In a message dated 2008/09/02, tchannel1@............ writes:

> Hi Folks,  This a.m. I checked my sensor screen and found my Lehman, at 20 
> seconds, was recording some kind of noise.  At first I thought it looked like 
> the long freqs. from a distant earthquake.  However, they built up over a 5 
> hour period, all in the quiet time of last night.  My other two short period 
> sensors showed nothing.   I looked of an earthquake, but found nothing to 
> attribute this to.

Hi Ted,

       This sounds much more like reverse convection noise. Did it stop when 
the room temperature warmed up during the morning?

       Do you have an Al plate + heater resistors in the inside top of your 
insulating seismometer case?

       If not, one thing that can happen is reverse convection noise when the 
external air in the room cools quite a bit below the temperature of the 
seismometer and the floor. Rolls of cold air peel off the inside of the housing on 
alternate sides, sink down to the floor and push the arm about. You can check 
this easily by putting a 'knee' type electric heater pad set to the lowest 
heat on the external top of the case. The noise will then cease quite quickly. 
Alternatively, do you have a 'single' electric blanket?

       Hope that this helps!

       Regards,

       Chrios Chapman   
In a me=
ssage dated 2008/09/02, tchannel1@............ writes:



Hi Folks,  This a.m. I che=
cked my sensor screen and found my Lehman, at 20 seconds, was recording some=
 kind of noise.  At first I thought it looked like the long freqs. from=
 a distant earthquake.  However, they built up over a 5 hour period, al=
l in the quiet time of last night.  My other two short period sensors s=
howed nothing.   I looked of an earthquake, but found nothing to a=
ttribute this to.




Hi Ted,



       This sounds much more like reverse conv=
ection noise. Did it stop when the room temperature warmed up during the mor=
ning?



       Do you have an Al plate + heater resist=
ors in the inside top of your insulating seismometer case?



       If not, one thing that can happen is re=
verse convection noise when the external air in the room cools quite a bit b=
elow the temperature of the seismometer and the floor. Rolls of cold air pee=
l off the inside of the housing on alternate sides, sink down to the floor a=
nd push the arm about. You can check this easily by putting a 'knee' type el=
ectric heater pad set to the lowest heat on the external top of the case. Th=
e noise will then cease quite quickly. Alternatively, do you have a 'single'=
 electric blanket?



       Hope that this helps!



       Regards,



       Chrios Chapman