In a message dated 31/08/05, ian@........... writes:

> I did a back-of-the-envelope calculation today for a 10 m long pipe, 
> 15cm in diameter, half filled with liquid, sitting on 1 support at each 
> end.  When the pipe is tilted by 1 micro radian, the difference in 
> loading between the posts is the equivalent of around 3 or 4 grams.
> 
> Could this be approached by monitoring the loading on the supports?
> 
> Ian Smith

Hi Ian,

       The total water mass would be about 88.3 Kgm, or 8.83 Kgm / meter, so 
even if you somehow just allowed the end 1m to flex, you would likely be on 
the limit of the accuracy / noise / drift of a force sensor at 4 gm, 1part in 
2000, which you may want to measure to 1%? An attempt to provide an offset force 
is limited by the thermal stabilty / compensation of the spring, which is 
done in a seismometer. 
       In general, you can measure very small movements to a much higher 
accuracy than the direct measurement of force. I don't immediatly see how weight 
measurement on a half filled trough would be practicable. You would also have 
dynamic inertia effects. Maybe totally fill the column and use a differential 
pressure sensor at it's centre? Measure just the direct inbalance in the system?

       Regards,

       Chris Chapman


In a message=20=
dated 31/08/05, ian@........... writes:



I did a back-of-the-envelop=
e calculation today for a 10 m long pipe,=20

15cm in diameter, half filled with liquid, sitting on 1 support at each=20

end.  When the pipe is tilted by 1 micro radian, the difference in=20

loading between the posts is the equivalent of around 3 or 4 grams.



Could this be approached by monitoring the loading on the supports?



Ian Smith




Hi Ian,



       The total water mass would be about=
 88.3 Kgm, or 8.83 Kgm / meter, so even if you somehow just allowed the end=20=
1m to flex, you would likely be on the limit of the accuracy / noise / drift=
 of a force sensor at 4 gm, 1part in 2000, which you may want to measure to=20=
1%? An attempt to provide an offset force is limited by the thermal stabilty=
 / compensation of the spring, which is done in a seismometer.=20

       In general, you can measure very sm=
all movements to a much higher accuracy than the direct measurement of force=
.. I don't immediatly see how weight measurement on a half filled trough woul=
d be practicable. You would also have dynamic inertia effects. Maybe totally=
 fill the column and use a differential pressure sensor at it's centre? Meas=
ure just the direct inbalance in the system?



       Regards,



       Chris Chapman