Magnetic damping as an alternative to oil damping for an AS-1 seismic sensor.

    Oil damping was ditched by the professionals somewhere in the late 1940s. It is relatively difficult to adjust, potentially messy, damps the higher frequencies selectively, is VERY temperature dependant and drowns bugs to give 'bug quakes'.

    Electromagnetic damping with quad NdFeB magnets is not expensive, not significantly temperature dependant, has a linear response, is clean, dead easy to adjust and very effective. Let me know if you want further advice / help.  Why accept operational problems / headaches which you do not need to have? 

    Considering magnetic construction materials, I can buy 2" wide by 0.25" thick bright rolled mild steel strip, which is adequately straight and flat. It seems to be readily available.  While you can tap some of the holes, it is not necessary. You use a hacksaw to cut the mild steel strip to length, file off the sharp ends, clamp the two plates together and drill clearance holes through both at the same time. This gets the bolt holes exactly aligned. I use a bolt with three nuts.

    I dip the cut, filed and drilled backing plates in Phosphoric Acid supplied for car repairs for a few minutes and then wipe and dry them - I do NOT rinse them. I coat most of the plate with Hammerite rust protecting paint, but give the magnet and bolt contact areas just a smear of CL chassis grease, which has rust inhibitor in it.

    For batch quantities, I would have the plates cut off on a milling machine. Then I would have them drilled, the edges finished, have the plates pickled and then Nickel plated.  Nickel plating is a common finishing process.

   I use three, 2" long, 1/4" diameter zinc plated mild steel bolts with UNC thread, matching nuts and washers.  Our general hardware stores carry this zinc plated type. Pan head or dome head will also do fine, but they are better zinc plated.

    I've checked for any magnetic field penetration through the 1/4" mild steel plates and they seem fine with the magnets and separations specified, but 1/8" thick mild steel is definitely NOT adequate. If you wished to reduce the backing plate area with less than a 1/2" border between the magnets and the edge of the plate, or chose thicker magnets, you might need to increase the backing plate thickness.



4 magnets                                                     $16.07


2 pieces of 2"x3"x1/4" steel                         $1.20


3 1.5"x1/4" bolts and 6 nuts                          $1.05


5"x7"x0.016" Cu sheet (K&S Engineering)        $6.49


2"x1/4" threaded brass rod & 2 brass nuts   


Damping arrangement.

Newer design to work with both the AS-1 and the EQ1 systems.

Construction details.  4 Magnets. 1" x 1/2" x 1/4". Wondermagnet: 0033, $3 ea. plus $4.07 handling and first class mail  K&J Magnetics: BXO84, $2.60 ea. plus $9.18 UPS ground.

The unit pictured here was manufactured by:
King Machine
4100 SW Research Way
Corvallis, Oregon 97333
541 752 8628 Phone

Magnet are slid to the right to expose copper damping vane.   Chris specifies copper plate that is 1/16" (0.0625") thick.  I used three layers of 0.016" for a total of 0.048".  I bought brass threaded rod, cut a slot with a hacksaw and soft soldered the joint with the damping vane.  You could equally well glue it together with two part acrylic cement. Epoxy does not stick copper or soft Al very well.

Magnets are in place for damping.

Fish-tank enclosure to shield from air currents.

Original design with oil damping.

Lahr's version of this system.