I like to use water as the weight.
----- Original Message ----- 
From: "Randall Pratt" 
To: 
Sent: Sunday, June 12, 2011 7:08 AM
Subject: RE: Seismograph Photo


> Hi Chris,
> 
> 
> 
> I am using strips of steel from a steel measuring tape for the pivot.  I cut
> strips about 1/8" wide across the tape and I use 2 pieces spaced about an
> inch to give side to side stiffness in boom motion.  A regular scissor will
> cut the tape and may also work to trim a double edge blade.  You are correct
> the boom is a bit high in the picture.  Before mounting the boom it can be
> hung from a thread in the pivot clamp and the line of the thread will point
> to the cg.  I think in most AS-1 types the cg is actually below the boom and
> the boom should be up and not level.  On my boom the coil below the arm and
> the extended damping plate below the arm tend to put the cg well below a
> horizontal arm at about the edge of the mass and the hinge side of the mass.
> The boom can also be balanced horizontally before mounting to get a cg for
> the lift test type of calibration or for a calibration coil computation.  
> 
> 
> 
> With my adjustable spring mounts I played with angles down to 30 degrees
> while I had the optical sensor.  Without temperature control it is to much
> of a struggle to maintain stability at lower angles and longer periods.
> The spring I have is negative length -0.191 inch.  I measured 6 weights
> between 7000 and 14500 grains and used linear regression on the
> measurements.  An easy way to do this is to hang a small plastic or paper
> cup from the spring and add lead bullets to the cup.   In the US many of us
> have reloading scales which can measure to 1/10 grain up to 500 at 7000
> grains per pound.  A vernier caliper can then measure extension from a fixed
> point to the cup rim allowing k to be determined fairly accurately.  After
> assembly, period measurement can then confirm the total boom mass since it
> is harder to weigh beyond the scale capacity.  
> 
> 
> 
> There is a hidden advantage to brazing the frame.  When the garage is 10
> degrees F the torch is warmer than a drill press!!!
> 
> 
> 
> Randy
> 
> 
> 
> 
> 
>
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