Calibration Techniques for Stopwatches and Timers NCSLI-2003, Tampa, FL August 21, 2003 Robert Graham Sandia National Laboratories P.O. Box 5800, M.S. 0665 Albuquerque, NM 87185-0665 (505) 845-0434 rmgraha@sandia.gov Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Disclaimer Use of manufacturer’s names or model numbers are for reference purposes only, and do not imply endorsement by Sandia National Labs, the Department of Energy, or the United States Government. The items depicted in this presentation were evaluated and found to meet the needs of our laboratory, but equivalent suitable items from other manufacturers may be available.

Introduction Topics to be covered: Stopwatch and Timer descriptions Instrument specifications Methods of calibration

Stopwatches and Timers Mechanical or digital Battery or AC line powered, or spring-driven Measures elapsed time from some initial starting point Can be stopped and elapsed time read Timers: Mechanical or digital Powered by battery or AC line voltage Desired time interval to be measured is set Counts down to zero Audible, visual, or electronic alarm signals when set time has elapsed

Types of Stopwatches (From NIST Handbook 105-5) Type I Digital Battery operated, quartz crystal Type II Analog Normally spring-driven, mechanical movement

Timers

Timer and Stopwatch Calibration Methods Method used depends on Device Under Test (DUT) and standards available Direct Comparison method Requires a telephone or short-wave radio receiver Totalize method Similar to Direct Comparison method Uses local calibrated standards Time Base method Requires a certified stopwatch calibrator Measures stopwatch time base directly Additional operational tests may be required

I. Direct Comparison Method A traceable time source is used to start and stop the stopwatch The source can be monitored either by telephone or radio receiver When the time tick occurs, start the stopwatch and record time Wait an appropriate length of time Time period depends on stopwatch accuracy Longer time intervals reduce the influence of the operator When target time tick occurs, stop the stopwatch and record the time Subtract start time from stop time and compare to stopwatch indication Do NOT use the local “Time and Temperature” number.

I. Direct Comparison Method Benefits: Minimum amount of additional equipment required Relatively easy to perform Can be used to calibrate all types of stopwatches Disadvantages: Can take a long time to complete For shorter time intervals, operator’s start/stop response is significant May incur long distance telephone charges Does not work well for calibrating timers

Sources for Traceable Time Signals NMI Location Telephone Numbers Radio Call Letters Broadcast Frequency(s) Time Zone National Institute of Standards and Technology (NIST) Fort Collins, Colorado, USA (303) 499-7111 WWV 2.5, 5, 10, 15, 20 MHz UTC Kauai, Hawaii, USA (808) 335-4363 WWVH 2.5, 5, 10, 15 MHz United States Naval Observatory (USNO) Washington, DC, USA (202) 762-1401 (202) 762-1069 N/A Colorado Springs, Colorado, USA (719) 567-6742 National Research Council (NRC) Ottawa, Ontario, Canada (613) 745-1576 (English) (613) 745-9426 (French) CHU 3.33, 7.335, 14.67 MHz EST Centro Nacional de Metrologia (CENAM) Querétaro, México (442) 215-39-02 (442) 211-05-06 (442) 211-05-07 (442) 211-05-08 (Spanish) XEQK 1.35 MHz CST MST PST

Other Sources for Traceable Time: W W V B W W V B Receiver Receives, decodes, and displays the time code from NIST’s W W V B radio station Do not try to use so-called “atomic clocks” that only synchronize themselves once or twice per day Their internal time bases may not be accurate enough to use as a calibration reference Traceability is directly to NIST

Other Sources for Traceable Time: GPS GPS Receivers Receives, decodes, and displays GPS time Should be calibrated to insure proper operation Traceability is directly to the U.S. Naval Observatory, and indirectly to NIST (since NIST monitors GPS and published the results)

II. Totalize Method Similar to Direct Comparison method Uses local, certified standards Laboratory frequency reference Synthesizer Time-interval counter Synthesizer uses frequency reference as external time base

II. Totalize Method Set the counter to TOTALIZE Apply a 1 kHz square wave signal from the synthesizer Open the gate of the counter and start the stopwatch at the same time Use the stopwatch’s start/stop button to press the counter’s gate button

II. Totalize Method After the required time, stop the stopwatch and close the gate of the counter at the same time Subtract the stopwatch’s indication from the counter’s indication The difference is the stopwatch error

II. Totalize Method Advantages: Can be used for all types of stopwatches and timers Measurement time can be customized for each unit Start/stop uncertainties can be reduced on some units Disadvantages: Requires more equipment for the calibration Frequency standard must be certified and traceable Start/stop uncertainties still apply on some units, especially timers Lower uncertainties require longer testing times

III. Time Base Method A certified stopwatch calibrator is used The stopwatch time base is measured directly 32,768 Hz (215 Hz) for most quartz crystal watches 4.19 MHz for older LED watches Uncertainties as low as ± 0.05 s/day (± 5.80 x 10-7 or 0.000 058%) Can also be used to calibrate mechanical (spring-driven) stopwatches

Stopwatch Calibrator

III. Time Base Method Advantages: Easy and fast – calibrations can be accomplished quickly Lower uncertainties No operator start/stop errors involved Disadvantages: Requires specialized equipment Commercial stopwatch calibrator Acoustic or inductive pickup, amplifier, and counter Separate functional test may be required (depending upon customer’s requirements)

Conclusion Method used depends on many factors If a lot of stopwatches are calibrated in a year, a commercial calibrator would be cost effective If only a few stopwatches are calibrated, a lower-cost (but more time consuming) method could be used Unit being tested also influences method Some AC-line powered units cannot be tested using stopwatch calibrator; another method must be used

Questions?