Improving the Video Totalized Method of Stopwatch Calibration Samuel C.K. Ko, Aaron Y.K. Yan and Henry C.K. Ma The Government of Hong Kong Special Administrative Region (SCL) 31 Oct 2015 1
Contents Introduction Calibration of Stop Watch Video Totalized Method Other application Measuring the Flashing Time Interval of LED Lights in Toys 2
1) Introduction The demand for stop watch calibration is high in Hong Kong They are from Test laboratory Food industry Chemical laboratory Manufacturing 3
Stopwatch can be calibrated using three methods: Direct comparison method Measured by a counter The counter usually is an audio signal broadcast by the official timekeeper of a region. The measurement uncertainty achieved by this method is larger than the other two methods Totalize method By comparison with time interval reference. 4
Time base method the frequency of the time base of the stopwatch, which is likely to be a quartz oscillator, is measured directly. However, since only the time base is tested, the functionality of the stopwatch is not checked. 5
2) Calibration of Stop Watch Using Video Totalize Method: The time interval reading of the stopwatch is compared to that of a laboratory time interval reference. 6
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A signal generator is set up to feed a 1 khz clock signal to the synchronous counter. 1 khz clock is phase locked with the laboratory caesium frequency standard A high speed digital camera is used to record a 10 second video clip at 240 fps or 420 fps for the stopwatch under test and the synchronous counter when they are both counting. 8
Start time: 11:09:54 vs 442.069 9
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Take the 10 s video capture after 7 hours Compare the time elapsed by the synchronous counter and the UUT stop watch 13
Measurement model: Measurand: Relative frequency correction RC Relative Correction Time elapsed reported by laboratory standard Time elapsed reported Time elapsed reported by laboratory standard by UUT RC (1 C) ( T ref_stop (1 C) T ( T ref_start ref_stop ) ( T T uut_stop ref_start ) T uut_start ) 14
Uncertainty components: 15
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Improvement Requirement: Clear image with big image resolution Small time resolution Reliable reference counter 17
Old video capture device New video capture device Image resolution 224 x 168 800 x 270 Frame rate 420 1000 Time resolution = 2.3 ms Time resolution = 1 ms 18
The relative measurement uncertainty obtainable by this calibration method is improved from 2 x 10 7 to 9 x 10 8. 19
Easy? Why in house designed synchronous counter? 20
Many commercial universal counters did not update synchronously with the input clock signal. Another issue revealed by this high speed video clip is that the universal counter does not display all digits of its reading simultaneously. Hard to read the digit at 0.001 21
Display of a universal counter captured by a high speed digital camera at 420 fps. 22
Is GUM framework valid for the measurement uncertainty evaluation? RC (1 C) ( T ref_stop (1 C) T ( T ref_start ref_stop ) ( T T uut_stop ref_start ) T uut_start ) 23
Two methods to derive the combined standard uncertainty: GUM MCM When applying the validation procedure on this measurement model, GUM framework failed. 24
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3) Other Application Measuring the Flashing Time Interval of LED Lights in Toys 28
Requested by toy industry in Hong Kong Measure multiple LEDs in toys at the same time Not destructive to the UUT Measure the duty cycle without considering the response of the light sensor and its circuitry. 29
Paper Measuring the Flashing Time Interval of LED Lights in Toys published in NCSLI Measure Vol. 9 No. 4 December 2014 30
Model: Uncertainty components: Relative correction of 1 khz clock Readings of synchronous counter Rect distribution with semi range = 2.4 ms (for 420 fps) or 1 ms (for 1000 fps) 31
CMC for measuring light flashing rate: The measurement uncertainty can be reduced significantly from 2 ms to 0.8 ms. 32