In a message dated 15/04/2005, 1goss@........... writes:

Chris  Chapman,

Thanks for the link to the ball bearings, I have them on the  way.
I am going to do a total rebuild. I have in hand =BD square aluminium,  will=
=20
this be ok for the boom. If it is not I will buy the =BD nominal stainless =20=
steel=20
water pipe you suggested.
Hi Bryan,
=20
    How flexible is your 1/2" Al? I would rate this as  a minimum size. I=20
prefer to 'do things the easy way', if there are no other  disadvantages. Be=
ing=20
able to buy compression ends for water pipe is an easier  than making plug e=
nds=20
for a square tube. Before you decide, see what pipe you  can easily buy? You=
=20
can mount your hard plate on the flat end and drill the  other end to hold t=
he=20
support plate for the coil. Check whether you can get  compression Xs withou=
t=20
too much trouble. They are available, but not from every  supplier.=20

I also  ordered 12*18*=BC inch aluminum plate for the base.
If I use a boom length of  30 inches where should I put the weight I will tr=
y=20
to attach the support wire  the way you suggested as well. It will take me=20
some time but I want the best  seismograph I can build.


I would mount the weight as far along the boom as  possible, just before the=
=20
end stop for the sensor. I would put a tongue of  damping copper horizontall=
y=20
under the boom with a gap of maybe 3/4", space  enough to slide a damping=20
fixing over it. The magnets are 1/4" thick and so are  the backing plates. =20=
The=20
tongue 'faces back' toward the pivot. Did I send  you a drawing of the fixtu=
re=20
that I use? Cancel this?
    Alternatively, put your existing damping plate  right on the end, using=20
your existing magnets and mount the coil under the arm  on the bearing side=20=
of=20
the mass. One query - your mass looks as if it is in a  tin can? Is this mad=
e=20
of steel or Al? If it is steel or has a steel lip, you  will have to watch o=
ut=20
for interactions with the magnets.
=20
**    Before you disassemble everything, I suggest  that you drain your oil=20
tray and check that you can get enough damping with your  existing magnets,=20
narrowing the gap as necessary. **=20
=20
    What thickness is your copper damping  plate?=20
    I saw one seismometer which used a 3/8" thick  horizontal copper plate 5=
"=20
x 5" for the mass and put the damping  magnets on top and bottom!  But I=20
don't know what it cost!=20
=20
    There is no 'perfect' way to build a seismometer,  but there are=20
constructional / instumentation problems which are best avoided.  Knife edge=
 bearings=20
and oil damping are two of them!
=20
    Regards,
=20
    Chris Chapman





In a message dated 15/04/2005, 1goss@........... writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Chris=20
  Chapman,

Thanks for the link to the ball bearings, I have them on t=
he=20
  way.
I am going to do a total rebuild. I have in hand =BD square alumin=
ium,=20
  will this be ok for the boom. If it is not I will buy the =BD nominal stai=
nless=20
  steel water pipe you suggested.
Hi Bryan,
 
    How flexible is your 1/2" Al? I would rate this=
 as=20
a minimum size. I prefer to 'do things the easy way', if there are no other=20
disadvantages. Being able to buy compression ends for water pipe is an easie=
r=20
than making plug ends for a square tube. Before you decide, see what pipe yo=
u=20
can easily buy? You can mount your hard plate on the flat end and drill the=20
other end to hold the support plate for the coil. Check whether you can=
 get=20
compression Xs without too much trouble. They are available, but not from ev=
ery=20
supplier. 
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>I also=20
  ordered 12*18*=BC inch aluminum plate for the base.
If I use a boom len=
gth of=20
  30 inches where should I put the weight I will try to attach the support w=
ire=20
  the way you suggested as well. It will take me some time but I want the be=
st=20
  seismograph I can build.


    I would mount the weight as far along the boom=20=
as=20
possible, just before the end stop for the sensor. I would put a tongue of=20
damping copper horizontally under the boom with a gap of maybe 3/4", space=20
enough to slide a damping fixing over it. The magnets are 1/4" thick and so=20=
are=20
the backing plates.  The tongue 'faces back' toward the pivot. Did I se=
nd=20
you a drawing of the fixture that I use? Cancel this?
    Alternatively, put your existing damping plate=20
right on the end, using your existing magnets and mount the coil under the a=
rm=20
on the bearing side of the mass. One query - your mass looks as if it is in=20=
a=20
tin can? Is this made of steel or Al? If it is steel or has a steel lip, you=
=20
will have to watch out for interactions with the magnets.
 
**    Before you disassemble everything, I sugge=
st=20
that you drain your oil tray and check that you can get enough damping with=20=
your=20
existing magnets, narrowing the gap as necessary. ** 
 
    What thickness is your copper damping=20
plate? 
    I saw one seismometer which used a 3/8" thick=20
horizontal copper plate 5" x 5" for the mass and put the damping=20
magnets on top and bottom!  But I don't know what it cost! 
 
    There is no 'perfect' way to build a seismomete=
r,=20
but there are constructional / instumentation problems which are best avoide=
d.=20
Knife edge bearings and oil damping are two of them!
 
    Regards,
 
    Chris Chapman