PSN-L Email List Message

Subject: Re: a possible seismometer?
From: "Daryl P. Dacko" mycrump@........
Date: Mon, 08 Dec 2008 18:35:25 -0500


In the old Scientific American "Amateur Scientist" column there is a 
discussion or two about using these as seismometers and tiltmeters.
They talked about using capacitive detectors at the ends to measure the 
displacement, if memory serves.
Some discussion on damping factors too, I believe.

You can get a compilation of all of the columns on CD for a very 
reasonable charge on the web, highly recommended...

Now where did I put that CD... ;')


Randall Peters wrote:
> I've been contemplating an instrument based on a piece of equipment we
> use in the physics department to teach hydrostatics.  It is a long tube
> that is sealed on one end, and the open end is immersed in a larger tube
> filled with water.  The smaller tube achieves an equilibrium position
> determined by its weight, overall length, and diameter of the internal
> air column.  If one lifts the inner tube and releases it, it oscillates
> in simple harmonic motion with a period greater than one second.  Also,
> because of the water, the motion is dampened.  The motion depends on the
> compressibility of the trapped air.
>     I've done an idealized calculation in which the water would be
> replaced with mineral oil and the plastic tube of the apparatus with a
> thinwall metal tube of length in the neighborhood of one meter.  It
> appears that the damping would be close to optimal and the period just
> under two seconds for the diameter of the inner tube being one
> centimeter and that of the outer tube holding the mineral oil about two
> cm's.
>     The inner tube should be constrained (perhaps with long triaxial
> threads, so that the motion is strictly vertical.
> I will be interested to know if any of you have considered such an
> apparatus.
>      Randall


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