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.

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