PSN-L Email List Message
Subject: Re: Simple broadband Seismometer
From: Gordon Couger gcouger@..........
Date: Fri, 02 Sep 2005 23:15:30 -0500
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
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.
> In a message dated 02/09/2005, gcouger@.......... writes:
> I am late to the discussion so I have missed things. Are
> the principle of two cans with large areas connect
together by a
> tube with a very small diameter bore and an air bubble
> 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
> surface of the bubble blocks nearly 100% of the light
shined on it
> so photocells or rotating photo paper can easily track
> 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
> the middle of a seismometer. Air bubbles are inherently very
> compressible. Maybe a heavily dyed oil 'bubble' or some
> work? You can choose the diameter of the main tube to give
> correct damping. Without knowing the dimension of what you
> '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
> tubes are used in some large tiltmeters. I don't think that
> found space on the back of his envelope to do detailed
> issues like the end fittings, the type of sensor, the limits
> and drift, or how such a system behaves dynamically, damping,
> 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
> it's performance.
> I did a back-of-the-envelope calculation today for a 10 m
> 15cm in diameter, half filled with liquid, sitting on 1
> each end. When the pipe is tilted by 1 micro radian, the
> in loading between the posts is the equivalent of around
3 or 4 grams.
> Could this be approached by monitoring the loading on the
> 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
> > changes. If some piezo sensor can withstand a
> by 44
> > Kg (2 supports), then 2 of them might give the
> signal. By
> > electrically differencing the signals from the 2 piezo
> sensors, the
> > remainder is the changing mass + noise. Just a
> > Hi Ian,
> > I don't really see why you need such a large pipe,
> > 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
> > and drifts just doesn't work to better than about 0.1%.
> > When it is dead easy to measure water levels to
> > 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.
> Chris Chapman
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