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

Section 4
Introduction to Calibration Methods
Calibration Methods
There are three generally accepted methods for calibrating a stopwatch or timer:
the direct comparison method, the totalize method, and the time base method.
The first two methods consist of time interval measurements that compare the
time interval display of the DUT to a traceable time interval reference. In the
case of the direct comparison method, the time interval reference is normally a
signal broadcast by an NMI, usually in the form of audio tones. In the case of the
totalize method, the time interval reference is generated in the laboratory using
a synthesized signal generator, a universal counter, and a traceable frequency
standard. The third method, the time base method, is a frequency measurement. It
compares the frequency of the DUT’s time base oscillator to a traceable frequency
standard [12]. The properties of the three methods are briefly summarized in
Table 5, and the following three sections are devoted to the three methods. Each
section explains how to perform a calibration using each method, and how to
estimate the measurement uncertainties.
Table 5 - Comparison of calibration methods.
Method
Time base
Direct comparison Totalize
Properties
measurement
Equipment Requirements Best Better Better
Speed Good Better Best
Uncertainty Good Good Best
Applicability Good Best Better
The methods used to estimate the uncertainty of measurement are described in
the ISO Guide to the Expression of Uncertainty in Measurement (GUM) [2].
This guide does not attempt to summarize the GUM, but does strive to produce
estimates of uncertainty that are consistent with the GUM. The resulting expanded
uncertainty of measurement is presented with a coverage factor that represents an
approximate 95 % level of confidence.
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