jonfr
Magnitude:2
Joined: May 19, 2006
Posts: 93
Location: Hvammstangi, Iceland
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I know everything about indoor noise. Since I have to have
my geophone inside also. It's a annoying noise, but
at current time there isn't alot that I can do
about.
I plan to add a resistance (ohm) into the seismometer, what type I haven't
figured out yet.
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Terry
Site Admin
Joined: Apr 18, 2006
Posts: 109
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Would the added resistance be for added damping or
possibly for noise reduction?
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jonfr
Magnitude:2
Joined: May 19, 2006
Posts: 93
Location: Hvammstangi, Iceland
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Terry wrote:
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Would the added resistance be for added damping or
possibly for noise reduction?
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I am going to increase the damping, electronic type
of noise is going to get filtered out by screened cable. The other source of
noise, well. I am just going to have live with that. But there isn't alot of noise at 2Hz and below, for instance, I lowpass my online plots down to 5Hz and they are flat
most of the time becose of that.
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tim_ga
Magnitude:1
Joined: Jun 14, 2006
Posts: 1
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Hi Terry:
I ran across your site a few weeks ago and have watched with interest the
discussion of the PVC seismometer of your design. It is an interesting design
with remarkable performance given the simplicity of construction. A few
questions:
1. How critical is maintaining vertical plumb when cementing the tube in the
hole? What technique did you use to insure alignment?
2. Any insight on how critical it is to establish a direct interface with
bedrock? In middle Georgia
this can be very problematic. One option I have considered is driving a rebar
rod eight to ten feet in the soil and then attaching the PVC base to a
concrete pad poured around the top of the rod.
3. Which amplifier board are you using (Larry’s or a custom board)?
I plan on constructing a similar unit this summer to compare with a “C Spring
Seismometer” designed by R. Sparks. This is a project for a small local high
school to introduce the students to basics of seismology.
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Terry
Site Admin
Joined: Apr 18, 2006
Posts: 109
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Hi, tim_ga
Welcome to the board ! I am glad to see you have an
interest in this seismometer . I hope I can answer
your questions.
1. How critical is maintaining vertical plumb when cementing the tube in
the hole? What technique did you use to insure alignment?
Well it needs to be vertical for the least amount of damping and maximum
period. However since this unit is “some what” tunable it can tolerate a
small amount of tilt. The down side is the more severe the tilt is the closer
the magnets need to be placed to maintain operation. With that being said I
leveled mine with an ordinary bubble level and it seems to work fine.
2. Any insight on how critical it is to establish a direct interface with
bedrock?
In middle Georgia
this can be very problematic. One option I have considered is driving a rebar
rod eight to ten feet in the soil and then attaching the PVC base to a
concrete pad poured around the top of the rod.
I understand your problem and I have found in the past that the larger the
mass is that you attach a seismometer to the better. However sometimes it is
just not possible but I would say that you have a workable solution with the
rebar rod .
3. Which amplifier board are you using (Larry’s or a custom board)?
The board that I am using is my own design and over the years it has gone though
several evolutions .
Terry
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jonfr
Magnitude:2
Joined: May 19, 2006
Posts: 93
Location: Hvammstangi, Iceland
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Basic list of what is needed to build this seismometer.
Antenna.
Concrete.
magnets.
A pot for the molde to build the pad for the pvc seismometer.
rods.
coil.
PVC (??)
I am going to add numbers to this list, when I am sure I got it right.
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Terry
Site Admin
Joined: Apr 18, 2006
Posts: 109
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Yes that’s a good start on the list there are some small
things left out. The magnet mounts - which are really antenna mounts soldered
to washers .
Remember to trail fit everything over and over .
I found this picture of the coil form (at the time of this picture the coil
had not been wound ) ,the bottom floater magnet with
the mount and the 3in. or 7.62 cm PVC pipe coupler . The magnet is 5.53 cm
across
Terry
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jonfr
Magnitude:2
Joined: May 19, 2006
Posts: 93
Location: Hvammstangi, Iceland
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I plan to start building this seimsmometer
next week, I am going to start with the base it self. What is the rod long
and thik that is in the concrete
?
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Terry
Site Admin
Joined: Apr 18, 2006
Posts: 109
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I think 5/16’s of an inch is .97536 cm. and 20 threads per
inch . The main thing is the base rod needs to fit
the bottom of the antenna mount and extend down in to the concrete
. The length of the base rod depends upon the depth of the mold . In my example I used a old flower pot as a mold . I also welded some small pieces of metal to the
part of the rod that was submerged in the base concrete. That will secure the
rod in the concrete and keep it from turning .
Also remember that the antenna rod you use must be .062 in or 1.5748 mm in
diameter. If you use a different diameter antenna rod it will change the
response of the seismometer .
Terry
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jonfr
Magnitude:2
Joined: May 19, 2006
Posts: 93
Location: Hvammstangi, Iceland
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I have made a small modifaction.
I am going to use a whole rod, the rod is going to be 50 - 60 cm and is going
to be whole from the concreate and is going to hold
the first magent. This is not a huge diffrent from your drawing, but I won't be using a PVC
antenna as you do.
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Terry
Site Admin
Joined: Apr 18, 2006
Posts: 109
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I would not think that would impact the operation of the
seismometer as long as the rod diameter is the same .
Once I made one without the coil spring at the bottom. The rod seemed to be
stiffer and the unit lost some sensitivity . However
by making the rod longer it might make up the difference .
This of course would change the period of the seismometer.
But with that being said, experimentation is what makes this hobby so much
fun.
I was running some test on another seismometer over the weekend
. The distance between the coil and magnet and with varying strengths
of magnets . I came up with some surprising results . I will post them at a later date.
Terry 
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jonfr
Magnitude:2
Joined: May 19, 2006
Posts: 93
Location: Hvammstangi, Iceland
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 Posted:
Fri Jul 0
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