Pressure for 8 Days    Click on Picture for full size.

This clearly shows the effects of Barometric Pressure on the clock over an eight day period. Each axis has been scaled to emphasise the relationship

The upper line is the red pressure curve, while the lower line is the blue rate curve of the clock.

Barometric error is a very real phenomenon that affects all pendulum clocks. On the majority of clocks the various errors that exist are all usually related directly to the quality, and thus the expense, of the movement. As these inbuilt errors are so great, and the barometric error so small, you will not notice any difference as the barometer pressure changes. Only on a precision clock, with its attendant accuracy, can it clearly be seen.

I won't explain a great deal as frankly I don't comprehend some aspects of this myself, but the rate of the clock is affected by both 'flotation' errors and 'accession to inertia' errors. The result is for the clock to gain in periods of low pressure, whilst loosing time in periods of high pressure.

In the graph above if you ignore the temperature variation (on this particular clock the temperature compensation is quiet good anyway) you can see that the lower line of the rate is closely linked to that of the top pressure line.

Pressure for 3 Days    Click on Picture for full size.

Here we have a 3 day period that shows a clear rise in the red pressure curve, followed by a similar drop. The clock's rate, shown by the lower blue rate curve, shows the rate closely following the pressure curve in reverse. The green line is the temperature inside the sealed case.

Here is another example shortly before, during, and after a storm.



The preliminary results so far point to a change of:-
0.025 secs/day per mm of mercury

0.01875 secs/day per millibar
(1 mmHg = 1.333223684211 millibar)

0.635 secs/day per inch of mercury
(1 mmHg = 0.03937 inchHg)





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