## PSN-L Email List Message

From: ian ian@...........
Date: Thu, 01 Sep 2005 22:05:09 +0100

```Hi,

one assumption I made was that the mean signal, generated by the mean
mass is subtracted and you are then only measuring the changes.  If some
piezo sensor can withstand a load generated by 44 Kg (2 supports), then
2 of them might give the required signal.  By electrically differencing
the signals from the 2 piezo sensors, the remainder is the changing mass
+ noise.  Just a thought...

Ian

ChrisAtUpw@....... wrote:

> 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
>

Hi,

one assumption I made was that the mean signal, generated by the mean
mass is subtracted and you are then only measuring the changes.  If
some piezo sensor can withstand a load generated by 44 Kg (2 supports),
then 2 of them might give the required signal.  By electrically
differencing the signals from the 2 piezo sensors, the remainder is the
changing mass + noise.  Just a thought...

Ian

ChrisAtUpw@....... wrote:
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.

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

```