On Tue, 5 Oct 2004 firmware@tds.net wrote:

>> 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.

Measure the period of a temperature and amplitude stabilised pendulum. Or 
look at the Eotvos gravimeter (torsion balance). As long as you don't 
expect to put the thing in a container hanging off a helicopter and fly 
with it above uncharted lands, and do not want absolute answers, you have 
a lot of options. Normal (Ascani etc) gravimeters use a weight on a spring 
and a servo to reset the position if it moves off the 'center'. The 
position is read using a microscope and a photocell pair and the whole 
item is temperature stabilised. Don't try to build one of these, maybe try 
to buy one second hand and have it professionally refurbished (an 
extremely time-consuming task).

Peter


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