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Test and Calibration Instrumentation

Instrument Calibration must be carried out to maintain and verify instrument accuracy. Test and Calibration instruments are available that cover all different instruments which measure typically Flow, Pressure, Temperature etc. They are also used for setting up final elements such as control valves. Test and Calibration instruments are available in portable versions for site calibration or bench versions for the workshop.   Calibration is the process of configuring an instrument to provide a result for a sample within an acceptable range. Eliminating or minimising factors that cause inaccurate measurements is a fundamental aspect of instrumentation design.

There are as many definitions of calibration as there are methods. According to ISA’s The Automation, Systems, and Instrumentation Dictionary, the word calibration is defined as “a test during which known values of measurement are applied to the transducer and corresponding output readings are recorded under specified conditions.” The definition includes the capability to adjust the instrument to zero and to set the desired span. Usually Calibration involves injecting an accurate signal from the calibrator into the instrument at 0%, 25%, 50%, 75% and 100% of range and adjusting the instrument zero and span to a point where the instrument is aligned with the calibrator for the specified range.

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Test and Calibration Instrumentation 

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Low Pressure Calibrations - When you're performing a low pressure calibration, there are some things you can do to make the job easier and better (more accurate) - From Martel Matters Newsletter.
Temperature Calibrations (Thermocouple Edition) -There are a few "gotchas" in temperature calibration when we're talking about T/Cs and RTDs. Each has its own problems, This article will focus on calibration of T/C instrumentation - From Martel Matters Newsletter.
Field Calibrators Make Everything Better - Users Are Opening Their Eyes to How Much Clearer Their Data Is - and How Much Better Their Processes Can Run - with Field Calibrators - From www.controlglobal.com 
Martel Process Calibrators Blog - Lots of Interesting Calibration Related posts.
Who Do You Trust? - Can you Trust your Vendor’s Calibration Certificate?
Martel Calibrator Newsletter - Martel Matters is an e-newsletter from Martel Calibrators. It provides information on new and existing products and calibration "tips". Highlights from the Latest Newsletter include;
- Tips on performing Low Pressure Calibration
- Getting Rid Of Leaks
- Adding Volume To Your System
- Watching Out For Temperature Effects
- Considering Head Pressure Errors
“How to” Choose the Right batteries for your Calibrator - Most of our calibrators are designed to use standard replaceable Alkaline batteries. You know, the ones you can buy at the corner store. That’s a good choice because of the relatively high power density and mostly flat discharge curve of these cells. They start out with a high terminal voltage (1.5 V  or more). A really strong point for these batteries is the long shelf life. If you don’t use your calibrator much, that would be a good reason to use Alkalines. New ones typically have a shelf life of 3 years or more.
Is It a Calibrator? - People actually in the calibration industry often hear things being called calibrators that obviously aren’t. The defining points for a calibrator are detailed here. 

Calibration Forms

1.13 The Following are from the very useful InstrumentationPortal.Com

Calibration Form - Calibration forms provide a  list of actions to be done when performing instrument calibration. Instrument Calibration is required to make sure that instrument will function properly prior to installation. Before shipping, vendor has already done the calibration after setting the range to pre-determined value as requested by the end-user. It is common by contractor to re-check the instrument by performing bench calibration. However, some end-users prefer to install the instrument without undertaking this test. Following are some typical calibration forms for transmitters, gauges and control valves.
Transmitter Calibration Form
Control Valve Calibration Form
Pressure Gauge Calibration Form

Calibration Principles

Calibration Principles - Details in this excellent chapter from the ISA on how to define key terms relating to calibration and interpret the meaning of each. Understand traceability requirements and how they are maintained. It describes the following;

• The characteristics of a good control system technician

• Differences between bench calibration and field calibration

• The differences between loop calibration and individual instrument calibration, listing the advantages and disadvantages of each.

• List the advantages and disadvantages of classifying instruments according to process importance—for example, critical, non-critical, reference only, OSHA, EPA, etc.

Flow and Level Calibration Notes -Thanks to INX Inc - These notes whilst being a little dated are still very useful.
Pressure & Temperature Calibration Notes- Thanks to INX Inc- These notes whilst being a little dated are still very useful.
Calibration Primer - From Omega.com - The most sophisticated industrial equipment will not be very useful unless it is calibrated. Through calibration, adjustments made to a piece of equipment ensure that it performs as expected—that it can be relied on to deliver predictable, accurate results that meet quality standards. This white paper from Omega Engineering explains what calibration is, why it is important, and how it works. NIST traceability is defined and discussed, and there is a step-by-step description of a basic calibration. This paper also discusses in-house vs. laboratory calibration, and it describes major types of calibration devices.
Some Notes on Device Calibration -From the University of Dublin - A comprehensive albeit a bit academic note.
Calibration - Calibration is the validation of specific measurement techniques and equipment. At the simplest level, calibration is a comparison between measurements-one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device. This article from Wikipedia covers all the basics pretty well.
A Beginner’s Guide to Measurement - This 30 page Beginner’s Guide from the National Physical Laboratory explains the fundamental concepts and basic facts about measurement, and in particular accurate measurement. It includes brief accounts of the role of measurement in modern and historical societies and explains the SI system, its base units and their relation to other units. The various organisations involved in measurement are introduced and their roles in linking all measurements to the SI base units through traceability chains explained. It includes general guidance about practical issues that affect the making of measurements, gives the meanings of key measurement terms, and explains the significance of such fundamental concepts as measurement traceability and calibration.

The Following Technical Tips are from Advanced Instruments Inc
What Is Instrument Calibration and What Does It Do? - Instrument calibration is one of the primary processes used to maintain instrument accuracy. Calibration is the process of configuring an instrument to provide a result for a sample within an acceptable range.  Eliminating or minimizing factors that cause inaccurate measurements is a fundamental aspect of instrumentation design.
Why is Calibration Important? - How a properly performed calibration can improve product performance.
What Factors Affect Calibration? - Once the benefits of a properly performed calibration are understood, it becomes evident that care must be taken during the process to prevent potential error sources from degrading the results. Several factors can occur during and after a calibration that can affect its result.

Calibration Terminology

Calibration Terminology - Details on calibration terms - from Beamex.

The following links are from Fluke
Glossary of Loop Calibration Terms
Glossary of Pressure Calibration Terms
Glossary of Temperature Calibration Terms

Calibrating Fieldbus Instruments

Calibrating Fieldbus Transmitters - Fieldbus is becoming more and more common in today’s instrumentation. But what is fieldbus and how does it differ from conventional instrumentation? Fieldbus transmitters must be calibrated as well, but how can it be done? Until now, no practical solutions have existed for calibrating fieldbus transmitters - from Beamex.

Calibration Frequency

Instrument Calibration- Glenn Carlson, Technical Support,  - Users frequently want to know how often they need to calibrate their In- Situ instrument. The most accurate answer to that question is “it depends”.  This article addresses this -thanks to In-Situ Inc . 
How Frequently should a Product be Calibrated? - The simple answer to this question, although not a very helpful one, is “when it needs it.”  From a more practical standpoint, daily or periodically testing the control solutions of known values can provide a quantitative indication of instrument performance, which can be used to establish a history - from Advanced Instruments Inc.
How often should Calibrators be Calibrated? - This article discusses some of the things to be considered when specifying the calibration period and provides some general guidelines. The same guidelines that apply to a calibrator also apply to other measuring equipment in the traceability chain. These guidelines can even be used for process instrumentation - from Beamex.
How often should Instruments be Calibrated? - Plants can improve their efficiency and reduce costs by performing calibration history trend analysis. By doing it, a plant is able to define which instruments can be calibrated less frequently and which should be calibrated more frequently. Calibration history trend analysis is only possible with calibration software that provides this functionality - from Beamex.
4.13 Calibration Intervals, A Manufacturer’s Perspective - David Deaver - The analysis tools that are currently available for Calibration Intervals focus on setting intervals to achieve a desired reliability target. This paper suggests there is another perspective that these tools do not currently address; consequence cost or accumulated liability. A case is made that sometimes the reliability target is a secondary consideration to managing this consequence cost. The paper also addresses how manufacturers establish calibration intervals. The paper presents, and defends, the practice of using no analysis whatsoever in establishing the manufacturer's recommended calibration interval - from Fluke.

Other General Test and Calibration Links

Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results- Barry N. Taylor and Chris E. Kuyatt - This is NIST Technical Note 1297 as it was published
.U. S. Army Corps of Engineers Process Instrument And Control Checklist - This is a very useful checklist which is designed to facilitate the performance evaluation of process instrumentation and control systems used to operate and monitor treatment processes and equipment.
On Site Flow Calibration is Painful but Necessary- by David W. Spitzer -thanks to ControlGlobal.com .Some new product introductions have raised doubt about what in-situ calibration for flowmeters is, and whether it can be duplicated with simulators and calibrators with expanded diagnostics.
Calibrating and Testing Control Components on your Heat Process - What, When and How Should I Calibrate? - Arthur Holland, Holland Technical Skills - an excellent explanation on the basics of calibration - from dcnz.
Calibrating Non Destructive Testing Instruments - NDT Resource Centre- Calibration refers to the act of evaluating and adjusting the precision and accuracy of measurement equipment. In ultrasonic testing, several forms of calibration must occur. First, the electronics of the equipment must be calibrated to ensure that they are performing as designed. This operation is usually performed by the equipment manufacturer and will not be discussed further in this material. It is also usually necessary for the operator to perform a "user calibration" of the equipment. This user calibration is necessary because most ultrasonic equipment can be reconfigured for use in a large variety of applications.
A Guide to Low Resistance Measurement - You have to register to obtain this handbook which gives an overview of low resistance measurement techniques, explains common causes of errors and how to avoid them. We have also included useful tables of wire and cable characteristics, temperature coefficients and various formulas to ensure you make the best possible choice when selecting your measuring instrument and measurement technique - from Cropico. 
Guide To Low Resistance Testing - Understanding and Measuring Low Resistance to Ensure Electrical System Performance - The purpose of this booklet is to help the engineer, technician or operator to understand the rationale behind low resistance, how to make a low resistance, how to select the proper instrument for the testing application and how to interpret and use the results - from Megger and Tech Rentals.
Getting Down to Earth - A Practical Guide to Earth Resistance Testing - Earth resistance is measured in two ways for two important fields of use:
- Determining effectiveness of “ground” grids and connections that are used with electrical systems to protect personnel and equipment.
- Prospecting for good (low resistance) “ground” locations, or obtaining measured resistance values that can give specific information about what lies some distance below the earth’s surface (such as depth to bed rock). It is not the intent of this manual to go too deeply into the theory and mathematics of the subject. As noted in the references at the end, there are many excellent books and papers that cover these. Rather, the information herein is in simple language for easy understanding by the user in industry - from Megger and Weschler Instruments
The Expression of Uncertainty and Confidence in Measurement - M3003 - The general requirements that testing and calibration laboratories have to meet if they wish todemonstrate that they operate to a quality system, are technically competent and are able to generate technically valid results are contained within ISO/IEC 17025:2005. This international standard forms the basis for international laboratory accreditation and in cases of differences in interpretation remains the authoritative document at all times. M3003 is not intended as a prescriptive document, and does not set out to introduce additional requirements to those in ISO/IEC 17025:2005 but to provide amplification and guidance on the current requirements within the international standard. This 82 page document is certainly comprehensive - from www.ukas.com.
The Internet Resource for the International Temperature Scale of 1990 - Temperature Scale and General Temperature information for metrologists, scientists, calibration engineers and those with an interest in the temperature scale and its realisation.
Calibration of Test Equipment for Maintenance Purposes - The responsibility of an Approved Maintenance Organisation (AMO) is to provide within its Maintenance Organisation Manual (Policy and Procedures Manual (or equivalent document) a list of all test equipment that must be calibrated and the process to track the calibration - from Civil Aviation Safety Authority.

The Following Papers are from BEAMEX
Automated Calibration Planning Lowers Costs - Calibration is an essential element of any instrumentation maintenance program. However, sometimes calibration operations can be long and time-consuming. By planning the process and adding the right tools, efficiency can be improved and costs lowered substantially. 
Traceable and Efficient Calibrations in the Process Industry - Today’s modern process plants, production processes and quality systems, put new and tight requirements on the accuracy of process instruments and on process control. Quality systems, such as the ISO9000 and ISO14000 series of quality standards, call for systematic and well documented calibrations, with regard to accuracy, repeatability, uncertainty, confidence levels etc.
The Safest Way to Calibrate - An Introduction to Intrinsically Safe Calibrators - There are industrial environments where calibrations should not only be made accurately and efficiently, but also safely. When safety becomes a top priority issue in calibration, intrinsically safe calibrators enter into the picture.

The Following Papers are from Fluke
Many Technical Calibration Papers including the following can be found thanks to Fluke, you have to register but it is worth it.
A Poor Man's Resistance Bridge
A Preliminary Assessment of the Effectiveness of 5700A Artifact Calibration
A Traceability Technique for Complex Waveform Generators
A Wheatstone Bridge for the Computer Age
An Application of the Guide to Measurement Uncertainty 
An Assessment of Artifact Calibration Effectiveness for a Multifunction Calibrator 
Calibration Data Management: Meeting the Reporting Requirements of ISO/IEC FDIS 17025 Future Developments in Oscilloscope Calibration 
Maintenance and Calibration of HART Field Instrumentation 
Why Calibrate Test Equipment? - You’re serious about your electrical test instruments. You buy top brands, and you expect them to be accurate. You know some people send their digital instruments to a metrology lab for calibration, and you wonder why. After all, these are all electronic — there’s no meter movement to go out of balance. What do those calibration folks do, anyhow — just change the battery? These are valid concerns, especially since you can’t use your instrument while it’s out for calibration. But, let’s consider some other valid concerns. For example, what if an event rendered your instrument less accurate, or maybe even unsafe? What if you are working with tight tolerances and accurate measurement is key to proper operation of expensive processes or safety systems? What if you are trending data for maintenance purposes, and two meters used for the same measurement significantly disagree?
Measurement Uncertainty - How does DMM Accuracy affect your next Measurement? - Measurement uncertainty is an estimate of the possible error in a measurement. It's also an estimate of the range of values which contain the true value of the measured quantity. It's also the probability that the true value lies within a stated range of values.

The following Calibration Links are from Dickson
Are All Metrology Labs Alike? - Short answer – NO!!! In fact, to those of us in the industry who truly know what it takes to recalibrate instruments to objectively defined standards, a better question might be— Are you using the equivalent of a meat thermometer to validate conditions in your processing plant or laboratories?
“Before” Calibrations Count More than Many Think - As most know, temperature and humidity dataloggers and chart recorders need to be recalibrated periodically to ensure this accuracy, and competent quality managers need to establish schedules for recalibrations that reflect due diligence to monitor that temperatures and humidity are kept within acceptable and pre-defined tolerances. But "recalibration" can mean different things, and what could be termed "recalibration on the cheap" does NOT demonstrate the accuracy of your recorded data (instruments).
Monitoring Revisited - While most pharmaceutical quality managers realize the importance of temperature and humidity tracking to guarantee both quality and compliance, the way in which many go about it is adding hidden costs. Technology for temperature/humidity tracking continues to evolve, and there are numerous time-saving features in recent temperature/humidity data loggers that can make a difference. On one hand, some quality managers are doing too much to track temperature /humidity data, while on the other hand some are doing too little. For many, it’s timely to revisit temperature and humidity monitoring. Here are some key points to consider.
Monitoring Temperature and Humidity - Monitoring temperature and/or humidity conditions is an essential ingredient of a wide range of quality assurance applications. There are many common methodological errors, however, in ways that this task is approached that either compromise quality standards or add unnecessary time and expense to the monitoring task. Insufficient calibration of temperature and humidity monitoring instruments is high on the list of problematic areas. Mismatching technology to the monitoring task at hand is another problematic area. This article revisits technology trends in monitoring instrumentation, provide tips on calibration and discuss common methodological errors that quality managers should avoid.
Get Before Data - Before data (a.k.a. "as received data") documents the accuracy of your data logger or chart recorder before it is calibrated. When your instrument is re-calibrated it is returned to original specifications. You cannot retroactively obtain "before" data.
1-Point, 3-Point, or Custom Point? - Choosing between 1-Point, 3-Point and Custom Point Calibrations is very straightforward when you know how calibration REALLY works... 
Calibration Glossary - A useful glossary explaining calibration terms.

4.13 Electrical Measurement Safety

Electrical Measurement Safety Program - Every day, an average of 9,000 U.S. workers suffer disabling injuries on the job.  Anyone performing electrical measurements should understand the safety standards and be certain their tools meet code. This page from Fluke is an excellent Safety Resource.
Video - Electrical Measurement Safety - This hour long session provides an awareness of electrical measurement hazards; a better understanding of the safety specifications for digital multimeters and testers; an understanding of the four installation measurement categories and; how to minimize and avoid electrical measurement hazards- You will need to register to see this video - from Fluke.
10 Mistakes People Make Working on Electrical Systems - Jim White - This list gets you thinking. We go through life making small mistake after small mistake and nothing happens, until we happen to get the wrong alignment of small mistakes and have an accident. Once the accident starts, we have no control over it, so the best thing to do is to avoid the small mistakes and tighten up the way we work – from Shermco Industries and Fluke.

Test and Calibration Standards

Standards Related to Temperature and Calibration - A list of ASTM standards.

Temperature Instrument Calibration

The following temperature calibration links are compliments of ISOtech
A Review of Some of the Best Articles Written about Water and its Triple Point - Details of articles on this subject.
Calibrating Thermometers -Dave Ayres and Anne Blundell - A thermometer without a traceable calibration route to recognised National Standards is fairly useless. Yet we all buy mass produced thermometers which are supplied without a calibration and use them. We all hope that the manufacturer has been conscientious and has at least carried out calibration checks on batch samples and has claimed a level of accuracy to the batch. But has the manufacturer used suitable standards for the calibration?
Improved Sterilizer Tests - Dave Ayres and Dave Hill - Scottish Healthcare Supplies Sterilizer Test Group assessed various established methods of on-site temperature calibration and realised there might be shortcomings in commercially available "complete" systems. The guidelines require tests on sterilizer systems to be carried out within a system uncertainty of ±0.5°C but the assessment showed that in many cases "complete" systems could produce a system uncertainty of ±1.0° or worse. (Temperature monitoring in sterilizer systems is critical ensuring that microbiological viability is eliminated from the product).
Industrial Measurements with very Short Immersion - J. P. Tavener, D. Southworth, D. Ayres, N. Davies.- One major problem that keeps recurring is the request to calibrate, or in some other way to evaluate, very short industrial temperature sensor assemblies. These sensors are so short that the sensor does not attain the temperature of its surroundings. Two distinct methods are possible, in method one the assembly is immersed in a comparison bath sufficiently to eliminate the stem conduction effect, even if this method creates a different result than achieved in-situ. Method two attempts to simulate the application in practice and provide a similar stem conduction error as the assembly sees in practice.
Automating Temperature Calibration Baths with Simple Low Cost Image Acquisition - David J. Southworth - A low cost video camera, “Web Cam” is used in conjunction with a PC and Temperature Calibration Bath to automatically calibrate handheld digital thermometers which have no provision to be connected to an external computer.
Stem Conduction and Light Piping in ITS-90 Fixed Point Cell Assemblies At A UKAS Laboratory - J. P. Tavener & A. Blundell - Standard Platinum Resistance Thermometers (SPRTs) with length-below-handle of only 480mm are regularly submitted for calibration at ITS-90 fixed points from –200 °C to +660 °C. The length of the thermometer limits the maximum size of fixed point cell that can be used to calibrate the thermometers. Stem conduction effects have been measured at zinc and aluminium temperatures in resealable cells. These have been quantified and eliminated by adopting a cell design with a very small connection between cell and gas supply.
Slim Cells an International Comparison - J. Tavener - This paper presents the results of the original primary laboratory inter-comparisons and the more recent inter- comparisons of PTB. The results show that the slim cells and apparatus offer good agreement with Primary and National Standards.
Temperature Calibration; Depths of Immersion - John P. Tavener -Of all the sources of errors and uncertainties in thermal calibration by far the largest source of error and least understood effect is that of immersion of unit under test, and the reference standard.
Primary Laboratory Comparisons -  The most accurate measurements made in a Primary Temperature Laboratory are during intercomparisons of ITS-90 fixed point cells, and in particular inter-comparing water triple point cells. To assess the stability of the water triple point, a laboratory ideally needs to be able to measure differences of just one or two micro degrees.At the Northern Temperature Primary Laboratory (NTPL) we found the spread of results too large to give a satisfactory result. Consulting the literature, and in particular Tischler & Prado [3] we eventually developed a 3 current technique from which we were able to calculate the zero current resistance to within 1 or 2 micro degrees. This paper describes in detail our method.
Recommended Book- Traceable Temperatures - An Introduction to Temperature Measurement and Calibration - J.V. Nicholas and D.R. White, John Wiley + Sons, 2nd Edition Download Chapter One: Measurement and Traceability  Purchase from Amazon.co.uk

Useful Books on Instrument Calibration and Measurement

Calibration: A Technician's Guide - This comprehensive review of calibration provides an excellent foundation for understanding principles and applications of the most frequently performed tasks of a technician. Topics addressed include terminology, bench vs. field calibration, loop vs. individual instrument calibration, instrument classification systems, documentation, and specific calibration techniques for temperature, pressure, level, flow, final control, and analytical instrumentation. The book is designed as a structured learning tool with questions and answers in each chapter. An extensive appendix containing sample P&IDs, loop diagrams, spec sheets, sample calibration procedures, and conversion and reference tables serves as a very useful reference. If you calibrate instruments or supervise someone that does, then you need this book.
Quality Calibration Handbook - Developing and Managing a Calibration Program - Quality calibration systems are the very foundation for improving research and development (R&D), production, and quality assurance arenas through accurate, reliable, and traceable calibrations of their test equipment. This book is about how to design, implement, maintain, and continuously improve a quality calibration system, with all the required documentation, traceability, and known uncertainty for each and every item of test equipment owned and used by any company, large or small. It will benefit companies that want to implement a program and also those that already have one in place. Calibration requirements vary across specific industries but every organization can use the quality calibration system described in this book as a foundation for its personalized program. By using the quality calibration system outlined and demonstrated, any organization can put together its own version to meet its specific requirements and/or regulations.
Loop Checking: A Technician's Guide - In today’s competitive markets, manufacturers strive to continually improve manufacturing performance to meet their business needs and goals. As process control loops have a major impact on a plant’s financial performance, focusing on loop performance is critical. This technician’s guide defines loop checking in the broader scope of control loop performance in addition to the more traditional terms of the plant startup. It discusses general methods and practices that can be applied across many processes/industries. Featured topics include: loop checking basics, factory acceptance testing, wiring and loop checks, performance benchmarking, and sustaining performance.
Troubleshooting: A Technician's Guide - Troubleshooting loops and systems is something all technicians must do, but that few truly master. This newly revised edition draws on the author’s long experience as an instrument and electrical engineer and his maintenance expertise to provide a detailed look at the skills and knowledge required for troubleshooting. Interspersed with a wealth of practical detail and real-world examples are Mostia’s no-nonsense discussions of what a good troubleshooter needs to know. He provides an in-depth discussion of the basic logical framework that underlies all troubleshooting as well as advanced troubleshooting techniques. He also explores the causes of failures and the techniques that engineers and technicians use to trace them down. This new edition covers troubleshooting methods, both basic and advanced, hints and troubleshooting aids, troubleshooting safety, basic maintenance concepts, information about training, and the developing troubleshooting skills. It also includes numerous examples of troubleshooting problems in mechanical systems, process connections, pneumatic systems, electrical systems, electronic systems, and valves. Mostia also explores test equipment, programmable electronic systems, communication circuits, transient problems, and software.
Start-Up: A Technician's Guide - When new plants or systems go online, the control systems technician (CST) faces special challenges. The author explores and explains the crucial role of a technician in this process. This book offers you a clear overview of typical start-up responsibilities. From the first team meeting to the last round of tuning and loop checking, Harris uses her extensive experience with process control plants to walk you through the issues and skills typically required. Each chapter includes self-study learning objectives, practice questions and exercises, answers, and listings of relevant standards. Written with the technician in mind, it is an application-oriented book that provides an overview of the scope of duties a technician must perform in real-world situations. Includes over 30 figures and tables; fully indexed.