> Ask your local particle physicist since this sort of timing
> is daily work for them.
> 
> R

Actualy, that's pretty close to the case. I'm examining the possibilities of an absolute gravity meter. I'm interested in exploring local variations and the effects of lunar and planetary motion. The basic concept is timing a falling object in a vacumn chamber. Sounds simple enough, until I calculated the neccesary precision to achieve useful results.

If this was a 'real' project with a customer (and a budget :-) I could just buy some standard timekeeping parts and move on. But this is just a hobby project. (one that's looking increasingly unlikley to be done) 

The research I've done seems to indicate that the effects of the sun and moon are in the 100 - 200 micro-gals range. (1 gals = 1 centimeter per second squared) The first plan was to measure velocity of a ball at two points as it fell in a vacumn chamber. I just did the math and to achieve 1 micro gals resolution with a one inch ball seems to need about 32 bits of resolution. (~72 msec at 40 psec resolution) Even at lower resolution, say 20 micro gals, I still don't think that's practical for an analog solution. Think I'll reconsider the mechanics of it, there's probably a way to measure a longer event with lower resolution and achieve the same overall precision.

And, yes, there are "official" instruments that can measure this. The ones I've discovered use lasers and inferometery to measure the velocity as the object falls, they're WELL outside my budget.

-Denny


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