PSN-L Email List Message
Subject: Re: Channel, 1,2 and 3?
Date: Thu, 27 Mar 2008 11:26:53 -0600
Hi John, This explanation paints a very clear picture, and I now
understand. Your are right, we learn be trying different things. This
failure is simply a good lesson on how, this pendulum moves, and how it does
I made a change: Let me try to anticipate what will happy, on the next
The bottom of the pendulum to pivot is 36". The pivot to the top of the
pendulum was also about 36", and the mass above and below the pivot are
I removed the threaded rod from the top, which makes up about 15", but it
was the major part of the top mass. This leaves 36" below the pivot and 21"
above the pivot. Now,The top 21" above the pivot contain almost no mass, it
is just a hollow tube. It now has a 2 second period.
I anticipate it will record earthquake, similar to the Ch1 which is a 42"
pendulum with nothing above the pivot.
I will post the results.
----- Original Message -----
From: "John Lahr"
Sent: Thursday, March 27, 2008 9:54 AM
Subject: Re: Channel, 1,2 and 3?
> 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
>>----- Original Message -----
>>Sent: Sunday, March 23, 2008 10:20 AM
>>Subject: Re: Channel, 1,2 and 3?
>>In a message dated 23/03/2008, johnjan@........
>>My understanding of a long period physical pendulum is that sensitivity to
>>ground motion is sacrificed in order to obtain the long period.
>> 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.
>> Chris Chapman
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