PSN-L Email List Message

Subject: Re: Lehman base material?
From: "steve hammond" shammon1@.............
Date: Tue, 18 Jan 2000 19:36:25 -0800

Chris, I just used a cheap cement drill to do this. Here is what i did on
mine and how I applied the design and set it all up.
Regards, Steve Hammond

      I used a combination of cement and Aluminum for the base of my Lehman.
I made  12 x 12 x 20 -inch deep hole in the ground and filled it with
cement. 50/50 mix.  I added a 12x12x4-inch box at above ground level so the
cement would extend above ground level by 4-inches.  After the cement set, I
drilled three 3/8-inch holes and installed (3) 6-inch x 3/8 machine anchors
in the cement in a triangular pattern. I then drill 3 holes to match the
pattern in the 1/2 x 8 x 14-inch Aluminum base plate on the Lehman.

     Using 6 nuts and 12 washers I 'm able to suspend the plate a few inches
above the cement and level it by adjusting the tripod captured nut
arrangement. This works very well for me. My Lehman has a 40-inch boom and a
5lbs mass weight. The upright wire angel is 42 degrees. The three bolt
pattern provides the initial coarse adjustment. I then added two 4-40
machine screws that extend through the left and right rear corners of the
aluminum plate. They butt into two metal blocks placed atop the cement pad
and under the plate. By applying equal amounts of force the Lehman pipe
uprights remain vertical at 90 degrees. When the ground conditions change
and the boom goes to stop, I apply a bit more force to one or the other to
adjust the boom back to center by tweaking the aluminum plate slightly
giving me very fine control over the vertical 90 degree adjustment. My
Lehman uses the standard oil nozzle in the upper cross pipe for the upper
guide wire, so I added a large hexhead screw as the lower pivot stop which I
use for positioning the boom forward and back in relation to the upper wire
oil nozzle. A turnbuckle is used in the upper guide wire to adjust the 0'
leveling of the boom. Installed in alluvium soil, I maintain an operational
period of 21 seconds. I have had the period as high as 40+ seconds, but I
needed to check it each day to keep it off the stops which become demanding
after a few months. I have found that with a 12-25 seconds period you can
record from California to New Zealand events in the range of 6.0 and higher.
If you would like to see a picture of the seismograph on the bench, there is
one on the PSN web site in San Jose. However this picture is taken when
I was making some measurements in the shop but it helps to visualize my
comments above.

To set it all up, I follow the following steps until I obtain the period I'm
after. It takes about 2 hours to dial-in 21 seconds the first time and over
the next 30 days the seismograph will find its own natural shape and require
readjusting several times. Once operational stag is reached, you only need
to adjust it as the ground conditions change.

The adjustments

Take a look at the web site picture this should help the discussion below.
The lower pivot screw is hidden by the lower cross plate that has been
drilled and tapped to receive the screw. The head of the screw is in the
back of the unit and by turning it in, you push the boom away from the pipe
uprights increasing the angel or  increasing the pendulum angel and reducing
the period of the pendulum.


1) Remove all damping.

2) Before starting, make sure the lower pivot nut is turned in well past the
imaginary vertical line passing between the the point in the oil nozzle
where the upper guide wire enters it and the pivot point on the end of the
boom. (I use a very fine point set in a cup drilled in the end of the lower
pivot screw.) If the position of the lower pivot screw gets to absolute zero
or - zero the boom will hang perfectly still or want to roll-off to the left
or the right and go to the stops and you will never be able to center it at
90-degrees to the pipe uprights. If you are unsure, turn the screw all the
way in and start there.

Adjustment loop

3) level the boom to 00' degrees using the turnbuckle.
4) center the boom to the pipe uprights at 90 degrees.
5) check the period. When you start it will be less than 1 second.
    5A) Exit -- go to damping adjustment step 7. Quit if the period is where
you want it   (mine is at 18 seconds undapened).
6) Unscrew the lower pivot screw 1/8 turn.
7) GoTo step #3

Section Adjusting the damping.

7) If you are using a magnet damping, a rule of thumb is to adjust your
damping to stop the seismograph after 3.5 cycles. A simple way to do this is
the breath method. Blow slightly on the side of the boom to set it in
motion. The center of the boom should come to rest after passing through the
imaginary center center line under the horizontal edge of the boom 8 times.
After blowing on it, the swing back would be numbered swing #1 passing
center on the initial swing back from being blown off center. Swing #2 is
the first half cycle swing #3 is the first full cycle swing back #4 is 1 1/2
cycles and so-on.

Regards, Steve Hammond  PSN Aptos, California

-----Original Message-----
From: ChrisAtUpw@....... 
To: psn-l@.............. 
Date: Tuesday, January 18, 2000 5:33 PM
Subject: Re: Lehman base material?

>In a message dated 13/01/00 09:23:33 GMT Standard Time, twleiper@........
>>> My daughter's Lehman is on a pier cast atop what is either an immovable
>huge bolder or exposed bedrock in the root / wine cellar. It is a very
>environment with natural temperature control and the seismo is made from
>thermally stable materials, such as a granite tool and die makers slab for
>base. With a natural period of 50 seconds it only needs seasonal centering
>Dear Mr.Leiper,
>    This all sounds great, but what is the price of granite engineering
>and how do you go about boring holes in them? I read through your account
>the construction of the original seismograph with interest.
>    I have been trying to think of a relatively cheap, heavy and robust
>material for a seismograph. I noted that people who used plate Aluminium
>seemed to have put weights on them, so presumably, some weight can be an
>    I was driving past some roadworks yesterday, when I had a bright idea.
>You can get 2" thick paving slabs made out of high density 'vibrated'
>concrete, with a reasonably flat finish on both sides. The workmen were
>cutting 3' x 2' slabs to size with a disk as I passed and I wondered if
>anyone had thought of using part of one for a seis base? You could probably
>stick flat metal base plates onto the concrete with epoxy. The smallest
>diamond core drill that I can hire is 1/2", which seems a bit too big. I
>haven't yet tried drilling a slab with a hammer drill and a carbide bit,
>from past experience, high silica aggregates are quite drill resistant.
>    Do you know if anyone has used a paving slab and if so, were there any
>problems? The price and weight seem about right. Can you comment, please,
>comparison the granite block?
>    Regards, Chris Chapman
>Public Seismic Network Mailing List (PSN-L)
>To leave this list email listserver@.............. with the body of the
>message: leave PSN-L


Public Seismic Network Mailing List (PSN-L)

[ Top ] [ Back ] [ Home Page ]

Larry Cochrane <cochrane@..............>