PSN-L Email List Message

Subject: Re: swinging gate vs. folded pendulum
From: "Geoff" gmvoeth@...........
Date: Sun, 2 Sep 2007 21:00:22 -0700

Hello Chris;

Is it possible to translate motion from vertical to
horizontal so the vertical velocity geophone
will be low and long instead of tall and long.

Sort of like an internal combustion engine
will translate linear motion to rotory motion.

loss of vertical sensitivity ?
increases in appearent noise ??

It might possibly be easier to handle such a device
when buring it in the ground.

A velocity sensor is little more than
a electric generator, I think.


----- Original Message ----- 
Sent: Friday, August 31, 2007 10:55 AM
Subject: Re: swinging gate vs. folded pendulum

> In a message dated 2007/08/31, ciburch@........... writes:
>> Being new to the home-built seismograph business I wonder about the 
>> advantages / disadvantages of the "swinging gate" vs. the folded pendulum 
>> designs 
>> for horizontal component seismometers.  The swinging gate appears to have 
>> the 
>> advantage of simplicity requiring just two pivots while the FP requires 
>> eight.
>> Chuck I. Burch
> Hi Chuck,
>       The swinging gate type is probably quite a bit easier to make and 
> likely requires less skill to set up. The setup tilt sensitivities are the same. 
>       You are aiming for a sensor with a 15 to 30 second period, to bring in 
> the long period Love and Rayleigh waves at ~20 sec.
>       I suggest that you use either ball on a plane or a crossed roller 
> design of hinge for a swinging gate, but you can also use a single fine piano wire 
> in tension.
>       The T frame can be made out of 3" x1" U channel Aluminum with 1/8" 
> thick corner plates. Make the whole seismometer as 'one item'. This is very much 
> easier to set up and adjust. Use NdFeB magnet + Copper plate damping. I use SS 
> nuts washers and bolts from a boat / marine chandler. They are a bit more 
> expensive, but they are OK with Al and they don't rust. Ordinary steel rusts when 
> in contact with Al. 
>       You will need a thermally insulated enclosure to protect it from air 
> movements / drafts. We usually use 2" thick Celotex, gaffer taped and stuck 
> together with foam grouting, both from a builder's merchant. You can make widows 
> with two sheets of mylar transparency film for laser printers. You want to be 
> able to visually check the lateral balance of the seismometer, which may drift 
> with time, without removing the cover.
>       You can also make a horizontal sensor using a water manometer and 
> plastic tube, but you will have to solder your own electronics to a prepared 
> design.
>       Regards,
>       Chris Chapman   

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