PSN-L Email List Message

Subject: RE: Reinforced concrete base for Lehman instrument
From: steve hammond shammon1@.............
Date: Sun, 9 Mar 2003 11:07:20 -0800

When we had a PSN meeting at Berkeley University, the folks in the lab told 
us that the broadband seismographs were placed on a 50/50 sand/cement mix.
Regards, Steve Hammond PSN San Jose, Aptos California

-----Original Message-----
From:	kpayea [SMTP:kpayea@............
Sent:	Sunday, March 09, 2003 10:36 AM
To:	psn-l@..............
Subject:	Re: Reinforced concrete base for Lehman instrument

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 
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 
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 
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,


----- Original Message -----
From: "beezaur" 
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
> > 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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