Stopwatch and Timer Calibrations - National Institute of Standards ...

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
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