Hi Chris,

It's good to hear from you.  Really glad to hear you're mending.

You are  absolutely correct about the need for a pressure 
container.  We wouldn't even think of trying to run one without 
putting it in such a case, though in fact, they sometimes don't do 
all that badly without, until you get some wind or a big weather 
system comes through.  Then it can get pretty exciting.  At a guess, 
the noise will go down by a factor of 10-50 when you add a good case.

Dave has made three designs.  The largest, the Inyo, uses 150g mass, 
and his latest prototype, the Napa uses about 60g, which we believe 
may be about the minimum for really good performance.  He carefully 
arranged to match their outputs to the characteristics of Larry's PSN 
A/D board http://www.seismicnet.com/serialatod.html
which uses WinSDR to collect the data.

Being fairly small, these can be enclosed in well-insulated boxes 
which greatly reduce the internal 
convection.  http://bnordgren.org/seismo/IMG_1697a.jpg
There's no doubt that the lighter design is more sensitive to 
convection, though sufficiently good insulation does appear to 
largely eliminate the problem.

I'll mail you off-list about the construction particulars.  Because 
building these requires access to a machine shop and the ability to 
assemble circuit boards, etc. you can understand why we haven't been 
encouraging the general community to take them on.  Though, there are 
now approaching a dozen which enterprising builders have constructed, 
which are apparently operating with some success.

We have begun to make noise measurements to understand how well they 
might be able to perform, and the early results are rather 
encouraging.  Next year we hope to discover how well they really can 
do by testing one alongside a very good instrument at a very good 
site.  As soon as I get the Web page done, I'll post a link which 
includes one of our first noise measurements.

Thanks for the Mylar offer.  Since we recommend baking the mechanical 
assembly before installing the electronics (to eliminate spring 
noise) we have to use a flexure material which doesn't mind the oven 
temperature, i.e. Kapton polyimide.

Regards,
Brett


At 06:22 AM 11/23/2011, you wrote:


>Brett Nordgren brett3nt@.............
>Subject: Re: Spring for AS-1 type vertical seismometer
>
>Hi Brett,
>
>     Vertical Seismometers which are NOT hermetically enclosed may be
>
>quite noisy when compared to horizontals. They are strongly effected
>
>by atmospheric pressure noise and by air convection, particularly if
>
>they have low mass. A Lehman type commonly has a mass of 1 to 2 kg.
>
>What mass does the FBV use, please?
>
>     Can you direct me to the plans for the FBV, please ?
>
>     What ADC do use, please ?
>
>     I have some sheets of 3 thou mylar / polyester, which looks like
>
>it could be used to make a seismometer bearing. Would you be
>
>interested in having a sample ?
>
>     I would be interested in trying to make a FBV. Can you confirm
>
>the cost of the board + postage to the UK, please ? I will need some
>
>help with the capacitors, the timing crystal and the ICs please.
>
>None of the UK suppliers that I can find have a 3 MHz crystal. The
>
>supplier which I was using has closed down. The pin spacings on
>
>capacitors are very variable. I think that I am OK for everything
>
>else. I do have a PayPal account.
>
>    I am continuing to recover medically, but progress is very slow.
>
>     Regards,
>
>     Chris Chapman


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