PSN-L Email List Message

Subject: Re: Short period spring vertical lower pivot question
From: "Geoffrey" gmvoeth@...........
Date: Fri, 17 Jul 2009 11:20:59 -0700

Hello PSN;

I have found by surrounding my electronics
with a series of small jars filled with
salt water I am able to eliminate some
low frequency undulations in the output.

I think surrounding anything with a quantity
of such water will moderate thermal changes
by adding thermal inertia to the system.
Sort of like a thermal capacitor.

Has anyone ever tried such a thing before
with one of your Very Low Frequency geophones?

Here it is like 115F outside anf 85F inside
the air conditioner will cycle with variations
up to 20F. I believe the water is greatly moderating
those thermal undulations and their effects
on the electronics.

If only I could surround the entire house with such
a water jacket maybe 3 ft thick.


----- Original Message ----- 
From: "meredith lamb" 
Sent: Friday, July 17, 2009 8:14 AM
Subject: Re: Short period spring vertical lower pivot question

> Hi Chris,
> Yes; the commercial available springs are a problem (unless one is satisfied
> with a 1 to ~ 6 sec period vertical); and/or
> use a form of feedback.  Even with a zero length spring (such as in my old
> Sprengnether), temperature changes made
> it tough to keep roughly centered within a series of 3 ~ enclosures with a
> set period of 15 seconds.
> Just noted a interesting web article this morning on the Physics Forums web
> site about "Designing and making springs
> with music wire" (Circa May-June 2009).  Its at:
> Although the wire was twisted prior to spring forming; it doesn't really
> sound like a "real" zero length spring too
> me....but....such might be ~ better ~ than a common hardware spring.  I
> don't have the expertise to evaluate the spring discussion technical
> implications to be frank.
> Take care, Meredith Lamb
> On Wed, Jul 15, 2009 at 4:58 PM,  wrote:
>>  In a message dated 15/07/2009, paleoartifact@......... writes:
>> Outside of buying anything.... versus home building,  the old Georgia Tech
>> vertical (Classroom
>>  Demonstration model) still remains as a quite interesting experimental
>> setup; especially because they claim it is capable of much longer
>> periods. It is rather strange that after some 11 years of its existence on
>> the web that there is still no known private individual that has garnered up
>> a web site on such with their opinion of merit.
>> Of course the original design was of wood and some metal, and lacked a
>> basic viable sensor and dampening; but the spring and pivot arrangement was
>> the main very interesting approach idea for a long period seismometer.
>> For readers it is at:
>>  Hi Meredith,
>>     I agree that this is a good design. The problem with all amateur
>> vertical sensors is the spring. Steel coil springs have quite a high
>> temperature coefficient of the modulus, so you are limited to periods of
>> less than about 6 seconds for simple systems, unless you add a full force
>> feedback loop. If you try to make a longer natural period, the system will
>> simply collapse as the room temperature increases.
>>     The second limitation is in the extension springs that you can buy.
>> They all have a relatively low initial tension, whereas seismometer springs
>> require quite a high initial tension. You can wind springs with a much
>> higher initial tensions by either twisting the wire as it is wound onto a
>> mandrel, or by pre-bending the wire in the direction of the spring axis, as
>> LaCoste did.
>>     This design can also be compensated for atmospheric pressure noise -
>> the principle source of noise by a large factor in uncompensated vertical
>> systems.
>>     Regards,
>>     Chris Chapman

Public Seismic Network Mailing List (PSN-L)

[ Top ] [ Back ] [ Home Page ]