In a message dated 10/07/01, dlstevens69@........... writes:

> 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 
> 

Dear Dan,

       There is no unique solution. Horizontal seis are less sensitive than 
vertical ones. Since the room is dark, having lights inside a transparent 
enclosure is likely to attract bugs. How constant is your 'constant 
temperature'? Does the temperature control switch off at any time? Do you 
have a min/max thermometer? The usual thinking is that the ground naturally 
provides the most stable temperature, so you put a well insulated rigid box 
over the seismometer and put a small heater in the top to maintain a 
reasonably constant vertical temperature gradient. This also provides some 
control over condensation. You can get rigid sheet foam with Al foil surfaces 
from building suppliers up to 3" thick, celotex?, which can be cut up using a 
fine wood saw and stuck together using polyurethane spray can foam. Gaffer 
tape will provide protection for the corners / edges. 2 mm Plexiglas is OK 
for the double windows, but check that it is clean before you stick it on. 
Polyurethane mastic sticks this well, epoxy does not. You may need to be able 
to 'semi seal' the box to the ground. A heater method which has not been 
mentioned is to use a sheet of Al inside the top of the box and stick some of 
the square section wire wound power resistors onto the topside with epoxy. 
The wiring and connections are then protected. My personal preference is to 
use a small transformer with tappings maybe up to 24 V, rather than having 
mains voltages anywhere near damp conditions. A block of camphor inside the 
case can be a reasonable insect deterrent. Shell Vapona flystrip is very 
effective. For lighting, gas discharge striplights generate much less heat 
than tungsten bulbs. You may want to provide a hex socket screw driver with 
an extended rod through the wall to be able to adjust the level without 
removing the box. See past PSN letters for lengthy discussions on enclosures! 

       Regards,

       Chris Chapman     


In a message dated 10/07/01, dlstevens69@........... writes:



When b
uilding 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?



Dear Dan,



       There is no unique solution. Horizontal seis are less sensitive than 

vertical ones. Since the room is dark, having lights inside a transparent 

enclosure is likely to attract bugs. How constant is your 'constant 

temperature'? Does the temperature control switch off at any time? Do you 

have a min/max thermometer? The usual thinking is that the ground naturally 

provides the most stable temperature, so you put a well insulated rigid box 

over the seismometer and put a small heater in the top to maintain a 

reasonably constant vertical temperature gradient. This also provides some 

control over condensation. You can get rigid sheet foam with Al foil surfaces 

from building suppliers up to 3" thick, celotex?, which can be cut up using a 

fine wood saw and stuck together using polyurethane spray can foam. Gaffer 

tape will provide protection for the corners / edges. 2 mm Plexiglas is OK 

for the double windows, but check that it is clean before you stick it on. 

Polyurethane mastic sticks this well, epoxy does not. You may need to be able 

to 'semi seal' the box to the ground. A heater method which has not been 

mentioned is to use a sheet of Al inside the top of the box and stick some of 

the square section wire wound power resistors onto the topside with epoxy. 

The wiring and connections are then protected. My personal preference is to 

use a small transformer with tappings maybe up to 24 V, rather than having 

mains voltages anywhere near damp conditions. A block of camphor inside the 

case can be a reasonable insect deterrent. Shell Vapona flystrip is very 

effective. For lighting, gas discharge striplights generate much less heat 

than tungsten bulbs. You may want to provide a hex socket screw driver with 

an extended rod through the wall to be able to adjust the level without 

removing the box. See past PSN letters for lengthy discussions on enclosures! 



       Regards,



       Chris Chapman