1. 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
(505)
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.

2. 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.

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

4. 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

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

6. Timers

7. 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

8. 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.

9. 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

10. 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)
WWV
2.5, 5, 10, 15, 20 MHz
UTC
Kauai, Hawaii, USA
(808)
WWVH
2.5, 5, 10, 15 MHz
United States Naval Observatory (USNO)
Washington, DC, USA
(202) (202)
N/A
Colorado Springs, Colorado, USA
(719)
National Research Council (NRC)
Ottawa, Ontario, Canada
(613) (English)
(613) (French)
CHU
3.33, 7.335, MHz
EST
Centro Nacional de Metrologia (CENAM)
Querétaro, México
(442)
(442) (442) (442)
(Spanish)
XEQK
1.35 MHz
CST
MST
PST

11. 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

12. 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)

13. 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

14. 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

15. 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

16. 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

17. 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 %)
Can also be used to calibrate mechanical (spring-driven) stopwatches

18. Stopwatch Calibrator

19. 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)

20. 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

21. Questions?