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

The Time Base Method
Section 7
The Time Base Method
The time base measurement method is the preferred method for stopwatch and
timer calibrations, since it introduces the least amount of measurement uncertainty.
Because the DUT’s time base is measured directly, the calibrating technician’s
response time is not a factor.
The exact method of measuring the stopwatch’s time base depends upon the type
of stopwatch or timer being calibrated. If the unit has a quartz crystal time base,
an inductive or acoustic pickup is used to monitor the stopwatch’s 32 768 Hz
time base frequency on a calibrated frequency counter (the pickup is fed into an
amplifier to boost the signal strength). If the unit is an older LED-type stopwatch,
the frequency is usually 4.19 MHz. An inductive pickup can even be used to
sense the stepping motor frequency of analog mechanical stopwatches, or the
“blink rate” of a digital stopwatch display. Or, an acoustic pickup can be used to
measure the “tick” of a mechanical stopwatch.
7.A. References for the Time Base Method
The reference for a time base calibration is the time base oscillator of the measuring
instrument. For example, if a frequency counter is used, the measurement
reference is the time base oscillator of the frequency counter. In order to establish
traceability, the frequency counter time base must have been recently calibrated
and certified. However, a better solution is to have the laboratory maintain a
traceable 5 MHz or 10 MHz signal that can be used as an external time base for
the frequency counter and all other test equipment. If an external time base is
used and its measurement uncertainty is known, it is unnecessary to calibrate the
internal time base oscillator.
7.B. Calibration Procedure for the Time Base Method
Two methods of calibrating a stopwatch time base are described below. One
uses a commercially available measurement system; the other uses a frequency
counter with an acoustic pickup. Note that neither calibration method requires
opening the case of the stopwatch or timer. Keep in mind that you should never
disassemble a stopwatch or timer and attempt to measure the time base frequency
by making a direct electrical connection. The crystal oscillators in these units are
very small, low-power devices. Their frequency can dramatically change if they
are disturbed or loaded down by the impedance of a frequency counter, and in
some cases they can even be destroyed by incorrect electrical connections.
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