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Stopwatch and Timer Calibrations Table 3 - Continued Description Type I Stopwatch (digital) Single control to start/stop Type II Stopwatch (analog) Start and stop Reset Split time (if equipped) Force to operate controls Audible signal of start/stop Must reset stopwatch to zero Must indicate whether display mode is regular or split time. Must not exceed 1.8 N (0.40 lbf) Dial and hands Face must be white Graduations must be black or red Hands must be black or red Required markings Unique, nondetachable serial number Manufacturer’s name or trademark Model number (Type I only) Digital display Provide delimiting character for hours, minutes, seconds (usually colon) Minimum increment < 0.2 s Minimum elapsed time at rollover Physical orientation 1 hour 30 minutes Stopwatch meets tolerance regardless of physical orientation 10
Description of Timing Devices Figure 2. Type I digital stopwatch. Figure 3. Type II mechanical stopwatch. 2.A.1. Basic Theory of Operation Every stopwatch is composed of four elements: a power source, a time base, a counter, and an indicator or display. The design and construction of each component depends upon the type of stopwatch. 2.A.1.a. Digital (Type I) Stopwatches The power source of a Type I stopwatch is usually a silver-oxide or alkaline battery that powers the oscillator and the counting and display circuitry. The time base is a quartz crystal oscillator that usually has a nominal frequency of 32 768 Hz, the same frequency used by nearly all quartz wristwatches. The 32 768 Hz frequency was originally chosen because it can be converted to a 1 pulse per second signal using a simple 15 stage divide-by-two circuit. It also has the benefit of consuming less battery power than higher frequency crystals. Figure 4 shows the inside of a typical digital device, with the printed circuit board, quartz crystal oscillator, and battery visible. The counter circuit consists of digital dividers that count the time base oscillations for the period that is initiated by the start/stop buttons [6, 7]. The display typically has seven or eight digits. 11
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Other Topics Related to Measurement
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Other Topics Related to Measurement
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