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Instrumentation Calibration, Design and Techniques
Instrumentation and instrument What is instrumentation? In general definition this can be defined as the art and science of measurement and/or control. Is achieved by using an “instrument” .
Instrumentation based on industrial application: “It is the application of instrument for the purpose of measuring, observing, transmitting, indicating, recording, monitoring, and controlling any industrial process variable”.
What is an instrument? Is any device used directly or indirectly in order to accomplish an objective or task. In Instrumentation, an instrument is any sensing, measuring, transmitting, indicating, or controlling device associated with a process or system. Ex. Measuring a body temperature using a thermometer.
Instrument application categories and functional divisions. Factory automation instruments Plant safety or safeguarding instruments Product Quality monitoring/control instruments Environmental condition monitoring /control instruments. Process variable measurement and control instruments.
Implementing instrumentation • How is instrumentation implemented? 1. Single or Stand alone System. 2. Complex System
Instrumentation system Instrumentation system - is an arrangement of two or more instruments connected together to perform a unified task. Each instrument operates independently according to its specific task. Failure from one member of instrument, means failure of the entire instrument system. Simplest form of instrumentation
Automation Is a system concept that utilizes instrumentation system to perform a certain task or sequences of operations in an automatic manner or without human intervention. Both maximizing quantity of production and quality and durability of produced goods is greatly improved.
AUTOMATED PROCESS Is a process or sequence of production activities done in an automatic manner. TYPES OF AUTOMATED PROCESS Highly Mechanized Process Chemical and Physical Process
Maintenance
What is Maintenance? All actions necessary for retaining an item, or restoring to it, a serviceable condition, include servicing, repair, modification, overhaul, inspection and condition verification. Keep system’s equipment in working order. To repair the equipment after FAILURE.
Question? Why do we need maintenance? What are the costs of doing
maintenance? What are the costs of not doing maintenance? What are the benefits of maintenance? How can maintenance increase profitability of company?
Purpose of Maintenance • Attempt to maximize performance of production equipment efficiently and regularly • Prevent breakdown or failures • Minimize production loss from failures • Increase reliability of the operating systems
Principle Objectives in Maintenance • To achieve product quality and customer satisfaction through adjusted and serviced equipment • Maximize useful life of equipment • Keep equipment safe and prevent safety hazards • Minimize frequency and severity of interruptions • Maximize production capacity – through high utilization of facility
Maintenance Objectives • Must be consistent with the goals of production (cost, quality, delivery, safety) • Must be comprehensive and include specific responsibilities
Maintenance Costs • Cost to replace or repair • Losses of output • Delayed shipment • Scrap and rework
Failure Failure – inability to produce work in
appropriate manner Equipment / machine failure on production floor – worn out bearing, pump, pressure leaks, broken shaft, overheated machine etc. Equipment failure in office – failure of power supply, air-conditioned system, computer network, photocopy machine Vehicle failure – brake, transmission, engine, cooling system
Types of Failure
Types of Failure Functional Failure
the inability to meet the specified performance standard Potential Failure a physical condition which indicates that the failure process has started Hidden Failure Failure is not apparent until the function is attempted
Current Maintenance Strategies Fix it when it fails or run until
failure Time based (calendar time or running time) Condition based
Types of Modern Maintenance
Types of Maintenance • Maintenance may be classified into four categories: • (some authors prefer three categoriesscheduled and preventive maintenances are merged) • • • •
Corrective or Breakdown maintenance Scheduled maintenance Preventive maintenance Predictive (Condition-based) maintenance
Corrective or Breakdown Maintenance • Corrective or Breakdown maintenance implies that repairs are made after the equipment is failed and can not perform its normal function anymore • Quite justified in small factories where: Down times are non-critical and repair costs are less than other type of maintenance Financial justification for scheduling are not felt
Disadvantages of Corrective Maintenance
• Breakdown generally occurs inappropriate times leading to poor and hurried maintenance • Excessive delay in production & reduces output • Faster plant deterioration • Increases chances of accidents and less safety for both workers and machines • More spoilt materials • Direct loss of profit • Can not be employed for equipments regulated by statutory provisions e.g. cranes, lift and hoists etc
Scheduled Maintenance • Scheduled maintenance is a stitch-in-time procedure and incorporates inspection lubrication repair and overhaul of equipments
• If neglected can result in breakdown • Generally followed for: overhauling of machines changing of heavy equipment oils cleaning of water and other tanks etc.
Preventive Maintenance (PM) Principle – “Prevention is better than cure” Procedure - Stitch-in-time It
• locates weak spots of machinery and equipments • provides them periodic/scheduled inspections and minor repairs to reduce the danger of unanticipated breakdowns
Advantages of PM • Advantages: –Reduces break down and thereby down time –Lass odd-time repair and reduces over time of crews –Greater safety of workers –Lower maintenance and repair costs –Less stand-by equipments and spare parts –Better product quality and fewer reworks and scraps –Increases plant life –Increases chances to get production incentive bonus
Predictive (Condition-based) Maintenance
• In predictive maintenance, machinery conditions are periodically monitored and this enables the maintenance crews to take timely actions, such as machine adjustment, repair or overhaul • It makes use of human sense and other sensitive instruments, such as –audio gauge, vibration analyzer, amplitude meter, pressure, temperature and resistance strain gauges etc.
Predictive Maintenance (Contd.) • Unusual sounds coming out of a rotating equipment predicts a trouble • An excessively hot electric cable predicts a trouble • Simple hand touch can point out many unusual equipment conditions and thus predicts a trouble
Effective Instrumentation Maintenance Approach Locating the real cause of a problem can be the most difficult part of the troubleshooting process. But taking a logical approach helps ensure a successful result.
Factors that Could Influence the Effectiveness of an Instrumentation System Maintenance: Familiarity of the process Proper understanding of the problem Proper evaluation of visible symptoms Knowledge in the application of different Measurement, Process Control and Maintenance Fundamentals Knowledge in the proper use of hand tools and equipment Use of Systematic Maintenance Approach
Understand properly the extent of the problem based on given facts, data and Effective Maintenance Approach: symptoms.
1.
2.
Start troubleshooting by first using Elimination by Deduction method. If the cause of the problem is highly identified, perform corrective action at once to solve the problem.
3.
Continue troubleshooting by applying Elimination by Input / Output Test method.
4.
Apply Root Cause Analysis (RCA).
Elimination by “Deductive Approach” Troubleshooting by eliminating one component from the other component in a loop by deduction or logical thinking method.
Elimination by Deductive Approach 1.
Understand properly the extent of the problem based on given facts, data and symptoms.
2.
Start troubleshooting by first using Elimination by Deduction method. If the cause of the problem is highly identified, perform corrective action at once to solve the problem.
3.
Continue troubleshooting by applying Elimination by Input / Output Test method.
4.
Apply Root Cause Analysis (RCA).
Guidelines in using Deductive Approach: 1.Analyze
the extent of the problem based on given facts or symptoms.
2.Come
up with a probability per element based on given facts and decide which element most likely to cause the problem.
3.Rectify 4.Apply
problem if already possible.
Root Cause Analysis (RCA).
Elimination by “Input/Output Test” or “Cause & Effect Method” Troubleshooting by applying an input and monitoring the output per loop component based on element’s input/output relationship table.
Guidelines in using Input/Output Test: 1. 2.
3. 4.
Established the details of each loop component of a given control loop. Using the degree of probability based on the result of your “Deductive Approach”, perform “Input/Output” Test. Rectify problem encountered while doing input/output test. Apply Root Cause Analysis (RCA).
Significant use of “Input/Output Test” or “Cause & Effect” Input/Output Test if properly administered is a very effective tool in identifying equipment functional, potential and hidden failures. Any failure identified during the test could trigger appropriate maintenance action/s.
Applying Root Cause Analysis (RCA)
What is a “Root Cause Analysis” It is a systematic approach to maintenance problem analysis. It emphasize mainly on the main cause or root cause of the problem not just the temporary solution. This concept could be well implemented by considering the ff questions: Is the problem clearly identified and understood based on given symptoms? Is the corrective action done really corrects the problem?
Other factors need to be considered in troubleshooting instrumentation system problem: • Loop configuration / system integrity. • Instrument type, installation, calibration & physical conditions. • Environmental conditions.
In general, the following simple guide questions will help an Instrument Technician perform effective maintenance: • What is the problem? • What do we think caused the problem? • What evidence do we have about the causes? • What solution(s) do you have in mind? • How will the solution(s) eliminate the cause of the problem?
Correcting Instrument Output response: What do you think will be your courses of actions if after doing an input/output test, the actual measured values are significantly different from the desired values?
Do Adjustment
Why Instrument Calibration is Necessary? The successful operation of any automated industrial process depends on the accuracy and performance of each instrument in the measurement and control loop. Instrument calibration helps to ensure that a process operates efficiently and safely within plant specifications and produces a product of optimum quality.
Example An Instrument technician is conducting an Input/Output test of an I/P converter shown in the figure. Input is 4-20 mA from TRCA and output is 3-15 psi. The resulting As Found I/O Table is shown below:
Findings Based on the As Found: I/O test Table below, the I/P shows an error of 0.2psi in every test point. In-order to eliminate the error, the instrument sensitivity was adjusted. After adjustment, another Input/Output test was conducted, and the result is shown on the I/O Table on the right. This time the error in psi per test point is 0. the process of adjusting the error is what is called CALIBRATION. The Table that contains the data after calibration As Left: I/O Test As Left: I/O Test Table As Found: is I/O called Test Table Table.
Calibration
Calibration “Calibration is an insurance policy that verifies the accuracy of test instruments.”
Calibration is the act of checking and verifying the accuracy of a measurement instrument by comparison with a reference standard. Properly calibrated instruments perform to manufacturers’ published specifications. Regularly calibrating measurement instruments ensures the accuracy of measurements that are relied upon during design and manufacturing test.
Why Calibrate? While most instruments that are evaluated and calibrated normally pass the test, instrument performance can change over time. There are several factors that can contribute to this change including drift, normal wear and tear, lack of proper maintenance, user error, and improper use and abuse of equipment. Regular calibration ensures that test and measurement instruments are operating at a known performance level.
Why is Calibration Required? By doing proper calibration procedure and through proper interpretation of the calibration results, instrument error/s can be identified and be corrected.
Calibration is required by law.
Instrument Calibration Cycles
Calibration is not a one-time occurrence. Instruments must be calibrated periodically to ensure specified performance. Each instrument requires a specific interval between calibrations. This interval is determined by the instrument’s owner and is often based on the manufacturer’s recommendations. The original equipment manufacturers’ (OEM) calibration intervals are typically based on conservative performance for the average user. For best results, the instrument owner should use several additional factors in determining the optimal calibration interval, including: The required accuracy for the application vs. the instrument’s specified accuracy. The business impact of using OOT
Calibration Quality System The International Organization for Standardization (ISO) is comprised of representatives from various national organizations and has 162 member countries. ISO develops standards for industry and trade. Partnering with ISO registered calibration providers ensures that the provider follows standard practices. ISO 9000 is a family of standards that provide a framework for managing an organization's processes and a set of standardized requirements for a quality management system. ISO/IEC17025 is a standard used by testing and calibration laboratories. Laboratories implement the requirements of ISO/IEC17025 to provide assurance
Calibration according to Legal Metrology Calibration according to R.A. 9236 of 2003 (REPUBLIC ACT NO. 9236 THE NATIONAL METROLOGY ACT OF 2003), is a set of operations establishing under specified condition, relationship between values indicated by a measuring instrument or measuring system, or values represented by material measure, and its corresponding known values of measure.
Instrument Calibration Block Diagram Input Measurement Standard (IMS)
Unit Under Test (UUT)
Output Measurement Standard (OMS)
b-a=c Note: IMS & OMS are commonly known as CALIBRATORS.
When is Calibration Required? 1.Over a period of time 2.Change in process parameters 3.Change in environmental conditions 4.Change in instrument mounting position 5.Before installation of new instrument 6.After any instrument repair 7.When process verification is required 8.Governmental Regulation (i.e.RA 9236) 9.Other reasons deemed necessary
How often instrument is calibrated?
By practice, the frequency of calibration depends upon the classification of the instruments: Critical: An instrument which, if not conforming to specification, could potentially compromise product or process quality and safety. (Typical is twice yearly) Non-critical: An instrument whose function is not critical to product or process quality, but whose function is more of an operational significance. (Typical is yearly) Reference Only: An instrument whose function is not critical to product quality, not
Cost and Risk of Not Calibrating
Calibration can be easily ignored or cycles extended beyond their recommended time frame, which may increase operational risk or regulatory compliance. Neglecting routine calibration schedules can lead to quality and regulatory issues, increasing downtime, and increase expenses. If a company is unable to meet its customer or regulatory requirements, they introduce significant risk of business interruption, loss of operating privileges, or compromised public safety. When compared with the significant business risks associated with non-compliance, calibration
Types of Calibration
Workshop, Laboratory or Bench Calibration Calibration utilizing deal conditions such as room temperature, humidity, room pressure, vibration & etc. A Bench Calibration is performed in the shop on the bench with power supplied from an external source. It may be performed upon receipts of new instruments prior to installation. This provides an assurance that the instrument received is undamaged. This also allows configuration and calibration in a favourable environment. Advantages Disadvantages 1. Instrument is removed, cleaned and inspected.
1. Problem may encounter during pullout and installation.
2. Calibration done in an ideal conditions. 2. Zero adjustment usually required after installation to compensate for field 3. Fixed calibration set-up and utilities. ambient operating conditions.
Typical Bench Calibration Set-up
Standards or Calibrators
Field Calibration Calibration utilizing actual field conditions such as field ambient temperature, barometric pressure, vibration, utilities, position & etc. Field Calibrations are performed “in-situ” or in-place, as installed. The instrument being calibrated is not removed from the installed location. Field calibration may be performed after installation to ensure proper connections and configuration. Periodic calibrations are more likelyAdvantages to be performed in the field. Disadvantages 1. May save calibration time. 2. May identify and allow troubleshooting of installation problems. 3. Done in actual field ambient operating conditions.
1. Loop elements performance/condition may not be individually checked.
Field Calibration
Characteristics of Calibration 1. Compliance to the Required Accuracy Ratio of Standards 2. Traceability of Calibration Standards 3. Uncertainty of Measurements 4. Compliance to ISO-17025
1. Accuracy Ratio of Standards This term describe the relationship between the accuracy of the calibration standard and the accuracy of the UUT. A good rule of thumb is to ensure an accuracy ratio of 4:1. This means that the accuracy of the calibration standard is four times better than the accuracy of the UUT.
Calibration Standards
The Importance of Calibration Standards (Calibrators): To determine whether a measurement is accurate and precise, it must be compared to a known STANDARD. A measurement standard is one that has been established as a model. Instruments that are used as measurements standards (Calibrators) are calibrated according to internationally accepted standards (Primary Std.). These certified standard instruments are then used to calibrate test equipment (Secondary Std.). Test equipment is, in turn, used to calibrate process instruments.
Instrument Calibration Block Diagram Input Measurement Standard (IMS)
Unit Under Test (UUT)
a = 50%
Output Measurement Standard (OMS)
b = 50.1% c = 0.1%
b-a=c Standards or Calibrators
What is Calibration Standard? Calibration Standard is an internationally accepted and traceable instrument or material used as reference in calibrating instruments.
Classification of Calibration Standard 1. Primary Reference Standard or Material -. Directly traceable to international standards. -. A standard which has highest metrological quality in a specified field. 2. Secondary or Certified Reference Standard or Material -. Traceable only to manufacturer’s reference standards. -. One which value is fixed by
2. Traceability of Standards All calibrations should be performed traceable to a nationally or internationally recognized standard. Traceability is defined by ANSI/NCSL Z540-1-1994 as “the property of a result of a measurement whereby it can be related to appropriate standards, through an unbroken chain of comparisons.”
Calibration Traceability
Hierarchy of Calibration Standards & Traceability (ISA)
Traceability – the property of a result of a measurement relating to appropriate standards, generally national or international through an unbroken chain of comparison.
In the Philippines, calibration is legally supported under R.A. 9236 – “the National Metrology Act of 2003”. It is an act establishing a National Measurement Infrastructure System (NMIS) for standards and measurements, and for other purposes.
National Metrology Laboratory of the Philippines (NMLPHIL)
3. Uncertainty of measurements Uncertainty analysis 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.
Measurement Uncertainty
Why Measure? The objective of a measurement is to determine the value of the measurand or the value of the particular quantity to be measured.
Measurement Errors, effects and corrections: In general, measurement has imperfections that give rise to an error in the measurement result. Commonly, an error is classified into three types, namely: a random error, systematic error and spurious error.
Random Error (Accuracy Random errorsError) are unavoidable errors, which are introduced into the measurement process at random or by chance. The effects of such variations known as random effects, give rise to variations in repeated observations of the measurand. Although it is not possible to compensate for the random error of measurement result, it can usually reduced by increasing the number of observations.
Systematic Error (Bias or Precision Error) Systematic error, like random error, can’t be eliminated but it too can often be reduced. If the influence of systematic error known as systematic effect can be quantified and if it is significant in size relative to the required accuracy of the measurement, correction factor or bias can be applied. After the correction, the expected value of the error arising from the systematic effect is zero.
Spurious Error Spurious errors are error, such as human mistakes or instrument malfunction, which invalidate a measurement. Such errors can’t be treated with statistical analysis and the measurement should be discarded.
Measurement and Measurand 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 the uncertainty of that estimate.
What is UNCERTAINTY? Based on definition under 2.2 of the ISO Guide to the Expression of Uncertainty in Measurement (GUM), the word “uncertainty” means DOUBT, and thus in its broadest sense “uncertainty of measurement” means doubt about the validity of the result of a measurement.
Measurement Uncertainty Concept
Measured Value = 99.89 mBar + Uncertainty of measurement
Common Sources of Uncertainty Environmental conditions Personal bias in reading values Finite instrument resolution Calibration of standards Rounding of measurement Methods & procedures of measurement Stability of power supply and etc.
Ishikawa (fishbone diagram): Environment
Equipment
Temperature
Man
Accuracy
Pressure Vibration & others
Bias
Resolution
Error
Stability & others
Direct or Inferred Others
Measurement method & procedure
Others
Connection & wire resistance Stability of
utilities Others
Measurement system & utilities
Uncertainty can be expressed in terms of the following: 1. 2. 3.
Standard Uncertainty: ui Combined Uncertainty: uc Expanded Uncertainty: U = uc (k)
Methods of Evaluating Standard Uncertainty ui Components: 1. Type A Evaluation (of uncertainty) – is the method of evaluating uncertainty by the statistical analysis of a series of observations. In this case, the standard uncertainty is the experimental standard deviation of the mean that follows from an averaging procedure.
2. Type B Evaluation (of uncertainty) – method of evaluation of uncertainty by means other than the statistical analysis of series of observations. In this case the evaluation of the standard uncertainty is based on some pool of information such as: •
previous measurement data;
•
experience with or general knowledge of the behavior and properties of relevant materials and instrument
•
manufacturer’s specifications
•
data provided in calibration and other certificates
•
other relevant information
Sample: Calculating Standard Uncertainty (ui) of Flowmeter and Proving Tank in terms of Standard Deviation (STDV): A 10-validation runs were conducted on a flowmeter calibration system using 3000 batch size. Results were tabulated and STDV of the flowmeter and the proving tank were calculated. Results shown on the left table. ui of Flowmeter is 3.13 ui of Proving tank is 1.05
The Combined Standard Uncertainty (uc):
The combined standard uncertainty of a measurement result, suggested symbol uc, is taken to represent the estimated standard deviation of the result. It is obtained by combining the individual standard uncertainties ui, whether arising from Type A or a Type B evaluation, using the usual method for combining standard deviations.
Calculating the Combine Standard Uncertainty (uc) of Flowmeter and Proving tank: ui of Flowmeter is 3.13 and ui of Proving Tank is 1.05
Combine Uncertainty (uc)
=
(ui of Flowmeter)² + (ui of Proving Tank )² (3.13)² + (1.05)²
Combine Uncertainty (uc) = 3.3
Expanded Uncertainty (U):
A quantity defining an interval about the result of a measurement that may be expected to encompass a large fraction of the distribution of values that could reasonably be attributed to the measurand. The expanded uncertainty denoted by U is obtained by multiplying the combined standard uncertainty uc by a coverage factor k. Thus; U=uc(k) TRUE Measurement = Observed measurement +/-U
Calculating the Expanded Uncertainty (U) of Flowmeter and Proving tank: Expanded Uncertainty (U)
= uc (k)
= 3.3 (2) = +/- 6.6 Where: uc = Combine standard uncertainty k = Coverage Factor
Statement of Uncertainty of Measurement in Certificates In calibration certificates , the complete result of the measurement consisting of the estimate y of the measurand and the associated expanded uncertainty U shall be given in the form (y +/- U). To this an explanatory note must be added The reported expanded uncertainty of measurement is stated which in the general case should have the as the standard uncertainty of measurement multiplied by the follow: coverage factor k=2, which for a normal distribution corresponds to a coverage probability of approximately 95% level of confidence.
Level of Confidence: Most of expanded uncertainty calculations are based on coverage factor (k=2) and confidence level of 95% (1 chance in 20 that the value of the measurand lies outside the interval).
4. Compliance to ISO-17025 Calibration technical requirements per PNS ISO/EIC 17025:2000 Human Factors (Personnel) Environmental Conditions Test & Calibration Methods and Method Validation Test/calibration equipment Traceability Handling of test and calibration items
Overall benefits of having a regular instrument calibration:
Enhances production efficiency Enhances product quality assurance Increases plant safety Reduction in production cost Improves profit margin
Instrument Range and Span
Range and Span To fully understand the concept of calibration, it is essential to understand the range and span of an instrument. Range is the set of values (LRV & URV) over which a measurement can be made without changing the instrument’s sensitivity. Span is the distance (or difference) between the upper range vale (URV) and lower range value (LRV).
Difference between instrument Measuring Range and Calibration Range Measuring Range – refers instrument measuring capability
to
Calibration Range – refers to range the instrument is calibrated to produce a scaled output.
Instrument Accuracy, Precision and Gain
Accuracy in Calibration Instruments are calibrated to make them accurate within manufacturer’s specifications. Accurate calibration therefore is an essential factor in instrument performance.
Ways of Determining Instrument Accuracy: Manufacturer’s specifications 2. By calculations (Calculated) 1.
1. As a percent of output span
Example: A pressure transmitter has an output span of 50 psi. It measures an actual tank pressure of 25 psig but reads 26 psi. In this case, the transmitter is accurate within 1 psi or 2% of span. Accuracy (%) = MV – TV/Span x 100
Example of accuracy calculation (% of span):
2. As a percent of measured value (MV) Example: A pressure transmitter has an output span of 50 psi. It measures an actual tank pressure of 25 psig but reads 26 psi. In this case, the transmitter accuracy is 4% of measured value. Accuracy (%) = MV – TV/MV x 100
Example of accuracy calculation (% of span):
What is an Accurate Instrument? An accurate instrument is an instrument in which the output accuracy always falls within manufacturer’s specifications every time an input is applied.
Sample: Accurate Instrument Manufacturer’s accuracy statement is +/- 0.5% of span
Control Chart (21-PT-020) FR Cement Corporation
Meaning and Importance of Instrument Precision Precision is another important factor in instrument performance. A precise instrument will produce the same output every time it receives an identical input. A transmitter that produces the same output signal from a constant input is precise.
Sample: Precise Instrument Manufacturer’s accuracy statement is +/- 0.5% of span
Control Chart (21-PT-020) FR Cement Corporation
Sample: Precise Instrument Manufacturer’s accuracy statement is +/- 0.5% of span
Control Chart (21-PT-020) FR Cement Corporation
Meaning and Importance of Gain in Calibration The level of accuracy to which an instrument can be calibrated is partially dependent on another factor known as Gain. Gain refers to the amount of output change for each increment of input change. It is a key factor in determining how accurately an instrument can be calibrated.
Gain Calculation Transmitter Gain is calculated by dividing the output span by the input span.
Sample Gain Calculations
Calibration Procedures
Calibration Procedures Calibration procedure refers to the way or manner on how calibration is carried out in relation to instrument’s input/output relationship. It could be either a 5point or 10-point input/output relationship.
5-point Calibration Procedures A calibration procedure which utilizes a 5 input and 5 output test points. This is the most widely used calibration procedure. Test points commonly used are 0, 25, 50, 75 and 100% of the input and output span.
5-point Input and Output relationship Table Example: Direct Acting Electronic Temperature Controller with calibration range of 100 – 500 °C
10-point Calibration Procedures A calibration procedure which utilizes a 10 input and 10 output test points. This is the most widely used calibration procedure. Test points commonly used are 0, 25, 50, 75, 100, 75, 50, 25 and 0% of the input and output span. This procedure is used to determine instrument error known as hysteresis.
10-point Input and Output relationship Table Example: Direct Acting Electronic Temperature Controller with calibration range of 100 – 500 °C
Basic Steps in Calibrating an Instrument: 1. Identify the type of UUT to be calibrated and record all necessary information required for the calibration job. 2. Identify and prepare the appropriate IMS, OMS and Utilities required for the calibration job. 3. Set-up the calibration system. 4. Calibrate UUT reference to UUTs’ maintenance manual or users’ established Work Instruction. 5. Evaluate/correct instrument error. 6. Do housekeeping. 7. Finalize Calibration Certificate.
Typical Bench Calibration Set-up
Types of Calibration Error
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 to 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.
Types of Calibration Error 1.
Linear Errors
Zero Shift
Span Error
Zero and Span Error
2.
Non – linear Error
3.
Hysteresis
Zero Shift (Linear) A zero shift refers to a situation in which an instrument signal output is consistently higher or lower than would be expected throughout the input span. It can also be described as a situation where the instrument is outputting consistently with inputs provided but starts at a point too high or too low on the output scale.
Span Error (Linear) Span error is another type of instrument error. The readings for an instrument with span error either do not represent 100% of the output span or the output span does not match the input span.
Zero Shift and Span Error Combination (Linear) Both zero shift and span error can occur in the same instrument. In such case, an input/output graph produces a signal line that agrees with neither the origin nor the angle of the ideal line.
Non – linear Error Non-linearity is a condition in which an instrument outputs signals that do not match inputs between upper and lower limits of the span. Severe nonlinearity is not a simple adjustment problem and may require instrument repair. And if the magnitude of the nonlinear error is unacceptable and it cannot be adjusted, the instrument must be replaced.
Non – linear Error
Hysteresis Hysteresis is another common instrument problem. In this case, instrument produces different signals depending upon the direction of the input procedure. Moving up or down through the input range produces different output signals.
Hysteresis
Hysteresis
The Calibration Certificate
Calibration Certificate/Report must at least contain the following elements as per 5.10.2 of PNS ISO/IEC 17025-2000. • A title • Name and address of the laboratory where calibration was carried out. • Certificate Identification • Name and address of client • Identification of the method being used. • Unit Identification
• Date Unit received & calibrated • Traceability, Uncertainty & Environmental Conditions • Test results & units of measurement • Findings & observations • Statement of the effect of the results relate only to item calibrated. • Signatures authorizing the certificate
In addition, calibration certificates shall include the following, where necessary for the interpretation of calibration results: 1. When an instrument for calibration has been adjusted or repaired, the calibration results before and after adjustment shall be reported. 2. A calibration certificate or label shall not contain any recommendation on the calibration interval except agreed with the client.
3. When a calibration work has been contracted, the laboratory performing the work shall issue the calibration certificate to the contracting laboratory. 4. The format of the calibration certificate shall be designed to accommodate data and to minimize the possibility of misunderstanding. 5. When it is necessary to issue a complete new calibration certificate, this shall be uniquely identified and shall contain in a reference to the original that it replaces. 6. Calibration certificates are part of the controlled documents in a company’s
Important Notes Regarding Certificates: 1. Hard copies of calibration certificates should also include the page number and total number of pages. 2. It is highly recommended that a statement specifying that the test report or calibration certificates shall not be reproduced except in full, without written approval from the issuing laboratory. 3. Calibration certificates must be controlled and considered legal documents.
Introduction to Validation/Verification Concept
What is Validation/Verification? Validation/verification – is the process of simulating an instrument with a known input and comparing the result to a Calibration Tolerance. If the difference is within the specified tolerance, no action shall be taken. However, if not, Calibration must be performed.
Characteristics of a Validation/Verification
1. Compliance to the Required Accuracy Ratio of Standards 2. Traceability of Calibration Standards 3. Uncertainty of Measurements 4. Compliance to ISO-17025 5. Acceptable Tolerance
5. Acceptable Tolerance Every Validation/Verification should be performed to a specified tolerance. The terms tolerance and accuracy are often used incorrectly. The definitions for each are as follows: Accuracy: The ratio of the error to the full scale output, expressed in % of span or the ratio of the error to the output, expressed in % reading. Tolerance: Permissible deviation from a specified value. Maybe expressed in measurement units, % of span or % of
Validation Tolerance should not be based on manufacturer’s accuracy statement only. It should include also the following: • Requirements of the process • Capability of available test equipment • Consistency with similar instruments at your facility.
Validation/Verification Block Diagram
Calibrate
y Is the diff. more than the Cal. Tolerance?
n
Quit
c a
b-a=c
b
Validation Acceptance Criteria: •Error is within the given calibration tolerance. •Types and magnitude of error indicated by instrument is acceptable based on application. •Measurement uncertainty is known/defined. •Other relevant criteria specified by process application.
5-point Validation/Verification Curve Acceptable Calibration Tolerance (+/- 0.2% of Span)
Measured Value
Quality Management System and Instrument Calibration
Quality Management System
Quality Management System is a part of the organization’s management system that focuses on the achievement of results, in relation to the product quality objectives.
QMS and Instrument Calibration
Since an accurately calibrated instrument contributes not only to the safety aspects of any automated industrial process but also much on efficiency and product quality, most of plant’s Quality Management System (QMS) deals calibration issue as one of its main concern. ISO-9000 QMS standard specifically item 7.6 thereof requires regular testing and calibration of any process instruments, which during malfunction could affects product quality.
Instrument calibration Standards shall be all in accordance with PNS ISO/IEC 17025:2000 “General requirements for the competence of testing and calibration laboratories.”