I know I'm butting in ,
but can someone please tell me why people never use a vacuum
which I would think would stop all direct air related problems,
but not the indirect problems like heavier air on the earths surface ?
No air pressure/density problems no convections problems,
possibly only radiated heat in the form of microwaves.
Even getting down to mars pressure like 5 to 7 mm
of Hg would be better than 760mm Hg ???
How many torr is a good vacuum ?


-----Original Message----- 
From: Christopher Chapman
Sent: Tuesday, December 07, 2010 12:03 PM
To: bryangoss@........ ; psnlist@..............
Subject: Re: Lehman seismometer






Thanks Chris,

I was able to remove the reverse convection problem.
I placed a heating pad on top of the wood box and inside the styrofoam box.

Hi Bryan,

    Check on how much power it consumes? The running cost could be significant.

The outdoor temperatures here was 76F 24 hrs ago to 25F tonight and it is staying in the 60s constant in the void between the two 
boxes. this stopped all the drifting or convection noise.

    This sounds like quite a large temperature increase. I try to provide the minimum heat required to keep the air column stable 
inside the seismometer case.

There seem to be a increase in the 6-second microseisms as a strong cold front moved through.

    This is a common observation. Microseisms can be generated by weather systems and by local storms as well as by the common deep 
ocean currents / storms.

Do you have pictures of your instruments? I would like to see them.

    My original seismometer used twin L booms of 1” x 1” x 1/8” mild steel angle, about 5” apart and cross braced to provide the 
horizontal and vertical frame. It seemed to work well and reliably. The bearing used a SS ball and a carbide tool counterface. The 
top suspension was a short 10 thou music wire. The base was 30” long and the column 18” high. The effective Al boom length ‘k’ was 
22” ~ a 1.5 second pendulum. These intermediate size pendulums are much easier to build and to operate, than the 1 metre, two second 
ones, while still giving an excellent performance for the 20 second surface waves. The practical limit to the set period is 
determined by the tilt drift experienced locally and is usually less than 30 seconds.




    I later cooperated with Stewart Bullen, a secondary school physics teacher, to design and test a seismometer for use in UK 
schools. He had been using a Lehman seismometer in teaching. The British Geological Survey are backing the project and SEP + MUTR 
produced the seismometer.

    I changed the construction of the frame to 3” x ¾” section Aluminum bars. One of my prototypes can be seen at 
www.jclahr.com/science/psn/chapman.html see ‘latest seismometer‘. Stewart and I had suggested satisfactory seismometer constructions 
to SEP, but their commercial ‘’experts’’ tried to redesign it, with very limited success. See the MuVentures photos with Paul 
demonstrating a later prototype.


     A prototype, which I still use, is shown at the ‘latest seismometer’ and it has a 3” x ¾” section Aluminum bar frame. The 
length is 30”, the vertical bar is 18” and the width is 12”. The arm is a bit over 22” long, giving a 1.5 second pendulum. Shorter 
pendulums are easier to construct, house and operate than 1 metre 2 second pendulums.

    A brass seismic mass of 1 kg is clamped to the Aluminum boom. The red sensor magnet frame uses ¼” thick mild steel backing 
plates and 4 off 1” x ½” x 1/8” NdFeB magnets. It is bolted together with 6 mm zinc plated mild steel bolts, which fit onto the 
outside of the base bar. The coil is wound on a rectangular former which gives good signal linearity, as opposed to the old type 
chunky round coils. The coil is bolted to the square ¾” thick mounting plate.

    The mounting plate is fitted with two ½” OD Al rod side extensions with V grooves. A V shaped 7 core SS cable made from fishing 
trace has loop ends crimped on, which fit into these grooves. This opposes any tendency of the arm to oscillate around it’s long 
axis when a quake occurs. The top loop is threaded around a groove on the edge of a 1.5” SS panel mount washer. A 10 thou OD music 
wire suspension is used.

    A horizontal 1/16” thick Copper damping plate is bolted to the mounting plate on the side facing the bearing. The red damping 
magnet block uses 4 off 1” x 1” x ¼” NdFeB magnets and it is mounted on the Al bar base. The block can be slid along the base to 
vary the length of the Cu damping plate covered by the field and so vairy the damping.

    The bottom bearing consists of a ½” OD SS ball bearing pressed into a hole drilled in the edge of the vertical column. The end 
of the Al arm is machined flat and a section of SS razor blade is glued to it with two component acrylic cement to provide the 
hardened counterface. This is a much better glue than Epoxy for mechanical constructions and it is tough when it sets, not brittle.

    The MUTR production seismometer together with application programs and technical notes are listed at 
www.bgs.ac.uk/schoolSeismology/seismometer.html
     There is also a 30 page technical manual for it, which is well worth reading for potential constructors, at 
www.mutr.co.uk/images/seismometer.pdf The sensitivity range is from 5Hz to over 20 seconds period. It uses crossed rod tungsten 
carbide bearings for both the top and the bottom suspensions.


    The UK schools’ seismic initiative has proved to be highly successful project with over 475 instruments having been sold since 
Easter 2007!

I hope that this description will be of help to constructors.

Regards,

Chris Chapman



__________________________________________________________

Public Seismic Network Mailing List (PSNLIST)

To leave this list email PSNLIST-REQUEST@.............. with 
the body of the message (first line only): unsubscribe
See http://www.seismicnet.com/maillist.html for more information.