PSN-L Email List Message

Subject: Re: Power Supply, Amplifier, and AD
From: ChrisAtUpw@.......
Date: Tue, 9 Dec 2008 21:30:14 EST


 
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@.......... 
(mailto: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_ 
(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. 
    
         
    





In a message dated 09/12/2008, sismos@.............. writes:
<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>Hello=20 Dayana,
Don't buy the black box that you see with an AS1.
you can do= =20 much better and probably less expensive or close with Larry's=20 boards.
Hi Dayana,
 
    I believe that Jeff Batten amateurseismo@......... will sel= l the=20 electronics box and power supply separately. 
 
    Seismometers like the AS1 and EQ1 try to use oi= l +=20 plunge plate damping. The viscosity of oil changes by about x10 for every 20= C=20 Deg / 36 F Deg change in temperature. If you set up the damping to be correc= t at=20 your room temperature, you will only have an operating range of about +/-4 F= =20 Deg. This is LESS than the normal daily variation. If the temperature falls=20= much=20 below this range, the damping increases dramatically and the seismometer MAY= =20 stop recording altogether. It's sensitivity will certainly decrease and the=20 calibration will NOT give the correct quake magnitude. I have designed a=20 replacement magnetic damper which is NOT effected by temperature and is=20 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 natura= l=20 period of about 1.5 seconds. So while the response is flat with velocity at=20 periods shorter than this (like a geophone), the sensitivity will fall off a= s=20 f^2 below 0.75 Hz when using a 'normal' seismic amplifier, like Larry's. The= =20 sensitivity to the 20 second period Rayleigh waves, will be down by a factor= of=20 about 180 - so you normally won't see any!
    A period extension circuit has been used in the= AS1=20 'black box' to extend the sensitivity ~flat from 1.5 to about 4.5 secon= ds.=20 This boosts the low frequency output. While it is OK for measuring=20 microseisms, it is otherwise rather limiting. A further perio= d=20 extension can be applied with the digital compenation filter supplied w= ith=20 AmaSeis, but it only works well on moderately strong signals. 
    I have extended the period to about 20 seconds=20 by changing four capacitors and three resistors. It now seems= to=20 work satisfactorily and covers most of the normal seismic range.
 
    On the EQ1 that we examined, the compensat= ion=20 covered about the same compensation period range as an AS1, but the output=20 seemed to be roughly proportional to f. This did NOT give a flat output. Sin= ce=20 the power laws are different, I would not expect the digital frequency=20 compensation built into AmaSeis to work adequately with an EQ1 trace. T= he=20 EQ1 electronics uses miniature surface mount components, so I would NOT=20 expect you to be able to modify EITHER the compensation range OR t= he=20 power law.
 
    The relatively long springs expand and contract= =20 with temperature and may well need adjustment with the seasons.
 
    The AS1 is designed with the centre of gravity=20= on=20 the horizontal arm.
    
         
    

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