Chris,

See: www.sas.org/E-Bulletin/2001-10-05/labNotes2/labNotes.html

The small tube is typically a glass tube drawn down to less than 
..01 inch diameter maybe a lot smaller. The bubble is injected in 
one end an moved to the center by adding fluid drop by drop.

The miscues formed by the air water interface makes a large dark 
band across a sensor just try it with a laser pointer and glass 
of water. In fact the shadow of the air water interface is so 
large it is a problem to get very much dynamic range over the 
range of a CCD sensor unless you look at  just one edge of the 
water bubble.

Temperature would be more of problem than pressure on the size 
of the bubble. But if the two edges of the bubble are both 
tracked the bubble size can be canceled out. This would take a 
rather complex machine vision project. Not beyond the means of 
an amateur  but rather ambitious on any level as a complete problem.

A more realistic solution may be use a very thin grade of oil 
and steel piston between a pair of coils on the tube to sense 
the its movement and position by using the coils in an AC bridge 
circuit. The tube could be brass, aluminum, or any non magnetic 
material and the piston any magnetic material.  The problem is 
with more crap than there is with a bubble. Sensing the potion 
of a bubble in a magnetic guild might be easier than sensing one 
with light if you can find a magnetic guild that keeps the 
partials in suspension over a long term.

Gordon



ChrisAtUpw@....... wrote:
 > In a message dated 02/09/2005, gcouger@.......... writes:
 >
 >     Chris,
 >
 >     I am late to the discussion so I have missed things. Are 
you using
 >     the principle of two cans with large 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
 >
 > Hi Gordon,
 >
 >     I am aware that it is possible to optically sense the 
ends of an air
 > bubble in a tube, but I am rather doubtful if this would be 
practical in
 > the middle of a seismometer. Air bubbles are inherently very
 > compressible. Maybe a heavily dyed oil 'bubble' or some 
mercury would
 > work? You can choose the diameter of the main tube to give 
roughly the
 > correct damping. Without knowing the dimension of what you 
are calling
 > 'a small diameter tube',  I can't really comment.
 >
 >     Ian was suggesting making a horizontal seismometer using 
a 15 cm dia
 > tube, half filled with water and weighing the ends. Half 
filled sensor
 > tubes are used in some large tiltmeters. I don't think that 
Ian had
 > found space on the back of his envelope to do detailed 
analysis of
 > issues like the end fittings, the type of sensor, the limits 
of accuracy
 > and drift, or how such a system behaves dynamically, damping, 
wave
 > formation, etc. I am not sure how you could support a 10m 
tube at the
 > ends, without the centre section oscillating like fun.
 >
 >     I hope that Ian can produce a full scale working model 
and measure
 > it's performance.
 >
 >     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
 >
 >     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 are plenty of problems in making precision
 >     measurements. 'Doing
 >      > things the hard way' is just plain dumb.
 >
 >     Regards,
 >
 >     Chris Chapman

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