PSN-L Email List Message
Subject: Re: Reinforced concrete base for Lehman instrument
From: "kpayea" kpayea@...........
Date: Sun, 9 Mar 2003 10:36:01 -0800
I built the base of my STM-8 style vertical Seismometer out of concrete
also, and I have had very good results from it. However, instead of casting
a solid piece, I used a combination of 8" x 8" and 8" x 16" concrete
stepping stones and a few concrete bricks. The base is made of two layers
of stepping stones - one of the 8x8 and one 8x16 in each layer - arranged so
the joints don't overlap. The finished base is 8x24 by about 3" thick.
The tower at one end to support the boom hinge is a stack of concrete
bricks. The parts are not mortared together, but are held with "liquid
nails" style adhesive. There is a special formula made for masonry. I let
all of the joins cure for about a week.
The advantage of this style is that there is no mortar to mix and no forms
to create.
I used steel "T" nuts for attachment points, epoxied into holes drilled in
the concrete parts.
I know this probably breaks several "rules" about seismometer construction,
but it works very well. I think the concrete acts as a thermal mass,
evening out small temperature variations and reducing the temperature
sensitivity. It has been running for a couple of years with no adjustments
to the sensor itself. I do adjust the offset in SDR seasonally as the
ambient temperature changes.
It sure is heavy!
Good luck,
Keith
----- Original Message -----
From: "beezaur"
To:
Sent: Saturday, March 08, 2003 8:48 PM
Subject: Re: Reinforced concrete base for Lehman instrument
>
> >
> >
> > Carbon could be OK, but how do you plan to make the fittings?
> > Metal is easier to fabricate and fit. You can easily get nickel plated
> > piano wire down to 8 thou. from a music shop, it does not have any
> > 'bending' memory and it works just fine. See http://www.daddario.com/
> > Suggest that you forget kevlar or other plastics. A ball bearing in the
> > end of the arm gives an excellent bottom suspension up against a
> > stainless steel razor blade glued flat onto the crossbar.
> >
> > Regards,
> >
> > Chris Chapman
>
> Chris,
>
> You are right that steel is more rigid than concrete, by about 8x, but a
> cement/sand base has easily 8x the cross sectional area of steel, so
> becomes more rigid because of its greater size.
>
> Arrow shafts can be ordered hollow. You would normally glue in a
> threaded insert (female) for the tip and push in a nock. One could
> easily glue in two tip inserts. Glue a ball bearing on one end and
> thread the other to the mass. You can drill a hole in a lead weight and
> glue an arrow tip in that. This way the boom (by this I mean just the
> compression member between the mast and weight) has a natural period in
> the tens of Hz and has next to no mass, so should not bother the pickup
> hardware, even if it does vibrate some. Also, making the boom so much
> lighter than the weight at the end should help sensitivity.
>
> As for tension member, My thought is that using bowstring material
> (650,000 psi breaking strength vs. maybe 100,000 psi for steel) simply
> reduces the mass and therefore increases the natural frequency. I have
> all this stuff anyway for my bow.
>
> My working hypothesis is that if I make the structure as stiff as
> possible and keep all mass on the boom concentrated, I will get a
> cleaner signal.
>
> Scott
>
> --
> A day without math is like a day without sunshine.
>
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