lehman seismometer

I have built a new lehman seismometer and I have a few questions.

A. I have a small outdoor building not temperature controlled thats why=20
I built the wooden box with insulation.
I am getting noise starting about 1:00am to about 9:00am Should I build=20
an outer box made of foam board and heat it with a heating pad or=20
something to control the temperature.

Hi Bryan,

     You need an airtight box to shield the seismometer from air=20
currents and drafts.

    This sounds like the classic reverse convection problem. The top of=20
the box cools below the floor temperature at night. Rolls of cold air=20
peel off the inside top of the case, fall to ground level and push the=20
arm about in the process.
     I use a 1/16=E2=80=9D Aluminum sheet fitted inside the top of the cas=
e and=20
I bolt on two Al-Clad power resistors dissipating about 10 to 20 watts.=20
This maintains a positive temperature gradient inside the case, at all=20
times.  I use a cheap 24 volt transformer. Check what you have=20
available?
      I advise against using light bulbs for heating. The light attracts=
=20
mice, spiders and a wide range of insect life, none of which you want=20
in, on or around your seismometer! The bulbs can be painted black, but=20
the temperature of the glass envelope is high and this can give=20
convection noise.
     Do not try to control the temperature of the seismometer. It is=20
very difficult to do this well enough and it is not necessary. It is=20
much better to allow the temperature of the seismometer to slowly=20
=E2=80=98float=E2=80=99 with the daily cycle.

link to the box and noise
http://www.1goss.com/s-box.jpg ;
noise example, I had it most of the afternoon although normally during=20
the day it looks like it should. http://www.1goss.com/noise.jpg

B. The first sensor I built several years back I donated to the local=20
elementary school when were moving. It used the razor blade knife edge=20
and steel all thread for the boom and did not have the small aluminum=20
rod I put in the new one.

     Do not use magnetic materials on the arm. This makes it sensitive=20
to changes in the Earth=E2=80=99s field and to switch pulses on the house=
=20
wiring. I mount the sensor coil on the boom and the drive magnet on the=20
base. I use four NdFeB bar magnets on =C2=BC=E2=80=9D mild steel backing=
 plates for=20
the drive magnet. This is cheaper than an Alnico U magnet and gives=20
over five times the output voltage. I also use a much smaller=20
rectangular sensor coil which gives a linear output, in place of the=20
old round fat coil.
     Razor blades do not make long lasting bearings. Hardened SS or WC=20
ball on a plane or crossed cylinders are very much better. A figure of=20
8 loop of wire around two cylinders also makes a very good bearing.

It worked well but I wanted to build a better one this time round so I=20
used a ball bearing for the pivot point, added the aluminum rod to help=20
with vertical stabilization and used square hollow tubing for the boom.

     One of the things you need to try to do is to stabilise the boom=20
against rotation. I use a square Al rod clamped to the boom with a=20
similar cross rod mounted at right angles to the boom. This rod has V=20
grooves at both ends. I use a twin V cable suspension made from 7 core=20
SS nylon coated fishing trace. This is supplied on a reel complete with=20
small crimp tubes. You can get a wide range of loads, up to 200lbs! I=20
form loops on both ends of the cable and these fit in the V grooves on=20
the ends of the cross bar. The top loop fits in the V groove around the=20
edge of a 1.5=E2=80=9D Al washer. This can be fitted with either a ball or=
 a=20
wire suspension.

Do you see anything wrong with this setup? I want criticism /=20
suggestions if you have any.
http://www.1goss.com/s-bearing.jpg
http://www.1goss.com/s-top.jpg
http://www.1goss.com/s-overall.jpg
http://www.1goss.com/s-boom.jpg
http://www.1goss.com/s-housing.jpg

     You would find it very much easier to build, set up and adjust the=20
seismometer if you built the base all on a single cross braced T/L=20
shaped frame, constructed from 1=E2=80=9D / 25mm steel angle bolted togeth=
er=20
with =C2=BC=E2=80=9D / 6mm SS bolts and lock washers, from a marine suppli=
er. You=20
set up the suspension, the arm, the sensing and the damping systems on=20
the rigid fixed frame. Then you tilt the whole frame to set the period,=20
and finally you trim the damping. The three piece tubular seismometer=20
design on psn almost guarantees both construction and operating=20
problems.

  I had a bit of trouble getting the 25sec period and am not sure it is=20
correct and haven't had a recordable local or teleseismic event to=20
compare with the local
Helicorder Displays.

     I am concerned about this. It suggests that your bearings have some=
=20
friction, somewhere. I have used a similar type of suspension and I had=20
no difficulty in setting a period of 60 seconds! I used a SS razor=20
blade glued to the flat on the end of the arm, as the counterface. You=20
can also buy hardened, polished SS break off knife blades, scalpel=20
blades, and WC triangular lathe tools, but the counterface surface must=20
be hardened.

C. Would a ball bearing at the top and all aluminum perform better than=20
a wire?

     Probably equally well, only provided that the wire suspension is=20
good. I drill a hole in the horizontal clamp bolt and thread the wire=20
between two vertical clamp washers.

And if I use all wire, what is the very best wire to use?

     Use steel music wire. You can buy this from about 7 thou OD=20
upwards. D=E2=80=99Addario have a catalogue. I can buy it in Europe. Wire=
 of 10=20
thou OD and above can be bought Nickel plated, which protects it from=20
rust. See what is available in a local shop which sells strings and=20
stringed instruments? Check out strings for mandolins? You can look up=20
the UTS strength in www.kayelaby.npl.co.uk

    I hope that this of help.

    Regards,

    Chris Chapman


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