[time-nuts] Regulating a pendulum clock

bg at lysator.liu.se bg at lysator.liu.se
Mon Aug 9 23:07:27 UTC 2010

Hi Ian,

> Unfortunately Gravity is not constant. Pendulum clocks show cyclic errors
> due to the influences of the Moon's and Sun's Gravitational fields. I
> forget
> the amounts but it is in the region of parts in 10 to the 7, which is
> easily
> measurable.
> This limits the compensations one can put into a pendulum clock.

http://en.wikipedia.org/wiki/Tidal_force gives 1.1e-7g and 0.52e-7g for
Moon and Sun acceleration at Earth.

Hmmm that is ca 0.1 microg (ug).

Leaving the pendulum clock line of thought... by instead measuring this
change of apparent gravity and the models for Moon-Earth relative
positions we have a source of time!

Hmmm... spec sheet of my best accelerometer

Temp stability         <30 ug/degC
1 year bias stability  <250 ug
Threshold/resolution   <1ug

The accelerometer clearly needs to be ovenized, and hopefully the spec is
is conservative and I need to have luck with a really good accel. You also
need  to mount it on a structure separate from where you walk. No nearby
heavy vehicles and so on. Well we will need a stable sampling clock, so
maybe the pendulum is back in the picture.

As the sensing element in the acc is made of quartz, and adding an oven -
there are quite a few similarities with an OCXO! Lesser models of these
accelerometers are operated as temperature compensated or without any
temperature compensations - compare with TCXOs and XOs...


Sorry if this went off topic!



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