PSN-L Email List Message

Subject: Re: Springs for Verticals
From: "Geoffrey" gmvoeth@...........
Date: Tue, 5 Jun 2007 00:20:48 -0700

To me, from personal experience, I say a very large mass
is your best bet for the best signal to noise ratio.

Just about any extension spring will give you
a one second period if the spring will
extent ten to twelve inches undere
the weights mass.
That is of course if the spring has no
inherent tension when fully relaxed.

It might be best to have a spring in the shape of a
flattened helix then use a plain old wire spring.
I think you can give such a spring a pre-tension
that means it will only hang extended a small bit
instead of a full 10 to 12 inches with mass attached.

Such a spring should be built by an industry or
research lab insted of a common man
because the lowest of temperature coefficients are
necessary for a good spring otherwise you will need to
control temps within tight specs. like having the mechanisim
located in a deep cave.

Sensitivity as well as signal to noise ratio are the two things
that make a good sensor.

Good Luck,

----- Original Message ----- 
From: "Ben Bradley" 
Sent: Monday, June 04, 2007 15:39
Subject: Re: Springs for Verticals

> 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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