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

The Totalize Method
6.A.1. Advantages of the Totalize Method
When using the stopwatch’s start/stop button to open and close the counter’s
gate, this method partially eliminates human reaction time, and therefore has a
lower measurement uncertainty than the direct comparison method.
6.A.2. Disadvantages of the Totalize Method
This method requires more equipment than the direct comparison method.
6.B. Uncertainties of Totalize Method
The factors that contribute to the measurement uncertainty of this method are
discussed below.
6.B.1. Uncertainty of the Frequency Input
A synthesized signal generator that was recently calibrated typically has a
frequency uncertainty ranging from 1 10 -6 to 1 10 -9 . If the signal generator
time base is externally locked to a laboratory frequency standard such as a cesium
oscillator or a GPS disciplined oscillator, the frequency uncertainty can be much
smaller, typically parts in 10 12 or less.
6.B.2. Uncertainty Due to Human Reaction Time
This source of uncertainty is due to any difference between the starting of both
the stopwatch and counter, and the stopping of both the stopwatch and counter.
In order to estimate this source of uncertainty, a study was conducted at Sandia
National Laboratories. Four individuals were selected and asked to calibrate a
standard stopwatch using the totalize method, using one hand to start and stop
the stopwatch, and one hand to start and stop the frequency counter. The time
base of the stopwatch was measured before and after each test (using the time
base method), and its offset from nominal was found to be small enough so that
it would not influence the test. Therefore differences in readings between the
stopwatch being tested and the standard would be due only to the operator’s
reaction time.
Each operator was asked to repeat the measurement process 10 times, and the
resulting difference between the standard and the stopwatch were recorded.
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