Ron Westfall wrote:
> 
> A simple cover will reduce air currents but it will not eliminate them
> altogether.  Convective circulation of air will still occur inside the
> cover due to small temperature differences.  To get truly stagnant air
> inside the cover, it is necessary to create a temperature gradient between
> the bottom and top of the cover.  The top should be warmer than the bottom
> to eliminate convective circulation.
> 
> A number of mechanisms can be created to do this (small light bulbs at the
> top of the box, etc.).  I found that most solutions generate too much power
> and the generated heat is too localized at one spot at the inside top of
> the cover.
> 
> What I ended up doing was to buy a bunch of ordinary (cheap) 1/4 (or 1/2)
> watt resistors and solder them end to end.  I also put heat shrink tubing
> over the chain of resistors for insulation.  I hooked the resistors directly
> across 115V AC power run to the box via a cord.  The cumulative resistance was
> chosen so that each resistor did not consume more than its rated maximum of
> 1/4 (or 1/2) watt and so that the chain of resistors in total generated the
> desired 5 or 10 watts.  Keep in mind that AC power is a sine wave which has
> to be taken into consideration when calculating power consumption i.e 115V is
> the peak voltage, not the average voltage.

There seems to be a bit of a misunderstanding here.  Domestic mains
power (the wall outlet kind) be it 110V AC, 117V AC, 120V AC (the most
common modern standard) or whatever your local system provides is the
voltage rating in R.M.S. (Root Mean Square) voltage.  Without getting
into a short course in electrical engineering, RMS voltage is the
equivalent of an equal DC voltage in terms of heating power.

The peak voltage of the sinewave is the square root of two times the
RMS.  For 115V AC mains this is about 163 Volts peak or 325 Volts peak
to peak.  Just hang a good o'scope on the line to verify this.

Thus the smallest value 1/4W resistor that can safely be placed across a
115V AC main is 52,900 Ohms.

P=E^2 / R == 115^2 / 52900 == 13,225 / 52900 == 0.250 W.

I agree that the technique of stringing together a large number of
resistors is a good approach.  Some 20 years back, I built a quartz
crystal based standard frequency oscillator.  Quartz crystals must be
oven stabilized for maximum stability.  I used a long string of
resistors epoxied to the oscillator circuit board as a heater and a
thermistor driven feedback loop to control the power input to the heater
string.  Results were quite good.

  Terminal strips can be used to
> anchor the chain (or chains) of resistors to the inside top of the cover.
> 
> One highly desirable aspect of the resistor chain is that the heat is
> generated in a distributed fashion.  By laying out the chain (or chains)
> evenly across the inside top cover, you get even heat generation and more
> evenly stagnant air.  The solution has the advantage of not requiring power
> supplies and the components are pretty cheap.
> 
> Keeping critters out is good too.  I have heard people on the
> list grump about spiders who like to build their webs using the seismo
> boom as an anchor point.
> 
> IMHO, I would get a cover material that is moderately stiff.  If you were to
> suddenly close the door of the room where the seismo is located, there will
> be a small impulse air pressure change in the room depending on the size of
> the room and the presence of any other openings.  A flimsy cover might move
> enough to generate a small amount of vibration.  I use 1/4" masonite which
> seems to work reasonably well.  When I open the door, I can see a very small
> amount of noise on the trace, but to tell you the truth, I am not sure if it
> is due to air pressure or vibration transmitted through the concrete floor.
> 
> Ron Westfall
> 
> > 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
> > thickness of the material used very crucial? Does anyone have any
> > pre-construction tips or pointers that would be helpful?
> > -dan
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Robert L. (Bob) Smith			Smith Machine Works, Inc.
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