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

Section 1
Introduction to Stopwatch and Timer Calibrations
Introduction
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 of time 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
sliced into fractions of a second such as milliseconds (10 -3 s, abbreviated as
ms) or microseconds (10 -6 s, abbreviated as μs). Table 1 lists these and other
1