Ian,

Why don't you use electromagnetic damping of the moving coil? Fluid
damping of seismometers went away in the 30's. It is mostly guesswork,
position dependent, messy, and it is very difficult to make an oil/vane
dashpot that critically damps a large mass because of the close 
clearances needed vs the imprecise movement of the large boom. And
just before THE great quake, a beetle will drown itself in the oil.

I have described electromagnetic damping previously; it simply involves
shunting the main coil with an appropriate resistor. Modern amplifiers
have no problem making up for the resulting signal loss.  If you know (or
can determine) a number of parameters of your coil/magnet, the damping 
can be exactly calculated. Once installed, you can forget about it, whereas
the oil dashpot is a constant p.i.a. 

You should find it in the PSN archives. If not, I will repeat it.

Regards,
Sean-Thomas
__________________________________________________________

Public Seismic Network Mailing List (PSN-L)