DRAFT RECOMMENDED PRACTICE

ISA-dRP105.00.01-201XCD4

Management of a Calibration Program For Monitoring And Control Systems

Approved xx month xxxx

ISA-dRP105.00.01-201X

Quality Management System for Implementation and Maintenance of an Industrial Calibration Program

ISBN:

Copyright © 201X by ISA, the International Society of Automation. All rights reserved. Not for resale. Printed in the United States of America. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means (electronic mechanical, photocopying, recording, or otherwise), without the prior written permission of the Publisher.

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Preface

This preface, as well as all annexes, is included for information purposes and is not part of ISA-dRP105.00.01-201X.

This document has been prepared as part of the service of ISA toward a goal of uniformity in the field of instrumentation. To be of real value, this document should not be static but should be subject to periodic review. Toward this end, the Society welcomes all comments and criticisms and asks that they be addressed to the Secretary, Standards and Practices Board; ISA; 67 Alexander Drive; P. O. Box 12277; Research Triangle Park, NC 27709; Telephone (919) 549-8411; Fax (919) 549-8288; E-mail: standards@isa.org.

The ISA Standards and Practices Department is aware of the growing need for attention to the metric system of units in general, and the International System of Units (SI) in particular, in the preparation of instrumentation standards. The Department is further aware of the benefits to USA users of ISA standards of incorporating suitable references to the SI (and the metric system) in their business and professional dealings with other countries. Toward this end, this Department will endeavor to introduce SI-acceptable metric units in all new and revised standards, recommended practices, and technical reports to the greatest extent possible. Standard for Use of the International System of Units (SI): The Modern Metric System, published by the American Society for Testing & Materials as IEEE/ASTM SI 10-97, and future revisions, will be the reference guide for definitions, symbols, abbreviations, and conversion factors.

It is the policy of ISA to encourage and welcome the participation of all concerned individuals and interests in the development of ISA standards, recommended practices, and technical reports. Participation in the ISA standards-making process by an individual in no way constitutes endorsement by the employer of that individual, of ISA, or of any of the standards, recommended practices, and technical reports that ISA develops.

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ADDITIONALLY, THE USE OF THIS DOCUMENT MAY INVOLVE HAZARDOUS MATERIALS, OPERATIONS OR EQUIPMENT. THE DOCUMENT CANNOT ANTICIPATE ALL POSSIBLE APPLICATIONS OR ADDRESS ALL POSSIBLE SAFETY ISSUES ASSOCIATED WITH USE IN HAZARDOUS CONDITIONS. THE USER OF THIS DOCUMENT MUST EXERCISE SOUND PROFESSIONAL JUDGMENT CONCERNING ITS USE AND APPLICABILITY UNDER THE USER’S PARTICULAR CIRCUMSTANCES. THE USER MUST ALSO CONSIDER THE APPLICABILITY OF ANY GOVERNMENTAL REGULATORY LIMITATIONS AND ESTABLISHED SAFETY AND HEALTH PRACTICES BEFORE IMPLEMENTING THIS DOCUMENT.

THE USER OF THIS DOCUMENT SHOULD BE AWARE THAT THIS DOCUMENT MAY BE IMPACTED BY ELECTRONIC SECURITY ISSUES. THE COMMITTEE HAS NOT YET ADDRESSED THE POTENTIAL ISSUES IN THIS VERSION.

The following people served as members of ISA Committee ISA105:

NAME COMPANY

This recommended practice was approved for publication by the ISA Standards and Practices Board on __________.

NAME COMPANY

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Table of Contents1 Scope .............................................................................................................................................. 9

2 Normative References ................................................................................................................10

3 Definition of Terms and Acronyms ............................................................................................10

4 Establishing a Calibration Program ...........................................................................................11

4.1 Calibration Program Concepts .........................................................................................11 4.1.2 Device Calibration ........................................................................................................11 4.1.3 Loop Calibration ...............................................................................................................11 4.2 Calibration Program Planning ..........................................................................................12

4.2.1 Calibration program development ......................................................................12 4.2.3 Calculating theoretical loop accuracy ................................................................12 4.2.4 Establishing Required Loop accuracy ...............................................................12 4.2.5 Calibration equipment requirements ..................................................................13 4.2.6 Calibration personnel requirements ...................................................................13 4.2.7 Loop Performance Verification ...........................................................................14 4.2.8 Verification Intervals (loop criticality) ................................................................14 4.2.9 Responsibilities ....................................................................................................15 4.2.10 Records System ...................................................................................................15

5 Calibration Program Activities ....................................................................................................16

5.1 Concepts ............................................................................................................................ 16 5.1.1 Implementation .....................................................................................................16 5.1.2 Program scope .....................................................................................................16

5.2 Activities ............................................................................................................................. 17 5.3 Functions ............................................................................................................................ 17

6 Calibration Program Management .............................................................................................17

6.1 Control ................................................................................................................................ 17 6.2 Assurance .......................................................................................................................... 17 6.3 Improvement ...................................................................................................................... 17

7 Examples ...................................................................................................................................... 17

Annex   A – Example Documentation ................................................................................................23

Calibration management

1. Purpose2. Scope3. References4. Definitions5. Establishing a calibration program

5.1. Calibration Program Concepts5.1.1. Process requirements5.1.2. Device calibration

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5.1.3. Loop Calibration5.2. Calibration Program planning

5.2.1. Calibration Program Development5.2.2. Calculating theoretical loop accuracy5.2.3. Establishing Required Loop Accuracy5.2.4. Calibration Equipment Requirements5.2.5. Calibration Personnel requirements5.2.6. Loop performance verification5.2.7. Verification Intervals (loop criticality)5.2.8. Responsibilities5.2.9. Records Systems5.2.10. Measurement capabilities including measurement uncertainty5.2.11. Instrument specifications, accuracy requirements.5.2.12. Development of calibration specifications

5.3. Calibration Documentation5.3.1. Calibration Procedure – requirements5.3.2. Calibration records – requirements

6. Calibration Program, activities 6.1. Procedures for calibration6.2. Calibration Record Forms6.3. Electronic / Paper-based program6.4. Measurement Standards

7. Calibration Program Management 7.1. Roles and Responsibilities7.2. Auditing of program7.3. Training 7.4. Measurement Standards7.5. Change management7.6. Third party suppliers/contractors7.7. Review of calibration documentation7.8. Training Requirements7.9. Change Management

8. Examples

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IntroductionPurpose

The purpose of this recommended practice is to provide the basic framework for developing and maintaining a consistent calibration program for industrial automation and control systems, including instrumentation used in safety instrumented systems. The recommended practice provides guidance for establishing a calibration program and establishes consistent requirements and methodologies related to verification and calibration of monitoring and control systems by considering the accuracy of each loop required by the process and then adjusting loop component(s) to achieve that loop accuracy.

Accurate, reliable, and repeatable operation of loops in monitoring and control systems is vital to maintaining the safety and reliability of a facility. A well-considered calibration program, properly implemented and maintained, can directly contribute to the assurance of the desired operation of the monitoring or control system for the facility. A calibration program establishes periodic assessments to be performed to monitor control system performance. Data acquired during these assessments not only aids in the establishment of future calibration intervals, but also is critical in the allocation of capital and operational resources. Clearly defined policy and procedures support the efforts of maintenance planners to schedule adequate labor and equipment for calibration both during and between facility outages. Calibration procedures reduce the likelihood of human errors due to improper practices, assure the desired results of the calibration effort, and promotes the proper operation of monitoring and control systems.

Workforce and other economic factors directly affect the maintenance levels in most industries. As a result, many facilities have increased the calibration intervals for monitoring and control systems and their components. In some cases, facilities have simply eliminated routine calibration checks. The result is decreased accuracy and increased failure rates, both of which negatively affect facilities’ operations in many ways, including safety.

In the process industries, more hardware faults occur in measuring instrumentation, transmitters, and control valves than in the logic systems itself. Reducing or eliminating calibration and maintenance of instrumentation and control systems increases the likelihood of system problems, including:

Reduced operator diligence in diagnosing failures of SIS equipment

Inability of maintenance personnel to respond to SIS failures in a timely manner

Increased errors in SIS equipment repair, calibration, inspection or testing

Reduced efforts in preventative maintenance

Reduced documentation of problem resolution

Maintenance practices for devices such as gauges and indicators, unfortunately, may occur only when the error in reading becomes large enough to be obvious to the operator or technician. Maintenance personnel routinely make decisions based on these devices. A faulty indication on such a device could lead to the release of energy. A well-considered calibration program that periodically measures actual loop accuracy should drive the calibration intervals for these devices.

Companies striving to maintain a safe working environment while ensuring the reliability of their facilities use calibration as a means of verifying the functionality and accuracy of their equipment.

In the coming years, companies that employ automation professionals will lose a great number of senior technical and engineering staff members. Outsourcing has also contributed to this

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problem, as many companies are no longer capable of producing qualified automation professionals. In fact, manufacturers and companies providing technical support are also facing a growing shortage of experienced automation professionals. Whether companies use internal resources or rely on contractors, following this recommended approach to developing a calibration program will enable them to capture critical knowledge on their automation instrumentation and systems. Like other aspects of maintenance, there are many things to consider when establishing a company calibration program. Certainly, this is the case with the calibration of monitoring and control loops in a control system. This document presents a recommended approach to developing, implementing and maintaining a calibration program that is intended to lead to increased accuracy and reliability of monitoring and control systems, decreased production costs, and quality control improvements. More important, this approach is also intended to lead to increased safety of operation.

This approach to calibration has proven successful when companies have adhered to the concepts set forth in these guidelines, enabling those companies to realize the full benefits from a standardized approach to calibration.

The intended audience for this document is any company or industry that utilizes instrumentation in the monitoring and control of a process or facility.

Organization

This recommended practice is organized ….

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1 ScopeGeneral Applicability

The recommended practice detailed in this document defines a baseline definition and model of a quality management system that can be utilized to implement and maintain a calibration program for industrial monitoring and control systems. It is applicable to all industrial monitoring and control systems.

Calibration Programs

Industrial measurement and control systems have a direct impact on safety and operating performance, both being directly related to operating costs. When a measurement and control system is not meeting the operating accuracy needs of the process, safety risks and operating costs increase.

A calibration program for an industrial monitoring and control system formalizes a methodology to periodically verify the performance accuracy of the components in that monitoring and control system and, when necessary, make adjustments to those components to bring them within their manufacturer rated accuracy and the loop within its required performance accuracy.

Each user company/facility must establish a calibration program specific to its needs. This recommended practice discusses the essential features of a calibration program and provides guidance how to establish such a program.

This proposed approach to a calibration program for automation instrumentation and systems takes into consideration all known loop measurement errors and establishes calibration tolerances based on the process requirements. Successful implementation of this approach requires management commitment to make this a living process. Critical stesp in the process include:

Developing a comprehensive list of loops and instrumentation equipment requiring calibration

Establishing criticality ratings for each of those loops

Establishing loop tolerance requirements

Proper selection and use of field calibration equipment

Measuring loop accuracy

Maintaining calibration intervals

Using qualified staff to perform calibrations.

Exclusions

This document does not provide or recommend manufacturer-specific calibration procedures for specific instruments as these are established by the instrument manufacturer and are outside the scope of this document.

Regarding monitoring and control loops that are part of safety systems, these guidelines are intended to be supplemental to and not modify the requirements of ANSI/ISA 84 and all its parts.

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How well a control loop performs its control task may be related to loop tuning, component design/application, etc. but is not related to or considered in this recommended practice on calibration. In other words, a final control element should be working properly to facilitate proper loop operation, but is it does not play a role in determining loop measurement accuracy. Proper operation of final control elements is critical to the proper operation of the entire facility and should be assured via a well-considered instrument maintenance program, which is outside the scope of this document.

2 Normative References ANSI/ISA-84.00.01-2004 Part 1 (IEC 61511-1 Mod), Functional Safety: Safety Instrumented Systems for the Process Industry Sector - Part 1: Framework, Definitions, System, Hardware and Software Requirements.

3 Definition of Terms and AcronymsAccuracy- the degree of conformity of a device’s output to its actual input value.

Calibration – the act of determining (by comparison with a standard) and, if necessary, adjusting the accuracy of a device.

The formal definition of calibration by the International Bureau of Weights and Measures is the following: "Operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties (of the calibrated instrument or secondary standard) and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication."

NIST Handbook 150:2001: 1.5.8 Calibration: Set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system, or values represented by a material measure or a reference material, and the corresponding values realized by standards.

NOTE 1 The result of a calibration permits either the assignment of values of measurands to the indications or the determination of corrections with respect to indications.

NOTE 2 A calibration may also determine other metrological properties such as the effect of influence quantities.

NOTE 3 The result of a calibration may be recorded in a document, sometimes called a calibration certificate or a calibration report.

According to international standards, calibration is a comparison of the device being tested against a traceable reference instrument (calibrator) and documentation of this comparison. Although calibration does not formally include any adjustments, in practice, adjustments are possible and often included in the calibration process.

CDS – Calibration data sheet

Error – the difference between an indicated value and the actual value.

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Loop – A combination of two or more instruments or control functions arranged so that signals pass from one to another for the purpose of measurement indication or control of a process variable. Alternately stated as all of the hardware and software components that work together for the purpose of measurement indication or control of a process variable.

Loop Accuracy - the degree of conformity of a loop’s measured or controlled variable indicated value to the variable’s actual value. Loop accuracy is accuracy of the loop measurement.

Loop Tolerance – the permissible limit of variation, from a known standard, in the loop indicated process measurement.

Verification – the act of checking the accuracy of a loop or loop component to determine whether it is performing within required tolerances.

4 Establishing a Calibration Program4.1 Calibration Program Concepts4.1.1 Monitoring and control instrumentation and systems currently used in industry range from pneumatics to digital electronics. These devices are as varied as the processes they monitor. Devices with moving parts require regular maintenance. These mechanical devices are much more susceptible to mechanical performance issues (e.g. binding and dragging due to environmental contamination) than are other automation instrumentation. Analog electronic instrumentation is subject to drift in settings and output. Digital instrumentation has multitudes of parameter settings that must be set properly to achieve desired operation. Devices not operating to their manufacturer’s specifications and/or not properly configured for the specific application can result in operational issues, such as off-spec quality, productivity issues, and safety issues. And then there is always the instrument failure. All of this results in a need for the loops important to safety, quality and correct operation of the facility be periodically calibrated.

Understanding and adhering to the following guidelines, explained in the sections that follow, is required to achieve the full benefits of the recommended approach to a calibration program set forth in this document.

4.1.2 Device CalibrationCalibration is commonly focused within industry on individual devices. Device calibration ensures that discrete components within the loop have been compared and adjusted, if necessary, to a reference standard. This is typically done before the component or instrument is installed and is used as an initial benchmark to ensure accuracy and then is periodically checked to ensure device accuracy.

4.1.3 Loop CalibrationA key concept of this recommended practice is establishing required tolerance and performing calibration of the entire loop, which includes multiple devices. Checking calibration is commonly focused within companies on individual devices. For example, a temperature transmitter could be calibrated as a single device but when installed with the sensor and indicator, the loop indication might not be within tolerance. However, the key parameter is whether the entire loop is providing a measurement within the tolerance needed for proper operation of the process. Each component in a loop has a rated accuracy. The inaccuracy or error of each component in the loop results in the total loop inaccuracy greater than that of any component.

The concept of loop calibration is, when the loop indication is not within the desired loop tolerance, the devices in the loop are adjusted as necessary to bring the loop indication within

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the desired tolerance. Device calibration simply adjusts a device to be within rated accuracy and does not reflect the accuracy of the entire loop.

Two other key concepts are (1) that the test equipment used to validate and calibrate a loop or component must be of sufficiently greater accuracy than that loop or component and (2) that persons performing this work must be properly trained in the process and use of the tools.

4.2 Calibration Program Planning

4.2.1 Calibration program developmentA calibration program should clearly establish and document:

Which devices are to have their calibration verified

What is the required tolerance for each loop to be verified

What is the calibration tolerance for each device to achieve the net required loop tolerance

What is the Loop Criticality Rating for each device

What is the time interval for each device to be verified

Which methods and equipment should be used for the verification and calibration for each device

What documentation is required to perform the verification and calibration tasks

What documentation is required to document the verification and calibration task results

What are the qualifications of staff members who will perform the verifications and calibrations

Who is responsible for the effectiveness of the calibration program

What are the requirements to establish and maintain verification and calibration equipment

Interval in which the calibration program should be reviewed and updated.

4.2.3 Calculating theoretical loop accuracyTheoretical loop accuracy is calculated by using the Root-Sum-Square (RSS) method, which combines the uncertainty of each component in the loop. Each of the listed effects on accuracy is squared and all squared effects are added together. The square root of this sum provides a combined uncertainty in either percentage or engineering units of the loop. Section 7 provides examples for calculating loop accuracy by using the manufacturers’ data and the RSS method.

4.2.4 Establishing Required Loop accuracyEach loop should be evaluated against and calibrated to a specified tolerance, which is the permissible deviation from the actual value or loop tolerance.

The tolerance for each loop may be established in either of two ways: (1) by establishing the required accuracy of the loop necessary to meet safety, quality, or production requirements of the process, or (2) by establishing the theoretical accuracy of the loop based on the rated accuracies of all of its components. In both methods, the theoretical accuracy of the loop must be

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calculated to demonstrate whether the loop is capable of meeting the required accuracy (in method 1) or to establish the target accuracy for the loop (in method 2).

Establishing an acceptable tolerance for each loop:

Clearly defines the level of acceptable performance for each loop

Provides a defined measure to use in the periodic check of loop performance

Facilitates tracking of loop performance over time

Focuses calibration efforts to the areas that provide the most benefits

Provides management with a measurement for auditing the calibration work (staff and equipment performance)

Clarifies loop performance expectations for operations and maintenance

This is obviously a very different approach to the more commonly encountered method of simply calibrating each device to its manufacturer’s accuracy specification, with no concern what the actual loop accuracy is or what loop accuracy is needed by the process.

4.2.5 Calibration equipment requirementsEquipment used to measure a loop’s tolerance or to calibrate a device must be:

Certified, typically by the manufacturer or a third party, at least annually, to be operating properly and within all manufacturers’ specifications. A sticker shall be affixed to the equipment documenting the most recent date of re-certification. For calibration equipment, the device’s calibration must be traceable to a national or other acceptable standard.

A factor of three times better than the specified tolerance of the loop being calibrated (for example, if the loop tolerance is 3%, the aggregate calibration equipment accuracy must be 1% or less.

Properly corrected for the effects of local conditions (e.g. ambient temperature, atmospheric pressure, etc.)

Adjustment control -Access to adjusting means and devices on confirmed measuring equipment, whose setting affects the performance, shall be sealed or otherwise safeguarded to prevent unauthorized changes. Seals or safeguards shall be designed and implemented such that tampering will be detected. The calibration process procedures shall include actions to be taken when seals or safeguards are found damaged, broken, bypassed or missing.

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4.2.6 Calibration personnel requirementsTo ensure the safety and reliability of the measurement and control system, only qualified, properly trained, personnel should be allowed to perform calibrations. Required areas of knowledge include:

Calibration

Loop checking

Troubleshooting

Instrumentation maintenance and repairs

Electrical safety

Operations and process safety

Certification of calibration personnel, either via a company’s internal training and certification program or via an independent third party certifier, are means to confirm the qualifications of the personnel involved in calibration. Personnel should be evaluated by a competent individual and supervisors should keep a register of qualified individuals by instrumentation type. It is important to understand that someone may be fully qualified to calibrate one type of instrumentation and not another.

4.2.7 Loop Performance Verification

All verifications and calibrations should be made using the “loop” calibration method.

4.2.8 Verification Intervals (loop criticality)Measurement and control systems can be divided into three general categories:

Critical systems: a system whose failure or misoperations may result in personal injury, equipment damage, on a negative impact on the environment.

Noncritical systems: a system whose failure or misoperations may degrade performance or operating capabilities.

Indication: a device or system used to monitor operating conditions

The verification frequency for a particular loop is a function of (a) the ruggedness of the device, (b) the performance history of the loop, and (c) the operating environment. Effective calibration programs base calibration intervals on these factors and on historical verification data for the loop. Some suggested starting points for verification frequencies of various loop criticalities are given below. These should be adjusted by the user with input from experience, historical performance information and/or Instrument Asset Management diagnostics.

Corporate primary pressure standards – 36 months

Corporate primary temperature standards – 36 months

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Corporate secondary standards (temperature and pressure) – 3 months

Field calibration devices (pressure, temperature, and electrical) – 12 months

Performance and compliance testing devices – before and after each use

Analytical devices – annual or as required by regulatory or operational requirements

Online performance monitoring devices – 12 Months

Critical systems – 18 months

Trips

Interlock

Control systems inputs

Non critical systems – 24 months

Online vibrations monitoring

Recorders, data acquisition systems, etc.

Indication – 36 months

Gauges

Indicators

Safety systems instrumentation - intervals for instrumentation involved in a Safety Instrumented Function will be the prooftest interval established for that SIF.

Records obtained using statistical data techniques for measurement frequency etc. can be useful in determining whether or not to modify calibration confirmation interval. Initial frequencies shall be set up as per the manufacturer’s recommendations provided that no data is available with regards tothe elements of duration and criticality.

4.2.9 ResponsibilitiesThe management of the plant should be responsible for establishing and owning the program and ensuring compliance with the automation instrumentation and system calibration program. This would include establishing a process for auditing compliance with, and updating , the process.

All aspects of the calibration program should be completely documented, including, but not limited to:

Calibration procedures (required method, equipment, personnel training/certification)

Required loop tolerances

Required forms

Verification and calibration record keeping system

Required verification frequencies

4.2.10 Records SystemThe documentation required for the calibration of measurement and control systems should comply with ANSI/ISA-84.00.01-2004 (IEC 61511 Modified), Functional Safety: Safety Instrumented Systems for the Process Industry Sector . These include a procedure, task list, and calibration data sheet.

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The verification procedure defines the overall steps necessary to check the performance of a type of loop. A task list provides specific steps necessary to perform the check, including equipment and methodology. A verification data sheet provides essential information on the particular loop and permits recording of the check results. An example of a verification data sheet is provided in the Annex.

The calibration procedure defines the overall steps necessary to calibrate a type of device (for example, a differential pressure transmitter). A task list provides the specific steps necessary to calibrate a specific loop. And, the calibration data sheet (CDS) provides essential information on the particular loop. This includes the calibration points with the acceptable loop tolerances. Using the CDS allows staff to capture the “as found” and “as left” data, which highlights how much drift has occurred in the loop since the last calibration. An example of a calibration data sheet is provided in the Annex.

Calibration confirmation is not achieved until and unless the fitness of the measuring equipment for the intended use has been demonstrated and documented. Calibration confirmation should include calibration and verification, any necessary adjustment or repair, and subsequent recalibration, comparison with the accuracy requirements for the intended use.

Software used in the calibration processes and calculations of results shall be documented, identified and controlled to ensure suitability for continued use. Calibration Software - any revisions to it, shall be tested and/or validated prior to initial use, approved for use, and archived.

5 Calibration Program Activities

5.1 Concepts

5.1.1 ImplementationGiven the number of loops in a typical industrial plant, fully implementing an automation instrumentation and calibration process is a major undertaking. For it to be successful, company management must be willing to provide support. To ensure consistency, a project team should be created to manage the project. The project will require support from the facility technicians, engineers and, most important, senior management.

5.1.2 Program scopeThe scope of an automation instrumentation and calibration program should include the following:

Develop instruments lists for each facility

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Assign a criticality rating to each device

Develop the process for each type of device

Determine the accuracy capability and requirements for each loop

Develop a task list for each loop

Develop calibration service records for each loop

Develop maintenance plans for each loop

5.2 Activities

Measuring actual loop accuracy (verification)

Calibration (in-situ and shop)

Documentation & Records

5.3 Functions

6 Calibration Program Management6.1 Control

Many third party software applications exist that can help automate your calibration program. Consideration should be given to providing a means to track progress, document calibrations, forecast calibrations required and trend results.

Include “due date tracking”

6.2 Assuranceadd discussion of audits (to ensure calibration / validation occurs on schedule)

6.3 Improvement

7 Examples Calculating theoretical loop accuracy examples

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Transmitter Accuracy 0.0750 0.0750System Accuracy 0.1000 0.1000Stability 0.0100 0.0100Ambient Temp Effect/oF 0.0036 0.0000 0.0036

Maximum Total Error 0.0886 0.1000 0.18860.0757 % 0.1000 % 0.1255 %2.27 PSI 3.00 PSI 3.76 PSI

Combined Uncertainties (±)

Transmitter Tolerance

Control System

Analog Input Card

Loop Combined Tolerance

Main Steam Pressure 0-3000 PSI

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Transmitter Accuracy 0.2500 0.2500System Accuracy 0.1000 0.1000Stability 0.1000 0.1000Ambient Temp Effect/oF 0.0100 0.0022 0.0122

Maximum Total Error 0.3600 0.1022 0.4622

0.2694 % 0.1000 % 0.2875 %6.74 PSI 2.50 PSI 7.19 PSI

Combined Uncertainties (±)

First Stage Pressure 0-2500 PSI

Transmitter Tolerance

Data Logger Analog Input

Card

Loop Combined Tolerance

Transmitter Accuracy 0.0750 0.0750System Accuracy 0.5000 0.5000Stability 0.0100 0.0100 0.0200Ambient Temp Effect/oF 0.0036 0.0100 0.0136

Maximum Total Error 0.0886 0.5200 0.6086

0.0757 % 0.5001 % 0.5062 %1.89 PSI 12.50 PSI 12.65 PSI

First Stage Pressure 0-2500 PSI

Transmitter Tolerance

Control System

Analog Input Card

Loop Combined Tolerance

Combined Uncertainties (±)

Switch Repeatability 3.0000Temperature Effect per oF 0.0200Stability % per Year 0.0000

Maximum Total Error 3.0200

3.0001 %90.00 PSI

Combined Uncertainties (±)

Turbine EH System Pressure 200-3000 PSI

Switch Tolerance

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Gauge Accuracy 2.0000Temperature Effect per oF 0.1000Stability % per Year 1.0000

Maximum Total Error 3.1000

2.2383 %67.15 PSI

Boiler Feed Pump Turbine Pressure 0-3000 PSI

Combined Uncertainties (±)

Gauge Tolerance

These examples highlight the large differences in the uncertainties from system to system and from device to device. Far less accuracy is required to calibrate gauge (combined uncertainties 2.238%) than is required to calibrate loops on the inputs to a control system (combined uncertainties 0.1255%). It is important to note that there are actually more identifiable factors to uncertainty in the systems. However, none of these factors is significant enough to make a recognizable increase in the uncertainty of the loop.

Implementation example

o In 1995, the natural gas supplier started providing their technicians loop tolerances for all of the utility power plants. Shown below is March 2006 audit of the power plant meter run number one.

o Providing acceptable tolerance for each loop was a new concept for the natural gas supplier. Over time, the company realized the following benefits of this practice:

o Clearly defined levels of acceptable performance for each loop

o Focuses calibration efforts to the areas that provide the most benefits

o Provides management with a measurement for auditing the calibration work (staff and equipment performance)

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Loop/instrument type examples

Calibration documentation forms

7.1

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Annex A – Example Documentation

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Calibration Service Record

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Developing and promulgating sound consensus standards, recommended practices, and technical reports is one of ISA’s primary goals. To achieve this goal the Standards and Practices Department relies on the technical expertise and efforts of volunteer committee members, chairmen and reviewers.

ISA is an American National Standards Institute (ANSI) accredited organization. ISA administers United States Technical Advisory Groups (USTAGs) and provides secretariat support for International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO) committees that develop process measurement and control standards. To obtain additional information on the Society’s standards program, please write:

ISAAttn: Standards Department67 Alexander DriveP.O. Box 12277Research Triangle Park, NC 27709

ISBN: