If a system has an acceptance criteria leakage rate of 10 and a PCMT evaluated leakage rate yields a result of 8 ±1, the test is acceptable since the variability of the leakage rate does not exceed the acceptance criteria. In another example, if a calculated leakage rate of 8 ±3 is obtained, the result is ambiguous because of the range for the result overlaps the evaluation criteria, that is, the possibility that the leakage rate is 11 cannot be ignored. When the uncertainty of the calculated leakage rate overlaps the acceptance criteria, the result cannot be designated as acceptable.

Conclusion (13)

(14)

The individually evaluated uncertainty components (Equations 9–14) are determined and substituted into the total combined uncertainty equation (Equation 8). The total combined uncertainty, UQ, is added to the measured leakage rate to yield the reported leakage rate with a high degree or stated degree of certainty using:

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It is essential that calculations be performed to ensure the testing system in total is capable of the required resolution, repeatability and accuracy, ensuring results are compatible with the expectations of the test outcome. As the test results require a higher degree of accuracy and precision, the more critical it becomes to ensure all of the measurement variables are properly accounted for. The resolution and repeatability of the test gages are calculated before testing commences. The test results, once calculated, in order to be complete, must be accompanied by a statement of uncertainty showing the degree of accuracy and precision with which test was performed. References ASME, Boiler and Pressure Vessel Code, Section V, Article 10, Appendix VI, American Society of Mechanical Engineers, New York, New York, 2011. ISO, Guide to the Expression of Uncertainty in Measurement, International Organization for Standardization, Geneva, Switzerland, 1995. v

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