1、CALIBRATIONA TECHNICIANS GUIDECable05.book Page i Wednesday, December 8, 2004 9:36 AMCable05.book Page ii Wednesday, December 8, 2004 9:36 AMCALIBRATIONA TECHNICIANS GUIDEMike CableISA TECHNICIAN SERIESCable05.book Page iii Wednesday, December 8, 2004 9:36 AMCopyright 2005 by ISA Instrumentation, Sy
2、stems, and Automation Society67 Alexander DriveP.O. Box 12277Research Triangle Park, NC 27709All rights reserved.Printed in the United States of America.1098765432ISBN 1-55617-912-XNo part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, elect
3、ronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher.NoticeThe information presented in this publication is for the general education of the reader. Because neither the author nor the publisher has any control over the use of the information
4、by the reader, both the author and the publisher disclaim any and all liability of any kind arising out of such use. The reader is expected to exercise sound professional judgment in using any of the information presented in a particular application. Additionally, neither the author nor the publishe
5、r have investigated or considered the effect of any patents on the ability of the reader to use any of the information in a particular application. The reader is responsible for reviewing any possible patents that may affect any particular use of the information presented. Any references to commerci
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7、 availability of any referenced commercial product at any time. The manufacturers instructions on use of any commercial product must be followed at all times, even if in conflict with the information in this publication.Library of Congress Cataloging-in-Publication Data in process.Cable05.book Page
8、iv Wednesday, December 8, 2004 9:36 AMvTABLE OF CONTENTSAbout the Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ixAcknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9、 . . . . . . . . . . xiiiChapter 1 Calibration Principles . . . . . . . . . . . . . . . . . . . . . . . 11.1 What Is Calibration?. . . . . . . . . . . . . . . . . . . . 11.2 What Are the Characteristics of a Calibration?. . 21.3 Why Is Calibration Required?. . . . . . . . . . . . . . 61.4 Who Perfor
10、ms Calibrations? The Control System Technician. . . . . . . . . . . . . . . . . . . . . 81.5 Characteristics of a Control System Technician . 91.6 Loop Calibration vs. Individual InstrumentCalibration . . . . . . . . . . . . . . . . . . . . . . . . . 101.7 Bench Calibration vs. Field Calibration . .
11、 . . . . 111.8 Classification of Instruments . . . . . . . . . . . . . 12Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . 13Review Questions . . . . . . . . . . . . . . . . . . . . . . . . 14Chapter 2 Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . 172.1 Calibration
12、Procedure Content . . . . . . . . . . . . 192.2 Calibration Data Sheets (See Examples in Appendix A-4) . . . . . . . . . . 212.3 P may be expressed in measurement units, percent of span, or percent of reading.As you can see from the definitions, there are subtle differences between the terms. It is
13、recommended that the tolerance, specified in measurement units, is used for the calibration requirements performed at your facility. By specifying an actual value, mistakes caused by calculating percentages of span or reading are eliminated. Also, tolerances should be specified in the units measured
14、 for the calibration.Cable05.book Page 2 Wednesday, December 8, 2004 9:36 AMCalibration 3For example, you are assigned to perform the calibration of the previously mentioned 0-to-300 psig pressure transmitter with a specified calibration tolerance of 2 psig. The output tolerance would be:The calcula
15、ted tolerance is rounded down to 0.10 mA, because rounding to 0.11 mA would exceed the calculated tolerance. It is recommended that both 2 psig and 0.10 mA tolerances appear on the calibration data sheet if the remote indications and output milliamp signal are recorded.Note the manufacturers specifi
16、ed accuracy for this instrument may be 0.25% full scale (FS). Calibration tolerances should not be assigned based on the manufacturers specification only. Calibration tolerances should be determined from a combination of factors. These factors include: Requirements of the process Capability of avail
17、able test equipment Consistency with similar instruments at your facility Manufacturers specified toleranceExample: The process requires 5C; available test equipment is capable of 0.25C; and manufacturers stated accuracy is 0.25C. The specified calibration tolerance must be between the process requi
18、rement and manufacturers specified tolerance. Additionally the test equipment must be capable of the tolerance needed. A calibration tolerance of 1C might be assigned for consistency with similar instruments and to meet the recommended accuracy ratio of 4:1.Accuracy Ratio: This term was used in the
19、past to describe the relationship between the accuracy of the test standard and the accuracy of the instrument under test. The term is still used by those that do not understand uncertainty calculations (uncertainty is described below). A good rule of thumb is to ensure an accuracy ratio of 4:1 when
20、 performing calibrations. This means the instrument or standard used should be four times more accurate than the instrument being checked. Therefore, the test 2 psig300 psig16 mA 0.1067 mA-Cable05.book Page 3 Wednesday, December 8, 2004 9:36 AM4 Calibration Principlesequipment (such as a field stand
21、ard) used to calibrate the process instrument should be four times more accurate than the process instrument, the laboratory standard used to calibrate the field standard should be four times more accurate than the field standard, and so on.With todays technology, an accuracy ratio of 4:1 is becomin
22、g more difficult to achieve. Why is a 4:1 ratio recommended? Ensuring a 4:1 ratio will minimize the effect of the accuracy of the standard on the overall calibration accuracy. If a higher level standard is found to be out of tolerance by a factor of two, for example, the calibrations performed using
23、 that standard are less likely to be compromised.Suppose we use our previous example of the test equipment with a tolerance of 0.25C and it is found to be 0.5C out of tolerance during a scheduled calibration. Since we took into consideration an accuracy ratio of 4:1 and assigned a calibration tolera
24、nce of 1C to the process instrument, it is less likely that our calibration performed using that standard is compromised. The out-of-tolerance standard still needs to be investigated by reverse traceability of all calibrations performed using the test standard. However, our assurance is high that th
25、e process instrument is within tolerance. If we had arbitrarily assigned a calibration tolerance of 0.25C to the process instrument, or used test equipment with a calibration tolerance of 1C, we would not have the assurance that our process instrument is within calibration tolerance. This leads us t
26、o traceability.Traceability: All calibrations should be performed traceable to a nationally or internationally recognized standard. For example, in the United States, the National Institute of Standards and Technology (NIST), formerly National Bureau of Standards (NBS), maintains the nationally reco
27、gnized standards. Traceability is defined by ANSI/NCSL Z540-1-1994 (which replaced MIL-STD-45662A) as “the property of a result of a measurement whereby it can be related to appropriate standards, generally national or international standards, through an unbroken chain of comparisons.” Note this doe
28、s not mean a calibration shop needs to have its standards calibrated with a primary standard. It means that the calibrations performed are traceable to NIST through all the standards used to calibrate the standards, no matter how many levels exist between the shop and NIST.Traceability is accomplish
29、ed by ensuring the test standards we use are routinely calibrated by “higher level” reference standards. Typically the standards we use from the shop are sent out periodically to a standards lab which has more accurate test equipment. The standards Cable05.book Page 4 Wednesday, December 8, 2004 9:3
30、6 AMCalibration 5from the calibration lab are periodically checked for calibration by “higher level” standards, and so on until eventually the standards are tested against Primary Standards maintained by NIST or another internationally recognized standard.The calibration technicians role in maintain
31、ing traceability is to ensure the test standard is within its calibration interval and the unique identifier is recorded on the applicable calibration data sheet when the instrument calibration is performed. Additionally, when test standards are calibrated, the calibration documentation must be revi
32、ewed for accuracy and to ensure it was performed using NIST traceable equipment.FIGURE 1-1.Traceability PyramidUncertainty: Parameter, associated with the result of a measurement that characterizes the dispersion of the values that could reasonably be attributed to the measurand. Uncertainty analysi
33、s is required for calibration labs conforming to ISO 17025 requirements. Uncertainty analysis is performed to evaluate and identify factors associated with the calibration equipment and process instrument that affect the calibration accuracy. Calibration technicians should be aware of basic uncertai
34、nty analysis factors, such as environmental effects and how to combine Process Instrument Primary Standards National Measurement Standard (e.g., NIST) Working Standards (“normal” shop instruments) Secondary Standards Cable05.book Page 5 Wednesday, December 8, 2004 9:36 AM6 Calibration Principlesmult
35、iple calibration equipment accuracies to arrive at a single calibration equipment accuracy. Combining multiple calibration equipment or process instrument accuracies is done by calculating the square root of the sum of the squares, illustrated below:Calibration equipment combined accuracyProcess ins
36、trument combined accuracy 1.3 WHY IS CALIBRATION REQUIRED?It makes sense that calibration is required for a new instrument. We want to make sure the instrument is providing accurate indication or output signal when it is installed. But why cant we just leave it alone as long as the instrument is ope
37、rating properly and continues to provide the indication we expect?Instrument error can occur due to a variety of factors: drift, environment, electrical supply, addition of components to the output loop, process changes, etc. Since a calibration is performed by comparing or applying a known signal t
38、o the instrument under test, errors are detected by performing a calibration. An error is the algebraic difference between the indication and the actual value of the measured variable. Typical errors that occur include:FIGURE 1-2.Span Errorcalibrator1 error()2calibrator2 error()2etc. error()2+sensor
39、 error()2transmitter error()2indicator error()2etc. error()+2+100%0100%ACTUAL VALUE% INPUT% OUTPUTSPAN ERRORSCable05.book Page 6 Wednesday, December 8, 2004 9:36 AMCalibration 7FIGURE 1-3.Zero ErrorFIGURE 1-4.Combined Zero and Span Error100%ACTUAL VALUE0100% INPUT% OUTPUTZERO ERRORS100%0100%ACTUAL V
40、ALUE% INPUT% OUTPUTCOMBINED ZERO ANDSPAN ERRORSCable05.book Page 7 Wednesday, December 8, 2004 9:36 AM8 Calibration PrinciplesFIGURE 1-5.Linearization ErrorZero and span errors are corrected by performing a calibration. Most instruments are provided with a means of adjusting the zero and span of the
41、 instrument, along with instructions for performing this adjustment. The zero adjustment is used to produce a parallel shift of the input-output curve. The span adjustment is used to change the slope of the input-output curve. Linearization error may be corrected if the instrument has a linearizatio
42、n adjustment. If the magnitude of the nonlinear error is unacceptable and it cannot be adjusted, the instrument must be replaced.To detect and correct instrument error, periodic calibrations are performed. Even if a periodic calibration reveals the instrument is perfect and no adjustment is required
43、, we would not have known that unless we performed the calibration. And even if adjustments are not required for several consecutive calibrations, we will still perform the calibration check at the next scheduled due date. Periodic calibrations to specified tolerances using approved procedures are a
44、n important element of any quality system.1.4 WHO PERFORMS CALIBRATIONS? THE CONTROL SYSTEM TECHNICIANA control system technician (CST) is a skilled craftsperson who knows pneumatic, mechanical, and electrical instrumentation. He or she understands process control loops and process control systems,
45、including 100%0100%ERRORS CAUSEDBY NONLINEARITYDESIRED VALUE% INPUT% OUTPUTCable05.book Page 8 Wednesday, December 8, 2004 9:36 AMCalibration 9those that are computer-based. Typically, he or she has received training in such specialized subjects as theory of control, analog and/or digital electronic
46、s, microprocessors and/or computers, and the operation and maintenance of particular lines of field instrumentation.A CST performs calibration, documentation, loop checks, troubleshooting, and repair or replacement of instrumentation. These tasks relate to systems that measure and control level, tem
47、perature, pressure, flow, force, power, position, motion, physical properties, chemical composition and other process variables.1.5 CHARACTERISTICS OF A CONTROL SYSTEM TECHNICIANHonesty and Integrity: A CST must possess honesty and integrity above all else. Most technicians work independently much o
48、f the time. Calibrations must be performed in accordance with procedures and must be properly documented. Additionally, the calibration department may be understaffed and production schedules may demand unrealistic completion requirements. These factors can have a real impact on proper performance a
49、nd documentation of calibrations. Remember: Nobody can take away your integrity; only you can give it away.Attention to Detail: Calibrations should be performed in accordance with detailed instructions. Each different make/model instrument is adjusted differently. Each instrument is installed in a different physical and loop configuration. Because of these and many other differences, attention to detail is very important. The minute a technician is not paying attention to detail, safety and proper performance are jeo
