Chris,

I am late to the discussion so I have missed things. Are you 
using the principle of to cans with largo areas connect together 
by  a tube with a very small diameter bore and an air bubble 
introduced into the bore will amplify the change in levels in 
one can related to the other and the change in the position of 
the bubble depends on the are of the can to the area of the tube 
connecting them.

Fantastic amplification is possible with very simple tools. The 
surface  of the bubble blocks nearly 100 % of the light shined 
on it so photocells or rotating photo paper can easily track the 
  earths movements.

Gordon

ChrisAtUpw@....... wrote:
 > In a message dated 01/09/2005, ian@........... writes:
 >
 >     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...
 >
 > Hi Ian,
 >
 >     I don't really see why you need such a large pipe, when 
something
 > nearer 1" would do fine. If you put on larger diameter end 
pots, you
 > increase the period.
 >     Trying to match up two sensors allowing for temperature 
coefficients
 > and drifts just doesn't work to better than about 0.1%.
 >     When it is dead easy to measure water levels to microns, 
why bother
 > with weight sensors? The inexpensive piezo sensors drift in 
output
 > voltage with temperature.
 >     There re plenty of problems in making precision 
measurements. 'Doing
 > things the hard way' is just plain dumb.
 >
 >     Regards,
 >
 >     Chris Chapman
 >

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