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

The Direct Comparison Method
Section 5
The Direct Comparison Method
The direct comparison method is the most common method used to calibrate
stopwatches and timers. It requires a minimal amount of equipment, but has
larger measurement uncertainties than the other methods. This section describes
the references used for this type of calibration and the calibration procedure.
5.A. References for the Direct Comparison Method
The direct comparison method requires a traceable time-interval reference. This
reference is usually an audio time signal, but in some cases a traceable time display
can be used. The audio time signals are usually obtained with a shortwave radio
or a telephone. Since time interval (and not absolute time) is being measured,
the fixed signal delay from the source to the user is not important as long as
it remains relatively constant during the calibration process. A list of traceable
audio time sources is provided in Table 6.
Table 6 - Traceable audio time signals.
National
Metrology
Institute (NMI)
Location
Telephone
numbers
Radio
call
letters
Broadcast
frequencies
National Institute
of Standards
and Technology
(NIST)
Fort Collins,
Colorado,
United States
(303) 499-7111 * WWV
2.5, 5, 10,
15, 20 MHz
National Institute
of Standards
and Technology
(NIST)
Kauai,
Hawaii,
United States
(808) 335-4363 * WWVH
2.5, 5, 10,
15 MHz
United
States Naval
Observatory
(USNO)
Washington,
DC, United
States
(202) 762-1401 *
(202) 762-1069 *
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