Hi Folks,  I did check things at 9:00am,  A different set of noise from =
the city started about 7:00 am.   I took an hour trace and found FFT =
showing the city noises at around 2 to 4 seconds? and very little in the =
10 to 30 seconds of the unknown ealier noise.   So this long freq. =
unknown noise was all but gone.  It started about 1:00am, got worse =
until 7:00am, and then faded out by 9:00am? =20

 I now don't think it was not a cold front, however Chris may be correct =
in pointing out it could be "Reverse Convection Noise"   Here is =
why.....
The temperature in the room, in the basement changes very little, maybe =
10 degrees.    Last night, it was unusually cool, got down to 39 =
degrees, day time maybe 68, in this room.   In the next room we had an =
open window, with the cool night air coming in.   Normally the window is =
open but the room does not make big swings in temperature, and I have =
not seen long freq noise before.  I don't think it was a draft coming in =
the window, I think it was the cooling of the basement and the room, and =
the enclosure, as Chris described.

The Lehman has a Styrofoam box, which mainly just keeps the drafts out.  =
It would not insulate much.   I will hope to see the same noise tonight, =
if I do I will close the window, to stop the cooling of the room.   I =
will wait 1/2 hour and then open the window again.

This noise is much like what happened a long time ago,when another =
sensor set in the garage and the cool night air would raise from the =
concrete floor.   That stopped when I enclosed sensor.  This sensor is =
enclose from draft, but I think it is responding to cooling.

Thanks, Ted


  ----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Tuesday, September 02, 2008 9:06 AM
  Subject: Re: New Noise


  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,








Hi Folks,  I did check things at =
9:00am, =20
A different set of noise from the city started about 7:00 =
am.   I took=20
an hour trace and found FFT showing the city noises at around 2 to 4 =
seconds?=20
and very little in the 10 to 30 seconds of the unknown ealier =
noise.  =20
So this long freq. unknown noise was all but =
gone.  It=20
started about 1:00am, got worse until 7:00am, and then =
faded out by=20
9:00am?  
 
 I now don't think it was =
not a cold=20
front, however Chris may be correct in pointing out it could be "Reverse =

Convection Noise"   Here is why.....
The temperature in the room, in the =
basement=20
changes very little, maybe 10 degrees.    Last night, it =
was=20
unusually cool, got down to 39 degrees, day time maybe 68, in this=20
room.   In the next room we had an open window, with the cool =
night=20
air coming in.   Normally the window is open but the room does =
not=20
make big swings in temperature, and I have not seen long freq noise=20
before.  I don't think it was a draft coming in the window, I think =
it was=20
the cooling of the basement and the room, and the enclosure, as Chris=20
described.
 
The Lehman has a Styrofoam box, which =
mainly just=20
keeps the drafts out.  It would not insulate much.   I =
will hope=20
to see the same noise tonight, if I do I will close the window, to stop =
the=20
cooling of the room.   I will wait 1/2 hour and then open the =
window=20
again.
 
This noise is much like what happened a =
long time=20
ago,when another sensor set in the garage and the cool night air would =
raise=20
from the concrete floor.   That stopped when I enclosed =
sensor. =20
This sensor is enclose from draft, but I think it is responding to=20
cooling.
 
Thanks, Ted
 
 

  
----- Original Message ----- 
  From:=20
  ChrisAtUpw@....... 
  
To: psn-l@.............. 
  
Sent: Tuesday, September 02, =
2008 9:06=20
  AM
  
Subject: Re: New Noise
  

In a=20
  message dated 2008/09/02, tchannel1@............=20
writes:


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

Hi =
Ted,

      =20
  This sounds much more like reverse convection noise. Did it stop when =
the room=20
  temperature warmed up during the=20
  morning?

       Do you have an Al =
plate +=20
  heater resistors in the inside top of your insulating seismometer=20
  case?

       If not, one thing =
that can=20
  happen is reverse convection noise when the external air in the room =
cools=20
  quite a bit below the temperature of the seismometer and the floor. =
Rolls of=20
  cold air peel off the inside of the housing on alternate sides, sink =
down to=20
  the floor and push the arm about. You can check this easily by putting =
a=20
  'knee' type electric heater pad set to the lowest heat on the external =
top of=20
  the case. The noise will then cease quite quickly. Alternatively, do =
you have=20
  a 'single' electric =
blanket?

       Hope=20
  that this helps!

      =20
  Regards,

       Chrios =
Chapman=20