PSN-L Email List Message

Subject: Re: Springs for Verticals
From: Ben Bradley benbradley@...............
Date: Mon, 04 Jun 2007 17:39:01 -0500

tchannel wrote:
> Hi Everyone,  I wanted to ask some questions about choosing a spring for a vertical spring sensor.
>    Pictures three situations, three different springs being pulled straight down by a mass. No triangular modifications, just straight down.
> 1 One is a strong spring like a screen door spring, pulled down by a large mass.
> 2  One is a weak spring  being pulled down by very little mass.
> 3  One is a rubber band being pulled down by a small mass.
> Just for comparisons, say they all had the same period of 1.5 seconds.   Is one of the three better for recording earthquakes?   Two questions here:  Is there an advantage in using a weak spring, or strong spring, if the resulting period is the same.  And Secondly is a rubber band spring every used?

   I understand that rubber bands are quite temperature sensitive (their 
force changes substantially with a change in temperature, many times 
more than a steel spring does), and for that reason alone I cannot 
imagine a rubber band would work well in a geophone or seismometer. The 
effect is so great that it can be put to good use in other devices, as 
in The Amateur Scientist column in the April 1971 Scientific American 
magazine, titled "Some Delightful Engines Driven By the Heating of 
Rubber Bands." You can get a CD of all the Amateur Scientist columns, 
which also includes several seismograph designs, at the bottom of this page:

   As far as 1 vs. 2, I can't say which one would be better, except to 
note that the larger mass/stronger spring combination of 1 would be less 
sensitive to air movements (there should be no such air movements in 
such a device, but I would still pick the more resistant combination 
just in case). Without any other observations or conclusions about which 
would be better, I would pick 1 for that reason.

> What prompted the question is seeing how small the spring in a geophones is.    Is the idea to have the smallest spring which would move under the smaller stimulus.

   For a "portable" device such as a geophone, I would think the idea is 
to have the smallest device that does the job. If a larger, more massive 
design does the job better in a fixed-location amateur (non-commercial) 
device, I would go with the larger design.

> Thanks, Ted


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