PSN-L Email List Message

Subject: Re: Channel, 1,2 and 3?
From: John Lahr johnjan@........
Date: Thu, 27 Mar 2008 08:54:01 -0700

Hi Ted,

The ideal sensor would be a mass that remains fixed in space while 
the ground moves about it.
The attachment to the Earth needs to be "loose," in the sense that 
the position of the mass will not quickly respond to a change in the 
position of the attachment of the mass to the Earth.  Thus, a long 
period, simple pendulum makes a good sensor.

Your propeller pendulum will tend to move as a whole, without 
rotating, when the ground moves back and forth.  Thus, there will be 
little or no signal recorded.  Moving the blades by hand will 
indicate that the period is quite long, but unlike a simple pendulum, 
this does not translate into a sensitive detector.

I think that you're on the right track, building and experimenting to 
see what works and what does not.  Keep up the good work.


At 07:57 AM 3/27/2008, you wrote:
>Hi Chris and Everyone,  At this point in time, I do believe the 
>"Nearly Balanced Pendulum" makes for a "LOUSY" Sensor.  Although it 
>does pick up local noise really well.
>I have samples of this Ch3 devise, set at 20 second period, events 
>from the North, and the East, and the sensor recorded nothing,  I 
>reset the period to 7 seconds and received the Honduras from the S/E 
>and still nothing.  I am going to reset the period to 2 seconds and 
>test it one last time,  then rebuild it into a Lehman.
>Some of you have a good understanding of why this did not 
>work.  Chris indicated (below) some of the reasons.
>I think I understand.  However could someone explain why ch1 works 
>okay, and ch3 does not work at all.
>Ch1 is a 42" vertical pendulum, just hanging there like a clock 
>pendulum, same coil, magnet and hinge as the sensor in question.
>Ch3 'THE LOUSY SENSOR' is a 36" below the pivot pendulum and about 
>36" above the pivot, similar to an airplane propeller.  It has an 
>adjustable mass which can go up or down the shaft above the pivot to 
>extend the period......and this it does very well, as I can get very 
>long periods up to 53 seconds.
>I tend to learn many things from my failures, and this seems to be 
>one of those opportunities.  I did post all three channels on 
>PSN.  ch2 is a vertical.
>Cheers, Ted
>----- Original Message -----
>From: ChrisAtUpw@.......
>To: psn-l@..............
>Sent: Sunday, March 23, 2008 10:20 AM
>Subject: Re: Channel, 1,2 and 3?
>In a message dated 23/03/2008, 
>johnjan@........ writes:
>My understanding of a long period physical pendulum is that 
>sensitivity to ground motion is sacrificed in order to obtain the long period.
>Hi Ted,
>     That is NOT my understanding in general.
>     But you are using a nearly balanced pendulum, with a high 
> moment of inertia but a small C of G to suspension distance, 
> correct? This type have a long period but a LOUSY sensitivity to 
> linear ground motion. They are however, sensitive to ground rotation.
>     Longer period pendulums like the Lehman and the Romberg are 
> very sensitive to linear ground motion and make good seismometers.
>     Regards,
>     Chris Chapman


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