Hi gang,
   There has been some recent discussion about calibrating seismometers. 
    Several years ago, my article on a calibrator using a meter movement 
as a force transducer was put on our 'home'  www.seismic.com.  Go to 
"Build Your Own Seismographic Station" then to "Article by Bob Barns".
   Jack Ivey built one these and I asked him to describe his experience. 
  His reply is below.  It includes an interesting remark about 
differences in behavior between large and small motions.  I have not 
seen this discussed before.
   We would benefit if others with experience with the calibrator were 
to report.
Bob Barns
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Bob,
I used the calibrator extensively in determining the response of a couple
of my Lehman derivatives.  I found it very easy to set up and use and
indispensable in determining the period and damping of the system, even
if you aren't terribly interested in the absolute calibration of your
seismometer.

This is because the return force and damping of a practical Lehman system
can be quite nonlinear, and if you adjust the period and damping by bumping
the boom with your thumb and eyeballing it, the behavior is quite different
than when you start measuring nanometer-scale movements (I found there
was way less damping for small displacements than for large).  The
calibrator is one of the only practical ways I've seen to apply the tiny
force needed.

I also think the quakes people post would be considerably more
interesting if they had an engineering units scale marked.

I tried to use your arrangement to calibrate my force-feedback add-on
to a 1-second vertical, however the force produced by the meter movement
was too small to be useful.  It would be nice to have an arrangement that
produces 10x or 100x the force for that application (I sacrificed several
types of meter in the attempt).

You're welcome to use these comments as you wish, no need to credit me.

Jack
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