In a message dated 27/06/2003, meredithlamb@............. writes:

> Suspect that the "natural" period would be rather short with rather a 
> severe limited mass weight of course. Would suspect that the older (1960's-1970's) 
> (big) meters with 4 extending base mount screws would be the handiest to 
> convert/utilize. 

Hi Meredith,

     A half second pendulum is about 6.2 cm long ~2.5", 1/3 sec pendulum is 
~1.1" long, so this is likely to be within the useful period range. One of the 
edge view meters might be OK.

 I think this type of meter suspension is also called a "taut band"..>  Would 
> also guess that any circuit might need some adequate isolation from the 
> meter.....if I'am putting it right. Theres actually another alternative rather 
> than just amplifying the meter coil output. One could use the meter coil and a 
> suitable potentiometer....just for dampening. One could use light and two 
> photocells around the end of the meter pointer (boom/mass) and likely see truer 
> displacement longer periods than the original velocity meter coil/magnet 
> output. I even wonder if the meter coil could be a nice feedback mechanism with a 
> light output approach. One would likely need a fine and gentle mechanical 
> touch to do this I'am sure.

     The meter movements have a copper former to give damping and a fairly 
high resistance coil. With a relatively inefficient sensor, you may need all the 
sensitivity that you can get. Trying to damp a light pendulum system using 
the coil + a resistor will cut the sensitivity a lot. Putting the meter straight 
into an inverting opamp input might be a better approach, but I suspect that 
this will still only allow you to use a very light mass and getting it 
reasonably 'right' could be tricky. Separating out the damping using a magnet actng 
directly on the pendulum mass plate would allow you to use a larger mass. With 
the light seismic mass, the sensor is going to be relatively noisy, 
susceptible to air movements.... workable, but way off optimum.
     I was thinking in terms of what a school science student might do to 
make something fairly easily and which 'works'. If you want a good seismic 
detector, it would be better to make a 1 sec SG pendulum type, instead of trying to 
modify a meter. Looking at the current cost of meters, this might also be a 
cheaper option.

    Regards,

    Chris Chapman
In a message dated 27/06/2003, meredithlamb@earthlin=
k.net writes:



Suspect that the "natural" peri=
od would be rather short with rather a severe limited mass weight of course.=
 Would suspect that the older (1960's-1970's) (big) meters with 4 extending=20=
base mount screws would be the handiest to convert/utilize. 




Hi Meredith,



     A half second pendulum is about 6.2 cm long ~2.5",=20=
1/3 sec pendulum is ~1.1" long, so this is likely to be within the useful pe=
riod range. One of the edge view meters might be OK.



 I think this type of meter=20=
suspension is also called a "taut band"..
 Would also guess that any circuit might need some adequate isolat=
ion from the meter.....if I'am putting it right. Theres actually another alt=
ernative rather than just amplifying the meter coil output. One could use th=
e meter coil and a suitable potentiometer....just for dampening. One could u=
se light and two photocells around the end of the meter pointer (boom/mass)=20=
and likely see truer displacement longer periods than the original velocity=20=
meter coil/magnet output. I even wonder if the meter coil could be a nice fe=
edback mechanism with a light output approach. One would likely need a fine=20=
and gentle mechanical touch to do this I'am sure
.



     T=
he meter movements have a copper former to give damping and a fairly high re=
sistance coil. With a relatively inefficient sensor, you may need all the se=
nsitivity that you can get. Trying to damp a light pendulum system using the=
 coil + a resistor will cut the sensitivity a lot. Putting the meter straigh=
t into an inverting opamp input might be a better approach, but I suspect th=
at this will still only allow you to use a very light mass and getting it re=
asonably 'right' could be tricky. Separating out the damping using a magnet=20=
actng directly on the pendulum mass plate would allow you to use a larger ma=
ss. With the light seismic mass, the sensor is going to be relatively noisy,=
 susceptible to air movements.... workable, but way off optimum.

     I was thinking in terms of what a school science st=
udent might do to make something fairly easily and which 'works'. If you wan=
t a good seismic detector, it would be better to make a 1 sec SG pendulum ty=
pe, instead of trying to modify a meter. Looking at the current cost of mete=
rs, this might also be a cheaper option.



    Regards,



    Chris Chapman