PSN-L Email List Message

Subject: Re: My thoughts on seismic data filtering
From: ChrisAtUpw@.......
Date: Sun, 29 Aug 2004 06:28:42 -0400

28/8/2004 From:"rem11560@..........."  

Since it is very difficult to build open loop vertical sensors having more than a few seconds natural period.

Hi Bob,

     This is only true if you use short arm Lehmans and a point on a plane bottom hinge. If you make the arm 1 m long and use foil or wire, crossed cylinder, or ball on flat suspensions, you should be able to get 20 to 30 seconds period with few if any problems. Having the 1 m arm reduces the admittedly rather large sensitivity to tilt. Getting reliable operation out of a 1 ft arm at 30 seconds requires very high quality hinges and precision slow motion levelling screws, but you have a high tilt sensitivity built in. 
     Crossed cylinder suspensions may be made from the shanks of solid tungsten carbide 0.125" drills. The finish seems to be good on the ones that I have tried, but you can lap / polish with diamond paste and a copper tube. Ball on a flat systems can be made using stainless steel ball bearings resting on a ~0.3" sided tungsten carbide flat topped lathe ''cutting tool insert''. Again, I have found the top finish adequate, but you can polish the surface to a mirror finish without too much effort. You do need these really hard corrosion resistant materials, but they are not too expensive and should be readily available from engineering suppliers.


     Chris Chapman


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