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

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Stopwatch and Timer Calibrations 3.B.1. Time Clocks and Time Recorders The specification for both overregistration and underregistration is 3 s per hour, not to exceed 1 minute per day. 3.B.2. Parking Meters The specifications for parking meters have no tolerance for overregistration. Parking meters with a time capacity of 30 minutes or less are specified to have a maximum underregistration error of 10 s per minute, but not to exceed 2 minutes over the 30 minute period. For parking meters with a capacity of greater than 30 minutes, but less than 1 hour, the tolerance for underregistration is 5 minutes, plus 4 s per minute for every minute between 30 minutes and 60 minutes. Parking meters that indicate over 1 hour have an underregistration tolerance of 7 minutes plus 2 minutes per hour for time intervals greater than 1 hour. 3.B.3. Other Timing Devices All other timing devices have an overregistration tolerance of 5 s for any time interval of 1 minute or more and an underregistration tolerance of 6 s per indicated minute. If the instrument is a digital indicating device, the tolerance is expanded by one half of the least significant digit. 24
Section 4 Introduction to Calibration Methods Calibration Methods There are three generally accepted methods for calibrating a stopwatch or timer: the direct comparison method, the totalize method, and the time base method. The first two methods consist of time interval measurements that compare the time interval display of the DUT to a traceable time interval reference. In the case of the direct comparison method, the time interval reference is normally a signal broadcast by an NMI, usually in the form of audio tones. In the case of the totalize method, the time interval reference is generated in the laboratory using a synthesized signal generator, a universal counter, and a traceable frequency standard. The third method, the time base method, is a frequency measurement. It compares the frequency of the DUT’s time base oscillator to a traceable frequency standard [12]. The properties of the three methods are briefly summarized in Table 5, and the following three sections are devoted to the three methods. Each section explains how to perform a calibration using each method, and how to estimate the measurement uncertainties. Table 5 - Comparison of calibration methods. Method Time base Direct comparison Totalize Properties measurement Equipment Requirements Best Better Better Speed Good Better Best Uncertainty Good Good Best Applicability Good Best Better The methods used to estimate the uncertainty of measurement are described in the ISO Guide to the Expression of Uncertainty in Measurement (GUM) [2]. This guide does not attempt to summarize the GUM, but does strive to produce estimates of uncertainty that are consistent with the GUM. The resulting expanded uncertainty of measurement is presented with a coverage factor that represents an approximate 95 % level of confidence. 25
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