Dear Mr.Leiper,

    Thanks for the quick reply. I thought that it might be something like 
this, for the last time that I looked at the price of a large piece of 
engineering quality polished granite, the cost was $ thousands, not hundreds.
    
>> In my case the price was ten bucks many years ago at a scrap yard. It is 
two by three and 4" thick and already had the levelling mechanism. All my 
attachment points are with
PC-7 epoxy, so no drilling was required. The idea was to preserve the 
geometry of the
components to the most ridiculous degree possible over long term temperature 
drifts
 (days/weeks). I had tried all manners of insulation, stratified heating 
elements, baffles, etc. with less than ideal success.
    Have you tried a heater in the top of the enclosure which is accurately 
regulated to a few degrees above the actual floor temperature?

>> Since my vault is highly stable, I decided to focus primarily on reducing 
drafts (relatively simple) and preserving geometric integrity over long 
period temperature swings
 through materials selection.
    I am not making the following points to quiz you, but just so that you 
can yourself reject any obviously inapplicable ones and maybe cross check on 
some others.  
    What are you observing in the behaviour of the seismograph which you 
think is a fault / wish to correct? Did you measure and plot the inside and 
outside temperature changes actually occurring and were the events correlated 
with temperature changes? Do the bottom arm, the uprights and the diagonal 
support arm materials have the same temperature expansion coefficient?  Do 
you also record air pressure, wind velocity, sunshine and rainfall? I am just 
thinking of other factors which might contribute to drift. You seem to be 
convinced that it is a temperature problem.... what if it isn't? How do you 
arrange the venting of the seis enclosure? If the air pressure falls 
suddenly, what flow can it cause inside the case? Do any of the effects tend 
to occur at around the same time of day? Are there any large electrical 
supply cables nearby? What sort of suspension systems do you use? Is there 
any magnetic material on the arm at all? I remember having to use Al rather 
than brass in one application because of it's weak magnetic properties. 

>>With a natural period of 50 seconds it only needs seasonal centring 
adjustment for what I theorise is increased weight of the frame house (the 
foundation rests on said bolder about 15 feet away) due to higher relative 
humidity in the summer, or the presence of leaves on a rather large beach 
tree about 30 feet away.
    With a 50 second response, the suspension angle is of the order of 0.1 
Deg, which has to be maintained to parts per thousand.... In winter, a large 
Beech tree will use very little water. In summer it may, a very rough figure, 
use >50 galls a day. Say you have three weeks of fine weather, could a 
reduction of 1000 galls on that side of the house be in agreement with an 
observed shift? 

    Just after I had sent my last EMail, I remembered where I had come across 
polished granite slabs before. A monumental stonemason.... Speaking gravely 
you say 'Yes, the array of holes is for security and to allow for flower 
baskets to be fitted.... No, no inscription please - in our situation, we 
consider it advisable that the stone should remain anonymous....'  You might 
even get a partially engraved one cheap.... even some stonemasons make 
monumental mistakes....
It is late and I am getting tired...

>> I think my next project will be a complete seismometer and recorder 
constructed entirely off materials available at Home Depot and Radio 
Shack.....
    I was looking at an old 4 lb glass bottling jar (Kilner) the other day 
and wondered if a mini S-G seis. could be fitted inside....

    Regards Chris Chapman 

_____________________________________________________________________

Public Seismic Network Mailing List (PSN-L)