Hi Roger,

I it very interesting that you have had a similar problem, and I 
appreciate your suggestions.  I my case, the sensor in in the basement, 
sitting on a concrete floor.  Several years ago I epoxied three small 
aluminum plates to the floor for the settling screws to sit on.  My 
concern at the time was that turning the leveling screw against the 
concrete was grinding into the floor, leaving concrete dust under the 
screw.  One of the probable flaws in my construction is that the 
leveling screw is very small (#4) and more than likely not really firm 
enough.  But, when I'm not being "haunted" the thing performs pretty 
well.  Seems like problems stemming from the mechanical design shouldn't 
be episodic the way I have having them.  I am using a 3 point mount, and 
the other two feet (base of the triangle) are sturdier.

One thing that your comments encourage me to revisit is the way the 
cover over the sensor is made.  It is made of 1/4/inch particle board 
and a little heavy on the heavy side.  I is just sitting on the base 
frame of the sensor, held down by it's own weight.  There are soft 
plastic feet attached where the contact is made to the frame.   I don't 
normally make a point of pressing everything down to reseat things after 
i adjust it.  Never occurred to me to do that.  One thing I did try 
since this last episode started was to put a little piece of tape under 
each foot, to introduce a little "sqisshyness" to prevent the kind of 
"micro-rocking" that you apparently had.  I didn't see any obvious 
difference.

Guess I'm going to have to embark on a real science project.

Larry




Roger Sparks wrote:
> 
> Hi Larry,
> 
> Sorry that you are having the noise problem after adjusting your
> seismometer.
> 
> I had a similar problem for a while that was caused by poor contact to
> the solid surface.  In my case, I was going through a rug to a concrete
> surface.  For a while, one leg did not properly contact the hard
> surface.  The microsiems caused a rocking that displayed as noise.
> 
> I did not know I had a problem until an FFT of the noisy signal showed a
> higher frequency spectra that was not present in the older traces.  The
> fix was simply to beef up my penetrations through the rug.
> 
> Are you using a three leg mount?  Are the contacts to the hard surface
> made with points?  Do you have a locking nut or other mechanism to fix
> the adjusting screws after adjustment?  Do you have any loose parts that
> could "rock" due to the microsiems?  Do you "set" your seismometer after
> adjustment by putting extra pressure to help reset the contact points?
> 
> Good luck on the repairs,
> 
> Roger
> 
> 
>>
>> .------ ------ ------ ------ ------ ------ ------ ------ ------ ------.
>> | Message 3                                                           |
>> '------ ------ ------ ------ ------ ------ ------ ------ ------ ------'
>> Subject: Seismograph noise problem
>> From:    Larry Conklin 
>> Date:    Sat, 09 Feb 2008 11:24:17 -0500
>>
>> Hi all,
>>
>> I think I must have a haunted system.  I have been running a SG 
>> seismometer for about seven years.  The mechanical design is very 
>> similar to the one described on the PSN web site, and I am Using 
>> Larry's electronics board.  Over nearly the entire time I have been 
>> running this system I have had repeated episodes of extreme noise or 
>> some sort of spurious signal.  When It occurs, the problem persists 
>> with little change for anywhere from a few days to several months, and 
>> then seems to fade away over the course of several days.  The problem 
>> reoccurred a couple of weeks ago.  It was apparently precipitated by 
>> the disturbance I caused by adjusting the leveling screw to recenter 
>> the pendulum (something I do frequently, with no problem).  Before 
>> making the adjustment, I was getting about +/- 30 counts of signal 
>> excursion from the low frequency channel.  Immediately after, I got 
>> around +/- 350 counts, rendering the system essentially worthless.
>>
>> I have put a lot of effort into trying to figure out what is going on, 
>> to no avail.  In one of the previous episodes, I disconnected the 
>> power to the oscillator that drives the antenna plate, and opened the 
>> loop for the feedback damping.  There was no significant change in the 
>> output, which led me at the time to conclude that there must either be 
>> something wrong with the electronics board,
>> or some sort of electrical/magnetic pick-up.  Despite a lot of 
>> diddling around, I couldn't determine a cause, and eventually, the 
>> system settled down without my having done anything specific to fixing 
>> it.  And, neither theory fits well with this current episode, which 
>> started when I mechanically disurbed the sensor a little by adjusting 
>> the leveling.
>>
>> So, do I have a poltergeist detector when I thought I had a 
>> seismometer?   I threw together a web page that shows the onset of the 
>> problem, as well as short time intervals  before and after the problem 
>> started this time.  If anyone cares to take a look at it and offer 
>> their thoughts (or condolences), I'd like to hear them.
>>
>>     http://home.twcny.rr.com/lwconklin/Seismograph_Noise_Problem.html
>>
>> Larry Conklin
>> Liverpool, NY
>> lconklin@............
>>
>>
>>     
> 
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