Convection Currents -
  I solved 'em for me

Karl Cunningham wrote:
> 
> Jim --
> 
> For the regular Lehman design, the amount of mass used does not affect the
> sensitivity of the seismometer, per se.  However, a larger mass makes the
> instrument less sensitive to convection currents and reduces the effect of
> any friction in the boom hinge.  On the other hand, a larger mass means a
> more heavy-duty suspension system and perhaps more friction in the hinge.
> So it is a tradeoff, and there is really no right answer.
> 

Ah - convection currents - the "3 A.M. blues" -
The seismometer runs fine *except* about 3 A.M.
Then, some nights, there are these slow swings 
all over the place for maybe 10 minutes (or longer!). 

The following insulated box and heater system 
eliminated the "3 A.M. Blues". It worked "perfectly" with a
light (?12 ounce?) pendulum garden gate type horizontal seismometer 
adjusted to a 30 second period.

------------------------------------

I figured the "3 A.M. blues" were air currents inside the 
insulated box protecting the seismometer.  I figured 
the air currents were caused when 
  1) the outside got colder than inside the box 
  2) cooled the top of the insulated box, 
  3) and the top of the air inside, 
  4) and the cool top air, now an unstable thermal inversion layer,
  5) "rotated" to the bottom of the insulated box
  6) until some sort of thermal equilibrium was established.

So - goals - make an insulated box that:
  1) the top of the inside of the insulated box was
     always warmer than the bottom of the insulated box
  2) keep out the creepy crawly things - spiders to professionals
  3) easy to examine (for level, etc.) without disturbing
  4) reasonably easy access for leveling, etc.

There is probably nothing original about the following box.
I imagine most stubborn long period folk wind up 
re-discovering the following box.

So - the bottom and sides of the box
  1) I got some 2" thick expanded Styrofoam from the local
     home depot type store,

  2) cut to generous box size - I tended to make "garden gate"
     seismometers about 12x14x14 inches - so 24 inch squares
     on the sides was fine - I am lazy and did not bevel the corners -

  3) "glue" the sides with the usual window caulking or paste.
     Just butt the end of one piece to the side of another piece -
     no particular art.
     (Real chemical glues dissolve the foam in an instant.)

  4) place the "glued" sides surrounding the area where you
     want to put the seismometer.  I am assuming a concrete slab
     or pillar.
     (I did not caulk the sides to the concrete, and had no 
       problem - but securing the sides to the cement would 
       reduce your worries of kicking the box over.)

  5) dose the concrete and insides of the box with all the 
     non-styrofoam dissolving insecticides in your house

  6) now - to keep out ground moisture, and a possible
     mildew problem, lay a sheet of 6x6 foot thin plastic -
     like 1 mil painter's "drop cloth" - on the concrete bottom
     and up the sides.  Trim and hold in place (seal) with
     your handy duct tape - (it will last in this application)

and - the top of the box
  7) I divided the top into 2 equal areas 
      - both made of the 2 inch styrofoam 
      - neither glued down to the sides or anything
      - just gravity holds them in place

         (OK - I cheated - styrofoam just floats away
          with the slightest excuse - 

          A big block Styrofoam can probably levitate 
	  from static electricity!
          
          I used a big old Mexican sombrero as a weight 
          to hold the two half tops on.
          It also helped keep the cat off the box!)

     a) One half of the top holds the heater (under the top)
          (two 4 watt night lights in electrical series 
             - reduce heating and lasts forever)
         I used a piece of sheet aluminum, like for roofing,
         to spread the heat of the lamps over the top of
         this half of the box. 

         The electric wires for the heater were twisted,
         and entered the box through a little notch in
         the top of the styrofoam side - on the other side
         from the electronics wires.  I laid cotton in the
         notch to help keep insects out.

         This half of the top is moved only when you have to
         do something really major with the instrument
     b) 
       - a thin plastic window, say 3/32" or 1/8 inch clear plastic
         covers the open part of the box.  You can look through
         this without disturbing the seismometer too much,
         and show your friends how you waste your evenings.
         (My wife said my friends were "victims" when I forced
          them to look at my machinery.)
       - another 2 inch thick piece of styrofoam normally  
         covers the above plastic window.

Now install the seismometer
  - place a hard plastic slab say 1/4 inch thick on the thin
    plastic at the bottom of the box - this will protect
    the thin plastic from the twisting effects seismometer
    leveling screws

  - place the seismometer on the thick plastic an do a 
    rough preliminary level, period, and damping adjustment.

  - turn on the heater and electronics

  - might as well let the system sit, warming and stabilizing
    for several days before thinking of serious adjustments.

General comments:
   a) I was afraid the air in the top would get heat, heat the
      copper pipe of the sensor, travel down the copper pipe
      and the heated pipe would create its own air currents.
      That did not seem to happen enough to notice.

      I had considered lining the inside vertical walls 
      of the box with sheet aluminum or copper to try to
      smooth the temperature difference from the warm top
      and the cooler bottom.  The box worked very  well
      for my purposes with out the temperature smoothing metal.

   b) I mounted all the electronics boards
         - selectable damping resistors
         - chopper op amp
         - analog to digital converter, etc.
       on a little stand so they would be high in the box.
       This way, any heat they produced would disturb air
       "only" in the top of the box.

   c) I tended to make quite light (1 pound or less) pendulums! 
        Hinges, pivots, flex wires, or what ever are interesting
        enough with 1 pound - seven pounds might as well be a ton.

   d) The above box and heater system worked "perfectly" with  
      a light pendulum garden gate type horizontal seismometer 
      adjusted to a 30 second period.

   e) Another remaining worry was that the expansion differential
      between the concrete, the plastic, and the copper frame
      of the sensor would cause slippage (and a big step function).  
      It seemed to be OK - I did not have to go to 
      temperature control.  I do tend to use thin leveling screws 
      that can probably bend enough to handle the strains
      due to differential expansion without slipping.

      The box was in a "protected" shed in the relatively
      stable climate of coastal California.  Potentially if
      the unit had to operate over a wider temperature range,
      some sort of thermostatically controlled environment
      might be necessary to handle differential expansions.

      (It would be wonderful to have your sensor in a cave of 
       bedrock and practically zero temperature changes.  :-)

-------------------------------------
You know, sometimes I miss those long slow rolling seismic waves
coming in from the Tonga islands.  A real kick.

Cheers
  Ed Thelen

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