PSN-L Email List Message

Subject: Re: Spring Constant and Temperature
From: Brett Nordgren brett3nt@.............
Date: Fri, 12 Dec 2008 23:26:40 -0500

Hi Gary,

As Chris mentioned in his reply, feedback can help deal with support-spring 
drift.  What can be done is to roll off the instrument response at low 
frequencies, in particular its response to forces/accelerations, and make 
it fall toward zero at DC.  The effect, then, is that the loop will tend to 
cancel out any spring changes which are slow enough.  You would probably 
want to put the seismo in a can and surround it with thick insulation so 
that ambient changes only get through to the seismo very slowly, and so can 
be cancelled by the loop.

The integral feedback branch is the thing that does the job and if you have 
a good integrator circuit it could probably do enough good to allow for 
using a steel spring.  It would be particularly effective with daily 
variations, though for longer-term seasonal shifts, you'd probably need to 
adjust the mechanical balance to re-null the loop.  You don't want to be 
running the instrument with the integrator continuously straining hard to 
offset a large, permanent, spring change.

The main problem with Ni-Span-C, assuming that you can get some, is that 
its performance is highly dependent on the way it is cold-worked and heat 
It is also only about 2/3 as strong as steel, and so will have 'lower' 
performance as a spring, by some measures.


At 05:44 PM 12/12/2008 -0800, you wrote:

>As we know, the spring constant changes with temperature for the typical 
>materials used for springs. For instance, for carbon steel, the modulus of 
>elasticity is 29.5E6 psi at 70 degrees F and then changes to 28.8E6 psi at 
>200 degrees F. This means that the spring constant changes with 
>temperature. Is this a problem that I should be concerned about. My 
>vertical seismometer is located in the wine cellar and so the temperature 
>does not vary during the day, but the temperature may change 10 degrees F 
>with the seasons. Does anyone use a spring made with Ni-Span-C 902? This 
>material is extremely stable with temperature. The modulus of elasticity 
>is 27.79E6 psi at 0, 100, and 200 degrees F. The only problem is that this 
>material is not used very much and it s a made to order situation. Peck 
>Springs in Connecticut ( 860-747-5715) has made springs with material from 
>time to time. They are checking to see what the cost would be.
>Any thoughts?
>Gary Lindgren
>585 Lincoln Ave
>Palo Alto CA 94301
>Check out Lastest Seismometer Reading


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