### Kew Observatory Trials Versus COSC - And why things will never get better

At Kew Observatory, the accuracy of a watch was measured by observation of the variations of its mean daily rate. For an 'A' certificate the trials lasted for 45 days, and included tests in temperatures varying from 40 to 90 deg F (4.4 to 32.2 deg C), in every position with dial vertical, face up and face down.

- The average daily departure from the mean daily rate could not exceed 2 s/d
- Maximum absolute deviation from mean rate in any position could not exceed 5 s.
- The errors should not increase more than 0.3 s/d per deg F. (0.54s/d per degC)

- Mean variation in rates ‘V’ is the arithmetic mean of the five absolute values of variations in rates obtained for the five positions of the watch during the first 10 days of the tests. Note: The variation in rate is the difference between two consecutive daily rates in identical environmental conditions.: Max of 2s
- Greatest deviation in rates ‘P’ is the absolute value of the greatest of the ten differences between one of the first ten rates and the mean daily test rate.: Max 10s
- Thermal variation ‘C’ is obtained by subtracting the rate at 8 deg C from the rate at 38 deg C, the whole being divided by the temperature difference, expressed in degrees Celsius. : error 0.6s per deg C

- Deviation in mean rate: Essentially identical to COSC
- Greatest devation from mean rate: Harder than COSC by a factor of '2' (which is really quite a lot tougher)
- Thermal control: Very similar to COSC

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