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Stopwatch and Timer Calibrations needs to be periodically compared to an even more accurate standard, and so on, until eventually a comparison is made against a national or international standard that represents the best physical realization of the SI unit that is being measured (in this case, the SI second). This measurement traceability hierarchy is sometimes illustrated with a pyramid as shown in Figure 1. The series of comparisons back to the SI unit is called the traceability chain. Metrological traceability is defined, by international agreement, as: The property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty. [3] The definition of metrological traceability implies that unless the measured value is accompanied by a stated measurement uncertainty, the traceability chain is broken. It is the responsibility of the calibration laboratory to determine and report the uncertainty of its measurements to its customers so that metrological traceability is maintained [4]. 6 Figure 1. The calibration and traceability hierarchy. The International Bureau of Weights and Measures (BIPM) located near Paris, France, is responsible for ensuring the worldwide uniformity of measurements
Introduction and their traceability to the SI. The BIPM collects and averages time interval and frequency data from more than 60 laboratories around the world and creates a time scale called Coordinated Universal Time (UTC) that realizes the SI second as closely as possible. Thus, UTC serves as the international standard for both time interval and frequency. However, the BIPM does not produce a physical representation of the second; it simply calculates a weighted average that is published weeks after the actual measurements were made. This document, known as the BIPM Circular T, shows the time offset of each contributing laboratory with associated uncertainties, and can be downloaded from the BIPM web site (www.bipm.org). The laboratories that provide data to the BIPM maintain the oscillators and clocks that produce the actual signals that are used as measurement references. Most of these laboratories are national metrology institutes (NMIs) that serve as the caretakers of the national measurement references for their respective countries. Thus, to establish traceability to the SI for time interval and frequency calibrations, the traceability chain for a measurement must link back to signals that originate from an NMI or a national timekeeping laboratory. The NMI that is chosen as a reference must contribute to the derivation of UTC by sending its measurement data (with associated uncertainties) to the BIPM. NIST is the ultimate reference point for most measurements made in the United States, and as such, submits time and frequency data to the BIPM. NIST provides it own real-time representation of UTC, designated UTC(NIST), that is distributed to the public using a variety of radio, telephone, and Internet signals. These signals are described in more detail in Section 5, and can serve as references for measurements that are traceable back to the SI. The traceability chain is easy to visualize if you think of it as a series of calibrations. Every link in the chain is a calibration; comparing a device to a higher standard, until eventually a comparison is made to the SI unit. Every calibration has some measurement uncertainty. At the top of the chain, the measurement uncertainties are so tiny they are insignificant to those of us who calibrate stopwatches and timers. For example, the difference between the best possible estimate of the SI second and UTC(NIST) is measured in parts in 10 16 . This represents a time offset of less than 0.1 ns (10 -10 s) over the course of a day. As we move down the traceability chain to the actual calibration of a stopwatch or timer, the uncertainties become larger and larger. For example, if the calibration laboratory uses an audio signal from NIST radio station WWV (Section 5) to calibrate its standard, the uncertainty of the received tones might be 1 ms. This uncertainty 7
Page 1: NIST Recommended Practice Guide Sto
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Page 7: Acknowledgements ACKNOWLEDGEMENTS
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Page 15 and 16: Section 1 Introduction to Stopwatch
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Required Uncertainty Section 8 How
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Other Topics Related to Measurement
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