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

The Direct Comparison Method
the uncertainty contributed by human reaction time to 0.02 %. However, if we
use a 1500 s interval, we could possibly be measuring the variation in human
reaction time, and nothing else. Therefore, we need to extend the time interval so
that human reaction time becomes an insignificant part of the measurement. For
example, NIST Handbook 105-5 [4] states that a stopwatch is considered to be
within tolerance if its time offset is 2 s or less in during a three hour calibration.
Three hours is a long enough time interval to exceed the 0.02 % requirement and
to ensure that the uncertainty contribution of human reaction time is insignificant.
There is no hard and fast rule, the length of the calibration can vary according to
each laboratory’s procedures, but it must be long enough to meet the uncertainty
requirements for the device being tested. If your uncertainty requirement is
0.01 % or lower, the direct comparison method might not be practical.
5.C. Uncertainties of Direct Comparison Method
The Direct Comparison Method has three potentially significant sources of
uncertainty that must be considered: the uncertainty of the reference, the reaction
time of the calibration technician, and the resolution of the DUT.
5.C.1. Uncertainty of the Traceable Time Interval Reference
If the reference signal is one of the telephone services listed in Table 6, two
phone calls are usually made. The first call is made to obtain the signal to
start the stopwatch, and the second call is made to obtain the signal to stop the
stopwatch. If both calls are made to the same service and routed through the
same phone circuit, the delay through the circuit should be nearly the same for
both calls. Of course, the delays will not be exactly the same, and the difference
between the two delays represents the uncertainty of the time interval reference.
In most cases, this uncertainty will be insignificant for our purposes, a few
milliseconds or less. For example, callers in the continental United States using
ordinary land lines can expect signal delays of less than 30 ms when dialing
NIST at (303) 499-7111, and these delays should be very repeatable from phone
call to phone call. Even in a theoretical case where the initial call had no delay,
and the final call had a 30 ms delay, the magnitude of the uncertainty would be
limited to 30 ms.
However, if an ordinary land line is not used, the uncertainties associated with
telephone time signals must be assumed to be larger. For example, wireless
phone networks or voice over Internet protocol (VOIP) networks sometimes
introduce larger delays that are subject to more variation from phone call to
phone call than ordinary land lines. If wireless or VOIP networks are used,
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