PSN-L Email List Message

Subject: Re: nonlinearities
From: Brett Nordgren brett3nt@.............
Date: Sat, 13 Aug 2011 18:38:41 -0400


I took some data from one occasion when you had baked a spring 
showing the noise before and after.



At 06:12 PM 8/13/2011, you wrote:
>The simple fact remains -- Feedback seismometers are the world 
>standard.  No other technology can come anywhere near their 
>performance and operational flexibility.
>Their noise levels and bandwidth of the typical off the shelf 
>instrument from several sources  are such that the only area where 
>significant improvement is desired relates to extremely long period 
>performance for the study of whole earth modes at 1000 seconds and longer.
>  Most of the development activity is related to making smaller and 
> less expensive instruments and greater  flexibility in 
> installation.  One significant exception is Metrozet  where a new 
> instrument to replace the STS-1 is under development.
>Non feedback instruments  are a relic of the past or short period 
>geophone or geophone-like instruments which have their niche in 
>local event monitoring and the amateur community.
>  The optical  open loop instrument is unlikely to have any success 
> (in my opinion) simply because it will never be stable.  When I 
> fist read the paper I concluded the project had no chance of 
> becoming an operational instrument but it was an interesting a 
> academic exercise. Others with the best credentials in the business 
> have shared that sentiment.
>Regarding creep effects --   When a spring is first installed in an 
>instrument there will be  "pops" related to what is probably 
>dislocation effects  in the spring material . Their frequency  will 
>gradually reduce in time. The solution is too bake the assembly, 
>with the spring at its operational stress, at ~ 160 C for several 
>hours. This will essentially  eliminate the effect.
>After the bake the spring can be removed and reinstalled as long as 
>the stress is applied in the same direction upon 
>reinstallation.  The springs in my instruments are typically at a 
>stress of 150,000 to 190,000 psi with a yield strength of ~300,000 
>psi. (17-7 stainless treated to CH900 condition )
>Dave Nelson
>Rolling Hills Estates , California


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