PSN-L Email List Message

Subject: General-Stability of "Lehman" sensor
From: "Connie and Jim Lehman" lehmancj@...........
Date: Thu, 28 Nov 2002 11:28:55 -0500



PSN-Friends--I have read with interest the exchanges regarding stability =
and modifications to the so-called "Lehman" seismic design.  The plans =
as published in July, 1979 Sci. American were the result of 5 years of =
running the system in a number of environments, ---if I remember we =
tried 4 different types of Milne cantilever long period units, and =
settled on the design published.  As I have said before, the system--as =
most seismic designs--lends itself to modifications & improvements.
   The BallBearing and Tensional Wire designs for the boom end sound =
ingenious--although I have never tried them.  I stayed with the 15-cent =
hardware 5/16 or so bolt ground to a knife edge.  When mounted properly =
against a hard plate or flattened bolt head epoxied to the upright, I =
never experienced any undo friction or flattening of the knife edge.  =
Microseisms were always there--in fact one would have to run fast to get =
away from microseisms.  One amateur enthusiast in Puerto Rico had to =
settle for his system on the 14th floor of a high-rise apartment =
building.  In windy weather the building swayed, and the microseisms =
just "rode the swells" as he put it.
    It is easy to "overkill" the location of the sensor.  Ground level =
on a concrete floor with little temperature or humidity changes is =
ideal.  Direct connection with bedrock is not necessary.  One of the =
best locations during the testing time was in an unused room of a =
laboratory building.  Several feet of fill clay had been  leveled, and =
several inches of gravel on top.  Any bedrock (limestone here is =
Virginia) was probably 10 or so feet away.  We laid down 3 small garden- =
shop masonry step-stones------mounted our system--let it settle for a =
day or two, and began recording.  Later the area was finished with =
concrete floor, etc, and placing our sensor there gave similar =
satisfaction--only now more building noise was evident--vibrations from =
the Chiller 150 ft. away, and stresses in the building and stairwells as =
students moved to and from classes.
   I do feel a kinship with persons frustrated in setting up and =
stabilizing a system especially in a closed space.  The Sci. American =
design should be stable for weeks at at least 15 second period.  A good =
heavy base material other than wood is best.  Composite material like =
laboratory table tops is ideal.  Rather than have adjustable tri --feet =
on the base, I would suggest firm solid feet--fixed bolts or similar, =
and then use thin shims to complete the task once you have the mechanics =
of the system in the "ball park".  Five or ten mil thick sheet metal =
pieces work great here.  Once you have a 10 second or so period =
centering ok, then all you have to do is add shims to the "front" leg =
and the period goes up in a nicely until you reach instability--then =
back off a shim or two.
    If there is a steady trend to drift to one side, chances are the =
base is tilting a bit  due to a structural weakness---or more likely the =
slab on which the sensor is placed is moving.  I have known of systems =
placed in the corner of a home basement, and periodically drift was =
noted as the house foundation settles a bit--and this can happen over =
years and not be detected  by sight.  If you really wish to overkill =
your sensor base you can do what amateur astronomers do--pour a concrete =
slab block to attach their scope base, and surround the block with =
several inches of sand---then there is no walkup tilt of the base, and  =
any lateral mechanical vibrations at minimized as well.=20
   Well I have rambled enough--good stability to all, and Season's =
Greetings!

                                                                      =
Jim Lehman=20







 
PSN-Friends--I have = read with=20 interest the exchanges regarding stability and modifications to the = so-called=20 "Lehman" seismic design.  The plans as published in July, 1979 Sci. = American were the result of 5 years of running the system in a number of = environments, ---if I remember we tried 4 different types of Milne = cantilever=20 long period units, and settled on the design published.  As I have = said=20 before, the system--as most seismic designs--lends itself to = modifications &=20 improvements.
   The = BallBearing and=20 Tensional Wire designs for the boom end sound ingenious--although I have = never=20 tried them.  I stayed with the 15-cent hardware 5/16 or so bolt = ground to a=20 knife edge.  When mounted properly against a hard plate or = flattened bolt=20 head epoxied to the upright, I never experienced any undo friction or = flattening=20 of the knife edge.  Microseisms were always there--in fact one = would have=20 to run fast to get away from microseisms.  One amateur enthusiast = in Puerto=20 Rico had to settle for his system on the 14th floor of a high-rise = apartment=20 building.  In windy weather the building swayed, and the = microseisms just=20 "rode the swells" as he put it.
    It = is easy to=20 "overkill" the location of the sensor.  Ground level on a concrete = floor=20 with little temperature or humidity changes is ideal.  Direct = connection=20 with bedrock is not necessary.  One of the best locations during = the=20 testing time was in an unused room of a laboratory building.  = Several feet=20 of fill clay had been  leveled, and several inches of = gravel on=20 top.  Any bedrock (limestone here is Virginia) was probably 10 or = so feet=20 away.  We laid down 3 small garden- shop masonry = step-stones------mounted=20 our system--let it settle for a day or two, and began recording.  = Later the=20 area was finished with concrete floor, etc, and placing our sensor there = gave=20 similar satisfaction--only now more building noise was = evident--vibrations from=20 the Chiller 150 ft. away, and stresses in the building and stairwells as = students moved to and from classes.
   I do = feel a kinship=20 with persons frustrated in setting up and stabilizing a system = especially in a=20 closed space.  The Sci. American design should be stable for weeks = at at=20 least 15 second period.  A good heavy base material other than = wood is=20 best.  Composite material like laboratory table tops is = ideal.  Rather=20 than have adjustable tri --feet on the base, I would suggest firm solid=20 feet--fixed bolts or similar, and then use thin shims to complete the = task once=20 you have the mechanics of the system in the "ball park".  Five or = ten mil=20 thick sheet metal pieces work great here.  Once you have a 10 = second or so=20 period centering ok, then all you have to do is add shims to the "front" = leg and=20 the period goes up in a nicely until you reach instability--then back = off a shim=20 or two.
    If = there is a=20 steady trend to drift to one side, chances are the base is tilting a = bit =20 due to a structural weakness---or more likely the slab on which the = sensor is=20 placed is moving.  I have known of systems placed in the corner of = a home=20 basement, and periodically drift was noted as the house foundation = settles a=20 bit--and this can happen over years and not be detected  by = sight.  If=20 you really wish to overkill your sensor base you can do what amateur = astronomers=20 do--pour a concrete slab block to attach their scope base, and surround = the=20 block with several inches of sand---then there is no walkup tilt of the = base,=20 and  any lateral mechanical vibrations at minimized as=20 well. 
   Well I = have rambled=20 enough--good stability to all, and Season's Greetings!
 
          &nbs= p;            = ;            =             &= nbsp;           &n= bsp;         =20 Jim Lehman 

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