A simple cover will reduce air currents but it will not eliminate them
altogether.  Convective circulation of air will still occur inside the
cover due to small temperature differences.  To get truly stagnant air
inside the cover, it is necessary to create a temperature gradient between
the bottom and top of the cover.  The top should be warmer than the bottom
to eliminate convective circulation.

A number of mechanisms can be created to do this (small light bulbs at the
top of the box, etc.).  I found that most solutions generate too much power
and the generated heat is too localized at one spot at the inside top of
the cover.

What I ended up doing was to buy a bunch of ordinary (cheap) 1/4 (or 1/2)
watt resistors and solder them end to end.  I also put heat shrink tubing
over the chain of resistors for insulation.  I hooked the resistors directly
across 115V AC power run to the box via a cord.  The cumulative resistance was
chosen so that each resistor did not consume more than its rated maximum of
1/4 (or 1/2) watt and so that the chain of resistors in total generated the
desired 5 or 10 watts.  Keep in mind that AC power is a sine wave which has
to be taken into consideration when calculating power consumption i.e 115V is
the peak voltage, not the average voltage.  Terminal strips can be used to
anchor the chain (or chains) of resistors to the inside top of the cover.

One highly desirable aspect of the resistor chain is that the heat is
generated in a distributed fashion.  By laying out the chain (or chains)
evenly across the inside top cover, you get even heat generation and more
evenly stagnant air.  The solution has the advantage of not requiring power
supplies and the components are pretty cheap.

Keeping critters out is good too.  I have heard people on the
list grump about spiders who like to build their webs using the seismo
boom as an anchor point.

IMHO, I would get a cover material that is moderately stiff.  If you were to
suddenly close the door of the room where the seismo is located, there will
be a small impulse air pressure change in the room depending on the size of
the room and the presence of any other openings.  A flimsy cover might move
enough to generate a small amount of vibration.  I use 1/4" masonite which
seems to work reasonably well.  When I open the door, I can see a very small
amount of noise on the trace, but to tell you the truth, I am not sure if it
is due to air pressure or vibration transmitted through the concrete floor.

Ron Westfall

> When building a cover for my Lehman what considerations should I take into 
> account? Obviously air currents are the main thing I'm trying to cut out, 
> but what about temperature and critters? The seismo is located inside a 
> windowless room that is kept at a constant temperature year-round. I'm also 
> hoping to use plexiglass, glass, or anything that I can see through. Is the 
> thickness of the material used very crucial? Does anyone have any 
> pre-construction tips or pointers that would be helpful?
> -dan
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