## PSN-L Email List Message

From: ChrisAtUpw@.......
Date: Thu, 1 Sep 2005 08:06:44 EDT

```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.

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

```