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Stopwatch and Timer Calibrations i ii mci \_anui auui id (2009 edition) Jeff C Gust Robert M. Graham Michael A. Lombardi [T Special Publication Ititutftitute of ind Technology 960-12 m.t of Commerce NIST Special Publication 960-12 Stopwatch and Timer Calibrations (2009 edition) Jeff C. Gust Richard J. Bagan, Inc. Robert M. Graham Sandia National Laboratories Michael A. Lombardi National Institute of Standards and Technology January 2009 U.S. Department of Commerce Carlos M. Gutierrez, Secretary National Institute of Standards and Technology Patrick D. Gallagher, Deputy Director Certain equipment, instruments or materials are identified in this paper in order to adequately specify the experimental details. Such identification does not imply recom- mendation by the National Institute of Standards and Technology nor does it imply the materials are necessarily the best available for the purpose. Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States. National Institute of Standards and Technology Special Publication 960-12 Natl. Inst. Stand. Technol. Spec. Publ. 960-12 80 pages (January 2009) CODEN: NSPUE2 U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 2009 For sale by the Superintendent of Documents U.S. Government Printing Office Internet: bookstore.gpo.gov Phone:(202) 512-1800 Fax:(202) 512-2250 Mail: Stop SSOP, Washington, DC 20402-0001 Foreword > FOREWORD Stopwatch and timer calibrations are perhaps the most common calibrations performed in the field of time and frequency metrology. Hundreds of United States laboratories calibrate many thousands of timing devices annually to meet legal and organizational metrology requirements. However, prior to the publication of the first edition of this guide in May 2004, no definitive text had existed on the subject. This NISTRecommended Practice Guide was created to a fill a gap in the metrology literature. It assists the working metrologist or calibration technician by describing the types of stopwatches and timers that require calibration, the specifications and tolerances of these devices, the methods used to calibrate them, and the estimated measurement uncertainties for each calibration method. It also discusses the process of establishing measurement traceability back to national and international standards. iii Acknowledgements ACKNOWLEDGEMENTS The authors thank the following individuals for their extremely useful suggestions regarding this new revision of the guide: Georgia Harris and Val Miller of the NIST Weights and Measures Division, Warren Lewis and Dick Pettit of Sandia National Laboratories, and Dilip Shah, the chair of the American Society for Quality (ASQ) Measurement Quality Division. v Table of Contents Contents Section 1 introduction to Stopwatch and Timer Calibrations 1 1 .A. The Units of Time Interval and Frequency 1 1 .B. A Brief Overview of Calibrations 4 1. C. Traceability and Coordinated Universal Time (UTC) 5 Section 2: Description of Timing Devices that Require Calibration 9 2. A. Stopwatches 9 2.B. Timers 14 2. C. Commercial Timing Devices 15 Section 3: Specifications and Tolerances 17 3. A. Interpreting Manufacturer's Specifications 17 3.B. Tolerances Required for Legal Metrology 22 Section 4: Introduction to Calibration Methods 25 Section 5: The Direct Comparison Method 27 5. A. References for the Direct Comparison Method 27 5.B. Calibration Procedure for Direct Comparison Method 31 5. C. Uncertainties of Direct Comparison Method 33 Section 6: The Totalize Method 41 6. A. Calibration Procedure for the Totalize Method 41 6.B. Uncertainties of Totalize Method 43 6. C. Photo Totalize Method 47 Section 7: The Time Base Method 51 7. A. References for the Time Base Method 51 7.B. Calibration Procedure for the Time Base Method 51 7.C. Uncertainties of Time Base Method 56 Section 8: How to Determine if the Calibration Method Meets the Required Uncertainty 57 Stopwatch and Timer Calibrations Section 9: Other Topics Related to Measurement Uncertainty 59 9. A. Uncertainty Analysis of Using a Calibrated Stopwatch to Calibrate another Device 59 9.B. The Effects of Stability and Aging on Calibrations of 32 768 Hz Crystals 60 9.C. Factors That Can Affect Stopwatch Performance 62 Appendix A: Calibration Certificates 65 Appendix B: References 67 61 List of Figures List of Figures Figure 1. The calibration and traceability hierarchy 6 Figure 2. Type I digital stopwatch 11 Figure 3. Type II mechanical stopwatch 1 Figure 4. Interior of digital (Type I) stopwatch 12 Figure 5. Inner workings of a mechanical (Type II) stopwatch or timer... 13 Figure 6. A collection of timers 14 Figure 7. Sample manufacturer's specifications for a digital stopwatch (Example 1) 18 Figure 8. Sample specifications for a digital stopwatch (Example 2) 20 Figure 9. Typical performance of quartz wristwatches using 32 768 Hz time base oscillators 21 Figure 10. Portable shortwave radio receiver for reception of audio time signals 30 Figure 11. Reaction time measurements (four operators, 10 runs each) for the direct comparison method 36 Figure 12. Averaging measurement results for four different operators.... 3 Figure 13. Block diagram of the totalize method 41 Figure 14. Using the start-stop button of the stopwatch to start the counter 42 Figure 15. Measured reaction times (four operators, 10 runs each) for totalize method 44 Figure 16. Mean reaction times (four operators, 10 runs each) for the totalize method 45 Figure 17. Photo totalize start reading. .47 ix :ch and Timer Calibrations List of Figures, cont'd Figure 18. Photo totalize stop reading 48 Figure 19. Ambiguous photo totalize reading 49 Figure 20. Time base measurement system for stopwatches and timers. ..52 Figure 2 1 . A time base measurement system for stopwatches and timers with an integrated graphics display 54 Figure 22. Graph of the frequency stability of two stopwatches 61 Figure 23. Factors that can change the quartz time base frequency 62 Figure 24. Stopwatch accuracy versus temperature 63 Figure Al. Sample calibration certificate, (page 1) 65 Figure Al. Sample calibration certificate, (page 2) 66 List of Tables List of Tables Table 1 - Metric prefixes (may be applied to all SI units) 2 Table 2 - Unit values, dimensionless values, and percentages 4 Table 3 - Type I and Type II stopwatch characteristics 9 Table 4 - Legal metrology requirements for field standard stopwatches and commercial time devices 22 Table 5 - Comparison of calibration methods 25 Table 6 - Traceable audio time signals 27 Table 7 - The contribution of a 300 ms variation in reaction time to the measurement uncertainty 32 Table 8 - Uncertainty analysis for direct comparison method (digital DUT) using a land line 38 Table 9 - Uncertainty analysis for direct comparison method (digital DUT) using a cell phone 39 Table 10 - Uncertainty analysis for direct comparison method (analog DUT) using a land line 39 Table 1 1 - Uncertainty analysis for totalize method 46 Table 12 - The effect of the length of the measurement time on stability (based on 25 readings) 53 Table 1 3 - Uncertainty analysis of using a calibrated stopwatch to calibrate another device 60 XI Intr lion Section 1 Introduction to Stopwatch and Timer Calibrations This document is a recommended practice guide for stopwatch and timer calibrations. It discusses the types of stopwatches and timers that require calibration, their specifications and tolerances, and the methods used to calibrate them. It also discusses measurement uncertainties and the process of establishing measurement traceability back to national and international standards. This guide is intended to serve as a reference for the metrologist or calibration technician. It provides a complete technical discussion of stopwatch and timer calibrations by presenting practical, real world examples of how these calibrations are performed. There are nine sections in this guide. Section 1 provides an overview, and serves as a good introduction if you are new to the field of metrology or to time and frequency measurements. Section 2 describes the types of timing devices that require calibration. Section 3 discusses specifications and tolerances. Sections 4 through 7 discuss calibration methods and their associated uncertainties. Section 8 provides additional information to help determine if the selected calibration method can meet the required level of uncertainty, and Section 9 discusses other topics related to measurement uncertainty. A sample calibration report and references are provided in the appendices. 1.A. The Units of Time Interval and Frequency Stopwatches and timers are instruments used to measure time interval, which is defined as the elapsed time between two events. One common example of a time interval is a person's age, which is simply the elapsed time since the person's birth. Unlike a conventional clock that displays time-of-day as hours, minutes, and seconds from an absolute epoch or starting point (such as the beginning of the day or year), a stopwatch or timer simply measures and displays the time interval from an arbitrary starting point that begins at the instant when the stopwatch is started. The standard unit oftime interval is the second (s) [1]. Seconds can be accumulated to form longer time intervals, such as minutes, hours, and days; or they can be 3 sliced into fractions of a second such as milliseconds (10~ s, abbreviated as 6 ms) or microseconds (10~ s, abbreviated as us). Table 1 lists these and other i Stopwatch and Timer Calibrations prefixes that can be used with seconds, as well as the
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