Chris,

Thinking about the damping, it seems like a basic coil and magnet sensor
like the AS-1 would be self damping due to the movement of the copper
winding in the magnetic field.  Or is it a case of it being self damping but
still underdamped and needing additional damping?

Thanks,

Bob

On Wed, Dec 10, 2008 at 5:30 AM,  wrote:

>  In a message dated 09/12/2008, sismos@.............. writes:
>
> Hello Dayana,
> Don't buy the black box that you see with an AS1.
> you can do much better and probably less expensive or close with Larry's
> boards.
>
>  Hi Dayana,
>
>     I believe that Jeff Batten amateurseismo@......... will sell the
> electronics box and power supply separately.
>
>     Seismometers like the AS1 and EQ1 try to use oil + plunge plate
> damping. The viscosity of oil changes by about x10 for every 20 C Deg / 36 F
> Deg change in temperature. If you set up the damping to be correct at your
> room temperature, you will only have an operating range of about +/-4 F Deg.
> This is LESS than the normal daily variation. If the temperature falls much
> below this range, the damping increases dramatically and the seismometer MAY
> stop recording altogether. It's sensitivity will certainly decrease and the
> calibration will NOT give the correct quake magnitude. I have designed a
> replacement magnetic damper which is NOT effected by temperature and is
> both easy to set up and CLEAN! No more oil creepage or spills!
>     See  http://jclahr.com/science/psn/chapman/as1%20damping/index.html
>
>     The AS1 uses a vertical swing arm with a natural period of about 1.5
> seconds. So while the response is flat with velocity at periods shorter than
> this (like a geophone), the sensitivity will fall off as f^2 below 0.75 Hz
> when using a 'normal' seismic amplifier, like Larry's. The sensitivity to
> the 20 second period Rayleigh waves, will be down by a factor of about 180 -
> so you normally won't see any!
>     A period extension circuit has been used in the AS1 'black box' to
> extend the sensitivity ~flat from 1.5 to about 4.5 seconds. This boosts the
> low frequency output. While it is OK for measuring microseisms, it is
> otherwise rather limiting. A further period extension can be applied with
> the digital compenation filter supplied with AmaSeis, but it only works well
> on moderately strong signals.
>     I have extended the period to about 20 seconds by changing four
> capacitors and three resistors. It now seems to work satisfactorily and
> covers most of the normal seismic range.
>
>     On the EQ1 that we examined, the compensation covered about the same
> compensation period range as an AS1, but the output seemed to be roughly
> proportional to f. This did NOT give a flat output. Since the power laws are
> different, I would not expect the digital frequency compensation built into
> AmaSeis to work adequately with an EQ1 trace. The EQ1 electronics uses
> miniature surface mount components, so I would NOT expect you to be able to
> modify EITHER the compensation range OR the power law.
>
>     The relatively long springs expand and contract with temperature and
> may well need adjustment with the seasons.
>
>     The AS1 is designed with the centre of gravity on the horizontal arm.
>
>
>
>
Chris,

Thinking about the damping, it seems like a basic coil and magnet sensor like the AS-1 would be self damping due to the movement of the copper winding in the magnetic field.  Or is it a case of it being self damping but still underdamped and needing additional damping?


Thanks,

Bob

On Wed, Dec 10, 2008 at 5:30 AM,  <ChrisAtUpw@.......> wrote:







In a message dated 09/12/2008, sismos@.............. writes:
Hello 
  Dayana,
Don't buy the black box that you see with an AS1.
you can do 
  much better and probably less expensive or close with Larry's 
  boards.


Hi Dayana,
 
    I believe that Jeff Batten amateurseismo@......... will sell the 
electronics box and power supply separately. 
 
    Seismometers like the AS1 and EQ1 try to use oil + 
plunge plate damping. The viscosity of oil changes by about x10 for every 20 C 
Deg / 36 F Deg change in temperature. If you set up the damping to be correct at 
your room temperature, you will only have an operating range of about +/-4 F 
Deg. This is LESS than the normal daily variation. If the temperature falls much 
below this range, the damping increases dramatically and the seismometer MAY 
stop recording altogether. It's sensitivity will certainly decrease and the 
calibration will NOT give the correct quake magnitude. I have designed a 
replacement magnetic damper which is NOT effected by temperature and is 
both easy to set up and CLEAN! No more oil creepage or spills!
    See  http://jclahr.com/science/psn/chapman/as1%20damping/index.html 
 
    The AS1 uses a vertical swing arm with a natural 
period of about 1.5 seconds. So while the response is flat with velocity at 
periods shorter than this (like a geophone), the sensitivity will fall off as 
f^2 below 0.75 Hz when using a 'normal' seismic amplifier, like Larry's. The 
sensitivity to the 20 second period Rayleigh waves, will be down by a factor of 
about 180 - so you normally won't see any!
    A period extension circuit has been used in the AS1 
'black box' to extend the sensitivity ~flat from 1.5 to about 4.5 seconds. 
This boosts the low frequency output. While it is OK for measuring 
microseisms, it is otherwise rather limiting. A further period 
extension can be applied with the digital compenation filter supplied with 
AmaSeis, but it only works well on moderately strong signals. 
    I have extended the period to about 20 seconds 
by changing four capacitors and three resistors. It now seems to 
work satisfactorily and covers most of the normal seismic range.
 
    On the EQ1 that we examined, the compensation 
covered about the same compensation period range as an AS1, but the output 
seemed to be roughly proportional to f. This did NOT give a flat output. Since 
the power laws are different, I would not expect the digital frequency 
compensation built into AmaSeis to work adequately with an EQ1 trace. The 
EQ1 electronics uses miniature surface mount components, so I would NOT 
expect you to be able to modify EITHER the compensation range OR the 
power law.
 
    The relatively long springs expand and contract 
with temperature and may well need adjustment with the seasons.
 
    The AS1 is designed with the centre of gravity on 
the horizontal arm.