Ted,

The kind of damping you need will generate a retarding force which is 
approximately proportional to velocity.  For very slow motions, it 
generates almost no damping force.  Most types of sliding friction are 
large until the joint 'breaks away' and starts moving, and then are 
relatively constant with velocity.  This is definitely not what you want 
for seismo damping.  Chris' magnets are a very good way of getting what you 
need--no static 'break away' and zero damping force at DC, but having the 
damping force linearly increasing with frequency.

As far as hinges go, flexures are pretty good, and some rolling designs are 
also.  Both have essentially no static, break away, friction.

An undamped sensor is actually more sensitive than a damped one.  It's just 
that an undamped sensor's output won't look at all like the actual earth 
motion you are trying to record.  With damping, you trade sensitivity for 
accuracy.

Regards,
Brett

At 12:21 PM 10/3/2008 -0600, you wrote:
>Hi Folks,
>
>I understand the necessity of using a Damper to keep the arm or boom from 
>oscillate or belling after a signal begins.   I would guess, after an 
>earthquake signal arrives, an undamper arm would continue to oscillate for 
>many minutes, overwriting most of the incoming signature.
>
>I also understand the importance of minimizing the friction of the 
>sensor's hinges.   The signals are very small and they must overcome the 
>friction before any movement of the arm can take place.
>
>My question is:  Does a Damper, oil or magnetic, not act the same as hinge 
>friction?, in that, the signal must, first, overcome the resistance of the 
>Damper?
>
>Is an undamper sensor  more sensitive than a dampered sensor?    I have 
>never tried this.   Again, I know the signature would be of little value, 
>but I am curious about the friction of the dampers.
>
>Thanks, Ted


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