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

Specifications and Tolerances
be used when dividing the absolute accuracy specification for shorter time
intervals than the period listed in the specifications. This is because for shorter
measurements periods, a new source of uncertainty, the resolution uncertainty of
the instrument, becomes an important consideration. For example, the absolute
accuracy of the example stopwatch (Figure 7) during a time interval of 30 s is
determined as
5 s
day
30 s
5 s
day
1
day
2880
0.0017 s
.
We can see from the specifications in Figure 7 that the stopwatch has a resolution
of 1/100 of a second, or 0.01 s. In this case, computing the absolute accuracy
specification for a 30 s interval results in a number (0.0017 s) that is about six times
smaller than the smallest value the stopwatch can display. Most manufacturers of
timing devices do not consider the resolution of the product in their specifications,
but we will include resolution uncertainty in our examples.
3.A.2 Relative Accuracy Specifications
While absolute accuracy specifications are helpful, sometimes it is more
desirable to specify accuracy relative to the measured time interval. This makes
its significance easier to understand. For this purpose, we define a quantity called
relative accuracy:
Absolute Accuracy
Relative Accuracy .
Measured Time Interval
Using the previous example from Figure 7, the stopwatch has an absolute accuracy
specification of 5 s per day, so the relative accuracy is:
Relative Accuracy
5 s
1day
5 s
86 400 s
-5
0.000 058 0.0058 % 5.810
.
Note that because the absolute accuracy specification and the measured time
interval are both expressed in seconds, the unit cancels out; leaving a dimensionless
number that can be expressed either as a percentage or in scientific notation.
Relative accuracy specifications can also be converted back to absolute time units
if necessary. For example, Figure 8 shows the manufacturer’s specifications for
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