PSN-L Email List Message

Subject: Re: Questions from a novice
From: ChrisAtUpw@.......
Date: Thu, 13 Sep 2001 22:29:43 EDT


In a message dated 14/09/01, dale.hughes@................ writes:

> The geophone uses 2 rare earth magnets attached to a 375gm aluminium
> cylinder supported by a spring inside a 50mm tube. The pickup coil is 12000 
> turns of fine wire on the outside of the tube. Damping has yet to be 
> optimised and is currently acheived by felt strips stuck to the side of the 
> Al cylinder so that friction brings the mass to rest in a few seconds - 
> 

Hi there Dale,

       Can you damp the motion by attaching a rod(s) with a disk on the end 
down from the mass and dipping it into an oilbath? You do need the weight to 
be critically damped - if you push the mass down a tiny bit, it should return 
to the rest position, but it should not oscillate at all. Sorry, but I very 
much doubt if the stick and slip friction given by the felt will be 
satisfactory. Suggest you use a brass weight for your next version. It has a 
relative density of 8.5, so you can submerge the whole mass into mineral oil 
which has a relative density of ~0.9, with not much loss of weight. Al has a 
relative density of only 2.7, so submerging it would reduce the weight to 
~1.8 / 2.7 times the weight in air. You do need to submerge the weight or the 
surface tension will probably pull it up against the cylinder wall.

       Good luck!

       Chris Chapman
In a message dated 14/09/01, dale.hughes@................ writes:


The ge ophone uses 2 rare earth magnets attached to a 375gm aluminium
cylinder supported by a spring inside a 50mm tube. The pickup coil is 12000 turns of fine wire on the outside of the tube. Damping has yet to be optimised and is currently acheived by felt strips stuck to the side of the Al cylinder so that friction brings the mass to rest in a few seconds - otherwise it oscillates for many minutes.


Hi there Dale,

      Can you damp the motion by attaching a rod(s) with a disk on the end down from the mas s and dipping it into an oilbath? You do need the weight to be critically damped - if you push the mass down a tiny bit, it sho uld return to the rest position, but it should not oscillate at all. Sorry, but I very much doubt if the stick and slip frictio n given by the felt will be satisfactory. Suggest you use a brass weight for your next version. It has a relative density of 8. 5, so you can submerge the whole mass into mineral oil which has a relative density of ~0.9, with not much loss of weight. Al h as a relative density of only 2.7, so submerging it would reduce the weight to ~1.8 / 2.7 times the weight in air. You do need to submerge the weight or the surface tension will probably pull it up against the cylinder wall.

      Good luck!

      Chris Chapman

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Larry Cochrane <cochrane@..............>