PSN-L Email List Message

Subject: Re: Seismometer Pivot Studies - email
From: Charles Patton charles.r.patton@........
Date: Thu, 16 Oct 2008 23:22:35 -0700

(Bob sent the following email and said it would be OK to share it and my 
reply with the PSN list.)
Bobhelenmcclure@....... wrote:
You advise the following:

Do NOT use either a 'point in a cup' or a genuine metal knife edge on a 
flat for a suspension system. Both apply stresses over or near the 
maximum strength of the metal and even if they work initially, they will 
not last long.  If the edges do not either roll over or shatter, they 
will dig into the counterface. These systems are a common cause of poor 
performance in amateur seismometers.
   I have violated this advice for years, and have never experienced any 
problems. I use pivots and sapphire cups salvaged from milliammeters. My 
horizontal pendulums weigh only 100 grams, but this is still a much 
higher load than the original meter coil assemblies imposed. My 
horizontal sensors are described at

   Please feel free to comment.



Hi Bob,
I hadn't seen your Lehman before. Nice.  I certainly won't argue with 
success, but I would make a couple of quick comments.

Certainly a great contributor to your success with jewel pivots was the 
sparing use of weight, but I wonder if that doesn't make you a bit more 
susceptible to drafts? Also there have been several questions to the PSN 
list from beginners in the vein of, " many pounds should my bob 
weigh?"  So there is a compromise somewhere there -- and I honestly 
don't know where it should be.  There is a minimum mass number that is 
set by thermal motion noise.  See:
and an excerpt of the paper by Erhard Wielandt, Institute of Geophysics, 
Stuttgart University is as follows:
“Brownian (thermal) noise
The resolution of an inertial seismometer is ultimately limited by the 
Brownian motion of its seismic mass. The energy of this motion can be 
concentrated near the mechanical eigenfrequency by minimizing the 
damping (maximizing the quality factor). Since this enhances at the same 
time the response to ground motion near the eigenfrequency, a high-Q 
suspension in fact has a reduced noise level at all frequencies when the 
noise is expressed as an equivalent ground motion. Treating the 
mechanical pendulum as a simple damped harmonic oscillator with mass M 
[kg], period T[s] and quality factor Q, one finds that the coefficients 
involved cancel approximately, and the condition for resolving minimum 
ground noise in the free-mode band is simply MTQ > 1 kg s. So 
theoretically, even a pendulum with a small mass or a small free period 
could be used for very-long period recording when the quality factor is 
high enough. However, this is not generally true. The 
harmonic-oscillator model neglects the fact that the mechanical damping 
(especially of thermally self compensated spring materials) is not 
viscous. For a given pendulum with an adjustable period such as a 
LaCoste suspension, the quality factor decreases more rapidly at long 
eigenperiods than it should do for viscous damping.  Therefore, the 
Brownian noise of mechanical suspensions increases at long periods, 
comparable to the 1/f-noise of semiconductors. Unfortunately, very few 
data are available on this phenomenon.”

I was recently reading a patent(?) on the invention of the taut-band 
panel meter movement for use in long term industrial service (such as 
power stations, etc.)  They knew that the meter life was limited by the 
jewel movement, and were touting the lifetime of the taut-band removing 
that limitation.  Your use of jewels with heavier loading than the 
original movement certainly brings into question operating life. 
Additionally, what kind of damping do you experience without the 
resistor loading and magnet assembly in place?  This would equate back 
to the pivot friction plus the air eddy loss.   If there is intent to 
use force feedback at some point, then the Q of the setup becomes of 
interest (as does the thermal noise for that matter.)
Chris remarked, “…What stiction levels do you get? Static friction is 
always greater than dynamic friction and it is always present in a 
mechanical sliding system. I can remember having to change meters which 
had 'sticky' suspensions.”

I can certainly identify with the copper magnetic properties.  Some 50+ 
years ago I made a scale with soda straws and straight pin on razor 
blade edges.  I attached a coil on the end to make a meter and noticed 
the attraction when I added the magnets.  But with regard to your 
implementation of the Lehman, you made the argument yourself for 
increased bob weight in order to get around the diamagnetic, and 
paramagnetic effects of your coil.  This is useful information to argue 
for the use of capacitance sensing with aluminum vanes?  (Although even 
there and with the use of varible reluctance, too, there are direct 
attraction effects from the voltages (or magnetics) at the levels we're 
interested in.
Chris remarked, “…Another point which Bob has not mentioned is that 
perspex is diamagnetic.  I am well aware of some copper wire having Fe 
traces.  I have had severe problems with aluminum vanes on Lehman 
seismometers giving instability when the sensor has drifted off centre. 
  Pure Cu seems to be OK.  You need to keep the outer magnet high field 
edges inside the damping plate at all times.

But back to the original point of the paper.  I don't pretend it's the 
end all of the discussion, but rather a rough guide to help center the 
builder's approach.  There are always other ways to accomplish an end 
goal, but some paths generally have more leeway while on the way to 
success. We're just trying to give some rough rules-of-thumb that might 
help guide the unwary.  Remember the experiments of Robt. Baker in Sci 
Am? He used glass slides, RTV and razor blade edges with optical (LED) 
sensors.  Fascinating approach.  Never have heard about whether it had 
long term stability problems, though, which would be my concern.
Chris had a bit stronger comment, “Vividly. A really rubbish 
construction which was sensitive to everything and did not enable you to 
separate out any single effect.”

If it is OK with you, I would really like to post this email to the PSN 
list, I think it's beneficial that #1, people see other approaches are 
possible, but #2 what I think some of the trade-offs are.  I certainly 
am not a Lehman authority, I just started this discussion/paper on 
pivots because of that very fact.  I wanted more insight into the 
possibilities.  Discussions with Brett and Chris certainly helped guide 
me.  The paper was a distillation of those discussions so the PSN 
community can share in some of the conclusions -- and de-bunk them if 

Charles R Patton

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