Seismology --- One of my other interests.....

[poor mystic]

Congratulations Dave!
I'm looking forward to seeing those photos.
Also I have been wondering about daddles' confusing remark (post before last) to the effect that he dropped "czechs" through his magnetic field on weekends. Excuse me? Did I hear that right?

 

[davenn]

drop pieces of brass and aluminum between the poles (czechs on the weekends).

I suspect he's having a play on words .... between the "poles" ( polish)

D

 

[davenn]

ok here's a pic

I may yet have to revise my dampening system. It doesnt quite follow the norm. as I am only using 1 magnet with the aluminium plate above it. I may yet have to go to the usual magnet on either side of the plate, if I cannot achieve enough dampening.
The first couple of quake recordings will clarify that. If I see too much oscillation of the pendulum, I will have to change the dampening.

Dave

PS a larger size of the image is viewable here

Its BIG for you to scroll around it for a close in look!!

 

[poor mystic]

Thanks, especially for that big picture Dave.
I wanted to know more, so I read Wikipedia's article on seismometers, where I found the following:

from https://secure.wikimedia.org/wikipedia/en/wiki/Seismometer

"The basic damped horizontal pendulum seismometer swings like the gate of a fence. A heavy weight is mounted on the point of a long (from 10 cm to several meters) triangle, hinged at its vertical edge. As the ground moves, the weight stays unmoving, swinging the "gate" on the hinge.

...The advantage of a horizontal pendulum is that it achieves very low frequencies of oscillation in a compact instrument. The "gate" is slightly tilted, so the weight tends to slowly return to a central position. The pendulum is adjusted (before the damping is installed) to oscillate once per three seconds, or once per thirty seconds. The general-purpose instruments of small stations or amateurs usually oscillate once per ten seconds.

...The hinge is very low friction, often torsion wires, so the only friction is the internal friction of the wire. Small seismographs with low proof masses are placed in a vacuum to reduce disturbances from air currents."

Wikipedia has much more to say; I recommend it as interesting reading.
Do you plan to build more instruments to complete an array, Dave?

 

[davenn]

Thanks, especially for that big picture Dave.
I wanted to know more, so I read Wikipedia's article on seismometers, where I found the following:

from https://secure.wikimedia.org/wikipedia/en/wiki/Seismometer

"The basic damped horizontal pendulum seismometer swings like the gate of a fence. A heavy weight is mounted on the point of a long (from 10 cm to several meters) triangle, hinged at its vertical edge. As the ground moves, the weight stays unmoving, swinging the "gate" on the hinge.

yup thats the way it works, The dampening is needed because the pendulum arm does start to move and that movement needs to be brouoght to a quick stop. Thats because it isnt totally isolated from the ground

...The advantage of a horizontal pendulum is that it achieves very low frequencies of oscillation in a compact instrument. The "gate" is slightly tilted, so the weight tends to slowly return to a central position. The pendulum is adjusted (before the damping is installed) to oscillate once per three seconds, or once per thirty seconds. The general-purpose instruments of small stations or amateurs usually oscillate once per ten seconds.

The trick for getting the longest period out of the unit is being able to lower that tilt angle of the boom to the lowest angle and still have the boom return to the central position rather than just swinging off to one side. Once I have got my one well adjusted, I should be able to get ~ 10 - 15 sec period.

...The hinge is very low friction, often torsion wires, so the only friction is the internal friction of the wire. Small seismographs with low proof masses are placed in a vacuum to reduce disturbances from air currents.

I will probably put a styrofoam box around my unit to stop the air draughts

Wikipedia has much more to say; I recommend it as interesting reading.
Do you plan to build more instruments to complete an array, Dave?

yes there's lots of info out there and the large group of guys in our PSN group have done much experimenting.
Yes I have a second frame welded up, but wont start on completing that unit till I iron out all the bugs in the first unit

cheers
Dave

 

[poor mystic]

I've always wanted to hear the seismograph recording of an earthquake, sped up to audio frequencies. Maybe, my chance is coming up!
Can I look forward to a post with a link to such a file?
(Can't find a smiley of an enthusiastic puppy with tongue hanging out.)

 

[davenn]

I've always wanted to hear the seismograph recording of an earthquake, sped up to audio frequencies. Maybe, my chance is coming up!
Can I look forward to a post with a link to such a file?
(Can't find a smiley of an enthusiastic puppy with tongue hanging out.)

I know some of the guys have converted to an audio file then sped it up. I will have to find out how that is done

Dave

 

[nbw]

You guys would love living in Christchurch!!! Lots of seismic activity: check out the quake drums at geonet.org.nz

 

[daddles]

Dave: if you can get me a digital sample of an earthquake signal, I'll convert it to a sound file (I can either do it directly with some tools I have or I'll write some code to do it). But it's probably just going to sound like something bumping against something else (i.e., not be terribly interesting). Or, send it to poor mystic, as he has my email address.

Sorry about the earlier czech thingy -- that was a play on word from an old joke about the papa bear eating the Czechoslovakian -- then the joke made a play on "the czech is in the male". I can't help doing such things and my family has disowned me for such behavior, so you'll just have to occasionally put up with it.

Did you do the welding on the aluminum supports? Nice job. I'm curious about the pivot. I see one ball bearing in the rod and one in the hole. It's not clear to me how it operates. Any more details?

 

[(*steve*)]

Sorry about the earlier czech thingy

Yeah, and so you should be! If those Czech's bounced, Dave would get very poor damping!!!

 

[davenn]

You guys would love living in Christchurch!!! Lots of seismic activity: check out the quake drums at geonet.org.nz

dunno mate, the Christchurch people are sick of living with the quakes !! 1000's have moved elsewhere. My 2 kids and granddaughter live in CHCH they are "so over it"

I did feel a 5.3 and a 4.0 whilst I was visiting over there in early May to see how the kids were doing.

geonet.org.nz has been at the top of my fav's list for many many years

Dave

 

[poor mystic]

yes, I heard that 50,000 people have permanently left Christchurch. It was a flat city when last I visited 5 years ago, but my brother says it hs been squashed into series of little rises and falls like a concertina.
They are living in interesting times over there, that's for sure.

 

[davenn]

Hi Daddles,

Pivot points and stuff......

yeah having to do a feww changes there. Things were not working quite as expected.

originally I was going to have the 2 steel balls against each other (the one in the vertical frame and the one on the end of the boom), but they wouldnt stay in place.

Then I was just using the one in the end of the boom up against the vertical but the steel ball indents the softer aluminium and almost stops very low freq motion of the boom.
So was suggested to my by fellow seismo constructors to use the steel ball in the frame up against a flat steel piece on the end of the boom.
Im also reducing the mass to either 1 lb or maybe 2 lbs max. Many of the guys were saying for this style of seismo the 4 lbs I was using is unnecessary and makes the system harder to critically dampen because of the additional inertia that that mass has. ( I hate using the old lbs system rather than metric. its just these diving belt weights are still sold as 1 lb and 2 lbs units.)

All these alterations are ok, getting the bugs ironed out to make this unit work will mean that the second one should be easier to get going.

An ex fellow workmate and I propane gas aluminium welded the frames.

There's only 2 of us left at the workshop now, the office girl and me, and we are working for the administrator.
Everyone else got their marching orders last monday. She and I have ~ 2 weeks left and we will both be gone as well. She is sorting out paper work and Im sorting out gear for the liquidation auction

cheers
Dave

 

[poor mystic]

ooh yummy!
Tell us about the auction please Dave!
(It's an ill wind that blows nobody any good.)
Tell us about test equipment for sale please!

 

[daddles]

Interesting about the pivot, as it looked to me like two balls and I couldn't see how it worked. For such things, you generally want hard steel against hard steel at least (steel balls from ball bearings are usually a 52100 steel, a rather ferociously tough steel). Even better is hardened steel against an even harder thing like sapphire, etc. (e.g., remember the "jeweled bearings" of watches?).

Here's something that appeared in my brain immediately upon looking at the design (see attached picture).

Part A is made from drill rod (in the US, either O-1 or W-1 (e.g.) hardenable steels). You machine it soft then harden it with a propane torch and an oil or water quench. The nice thing is that it's simple to make on a lathe -- turn down one end for inserting into the long arm and securing with a set screw. Then the other end is center-drilled. Harden it. Then you can make a simple brass lap and lap the center drilled hole's face to a nice polish. This is the hardened surface that bears against the ball bearing. The ball bearing could be held with wax or somesuch in a hole in the support (I'd use a ball end mill because I have some, but a center drill would work fine here too).

Even without a lathe, it wouldn't be too hard to make such a thing with files and a drill press -- or even a portable electric drill. The turned-down section could instead be made by Loctiting a pin into a drilled hole (or use a spring pin -- virtually no work).

I'd make a first one without bothering with the lapping. Total time to make the steel piece would be less than 10 minutes with most of the time for heat treating. Stop by the house and I'll make you one...

 

[poor mystic]

Y'know
I've been wondering about gemstone pivots for your boom.
it's easy to find say naturally faceted amethyst or (preferably) topaz. The boom still needs to be suspended somehow, but a couple of really hard knife edges would surely help with friction?

 

[davenn]

ooh yummy!
Tell us about the auction please Dave!
(It's an ill wind that blows nobody any good.)
Tell us about test equipment for sale please!

lots of test meters I have sorted into lots

have 4 lots of ....
1 of each of impedance meter ( speaker lines etc ), LCR Meter. 1000V Electronic Megger

there is a UNI-T 100MHz DSO only ~ 8 months old

several cable tracers, coax cable connector crimp tools

1 x Audio Function gene
1 x 2.7 GHz freq counter
1 x IRF 1200S Comms Test Set

lots of other electrical and other workshop tools, drills,saws, screw drivers etc etc

 

[davenn]

Y'know
I've been wondering about gemstone pivots for your boom.
it's easy to find say naturally faceted amethyst or (preferably) topaz. The boom still needs to be suspended somehow, but a couple of really hard knife edges would surely help with friction?

That would be interesting, never heard of anyone using gemstones

knife edge pivots on the lower pivot are common. you can still have the sharp knife edge digging into the surface and causing increased friction. A lot of the success "hinges" (pun intended ) on the quality of the bottom pivot.


Ohhh what I also intend doing as well is to move the suspension wires further out to the other end of the frame that holds the magnets ... ie. closer to the mass. hopefully that will help with my balancing problems.

Dave

 

[poor mystic]

(thinks...)
So the beam needs lots of mass to counter disturbances from air movement, but a heavy beam tends to bend the pole it depends from so you are forced to adjudicate a trade-off.
I find it hard to get past my jewelled bearing. Amethyst crystals terminate in sharp pyramids which could provide vertical support too, against a dimple in a suitable surface.
It is a bit surprising that something as flexible as aluminium is used. It would have been less surprising to me if you used rock for both the supporting and the oscillating elements of the seismometer.

 

[daddles]

Actually, on reflection, the material of choice for the pivot would probably be a carbide because a variety of shapes are available already fabricated and relatively inexpensively (carbide balls are quite common in tooling rooms and are e.g. used for "ballizing"). Some of the carbides are up there in hardness with things like aluminum oxide (which makes up things like sapphire, a material that was often used for jeweled pivots). Some of the ceramic cutting tools used in industry might be even harder. And industrial diamonds are available and aren't horribly expensive. For a plethora of examples, consult any tool supplier (such as MSC or McMaster-Carr).