In a message dated 18/11/2007, skmort@............ writes:

Small Space Heater  Control.



Hi Stephen,
 
    The problem with this is that the switch on  temperature is fixed. You 
would be OK if you used a differential thermometer,  one sensor on the ground 
and the other in top of the box. These could be two  diodes, two Pt elements or 
two Si thermometer micorcircuits with an amplifier  and offset control.
    The difference between not supplying enough heat  and getting reverse 
convection which upsets the seismometer and providing too  much heat which stirs 
up the air in the whole case is not that great. It depends  on the ground and 
outside air temperatures. The easiest method is to provide a  constant power 
to a heated Al panel and vary this until the convection just  stops in your 
particular box.
    If you use the Al cased resistors bolted to an Al  sheet, solder on 
insulated wires and cover the joints with adhesive  heatshrink tube, you can get a 
completely safe system. It will tend to  track the environmental changes 
fairly well and keep the top of the box just  warm enough, but setting it up in the 
first place may take a few tests with  different heater powers.
    Another alternative is to use a commercial  energy control module, such 
as is provided by a light dimmer control and use  this to drive your heater 
resistors. But these do tend to generate RFI and  you may need some protection 
capacitors.
 
    Regards,
 
    Chris Chapman



   






In a message dated 18/11/2007, skmort@............ writes:
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  style=3D"BACKGROUND-COLOR: transparent" face=3DCourier color=3D#000000 siz=
e=3D4>Small Space Heater=20
Control.


Hi Stephen,
 
    The problem with this is that the switch on=20
temperature is fixed. You would be OK if you used a differential thermometer=
,=20
one sensor on the ground and the other in top of the box. These could be two=
=20
diodes, two Pt elements or two Si thermometer micorcircuits with an amplifie=
r=20
and offset control.
    The difference between not supplying enough hea=
t=20
and getting reverse convection which upsets the seismometer and providing to=
o=20
much heat which stirs up the air in the whole case is not that great. It dep=
ends=20
on the ground and outside air temperatures. The easiest method is to provide=
 a=20
constant power to a heated Al panel and vary this until the convection just=20
stops in your particular box.
    If you use the Al cased resistors bolted to an=20=
Al=20
sheet, solder on insulated wires and cover the joints with adhesive=20
heatshrink tube, you can get a completely safe system. It will ten=
d to=20
track the environmental changes fairly well and keep the top of the box just=
=20
warm enough, but setting it up in the first place may take a few tests with=20
different heater powers.
    Another alternative is to use a commercial=
=20
energy control module, such as is provided by a light dimmer control and use=
=20
this to drive your heater resistors. But these do tend to generate RFI=20=
and=20
you may need some protection capacitors.
 
    Regards,
 
    Chris Chapman