In a message dated 09/02/2005, rsparks@.......... writes:

I am  looking for some feedback on my thoughts on liquid damping systems 
for  amateur seismometers.



Hi Roger,
 
    Consider the following
 
    1. The viscosity of oil varies by about an order of  magnitude for every 
20 C Deg change. It is NOT 'nearly constant', as one account  claims. The 
variations over 'normal' ambient house temperatures are huge.
 
    2. The viscous drag varies with the shear rate,  making the response 
strongly frequency dependant. 
 
    3. It is not a linear dependance.
 
    YOU JUST DO NOT NEED THESE PROBLEMS ! For these  reasons, professional 
equipment stopped using fluid damping in the 1940s to  1950s and now uses 
passive magnetic damping or active feedback magnetic  damping.
    With the ready availability of inexpensive NdFeB  magnets, it is simple 
and easy to make very efficient dampers, which  are dead easy to adjust,  using 
two soft iron backing plates with  NS and SN opposing bar magnets and an 
overlapping Cu or soft Al vane. The  damping is hardly effected at all by ambient 
temperature changes. 
    1"x1/2"x1/4" thick NdFeB bar magnets with the poles  on the large face do 
the job very nicely. 1/4" to 3/8" thick mild steel backing  plates are fine. 
The only waring is that you do need to clean up the magnet  faces with gaffer 
tape before final assembly to remove any fine wiskers of  magnetic material.
    Magnetic damping is cleaner, cheaper and  easier than oil damping systems.
 
    Regards,
 
    Chris Chapman






In a message dated 09/02/2005, rsparks@.......... writes:
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=3D2>I am=20
  looking for some feedback on my thoughts on liquid damping systems 
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  amateur seismometers.


Hi Roger,
 
    Consider the following
 
    1. The viscosity of oil varies by about an orde=
r of=20
magnitude for every 20 C Deg change. It is NOT 'nearly constant', as one acc=
ount=20
claims. The variations over 'normal' ambient house temperatures are huge.
 
    2. The viscous drag varies with the shear rate,=
=20
making the response strongly frequency dependant. 
 
    3. It is not a linear dependance.
 
    YOU JUST DO NOT NEED THESE PROBLEMS ! For these=
=20
reasons, professional equipment stopped using fluid damping in the 1940s to=20
1950s and now uses passive magnetic damping or active feedback magnetic=20
damping.
    With the ready availability of inexpensive NdFe=
B=20
magnets, it is simple and easy to make very efficient dampers, w=
hich=20
are dead easy to adjust,  using two soft iron backing plates =
with=20
NS and SN opposing bar magnets and an overlapping Cu or soft Al vane. T=
he=20
damping is hardly effected at all by ambient temperature changes. 
    1"x1/2"x1/4" thick NdFeB bar magnets with the p=
oles=20
on the large face do the job very nicely. 1/4" to 3/8" thick mild steel back=
ing=20
plates are fine. The only waring is that you do need to clean up the magnet=20
faces with gaffer tape before final assembly to remove any fine wiskers of=20
magnetic material.
    Magnetic damping is cleaner, cheaper and=20
easier than oil damping systems.
 
    Regards,
 
    Chris Chapman