PSN-L Email List Message

Subject: Re: gain for lehman type sensor ?
From: ChrisAtUpw@.......
Date: Sun, 20 May 2007 08:55:36 EDT


In a message dated 2007/05/20, jonfr@......... writes:

> Hi all
> What is the best gain for Lehman type sensor? I am going to use coils
> and magnet from Larry. The gain on the board is set on 1000.
> J=F3n Fr=EDmann

Hi Jon,

       The 'best' gain is determined by the instrument period, the strength=20
of the magnet and the coil shape, area and turns. In practice you set the=20
Lehman to give a period of maybe 25 seconds and select the gain to give coun=
ts of=20
say 200. The signal will be mostly background microseisms, which you have to=
=20
filter out, leaving the quake signals.
       Can you not get any NdFeB square magnets? I use four off 1" square x=20
1/8" thick ones on two 6mm mild steel backing plates and wind my own=20
rectangular coils to suit, made from old glass circuit board. See 'Sensor Co=
il / Magnet=20
System' at the bottom of=20
http://jclahr.com/science/psn/chapman/lehman/index.html
       This allows about +/-12 mm tilt drift to still give the same output=20
sensitivity. You should be able to buy 0.125 mm dia insulated copper wire re=
f.=20
YN92A from your  Maplins agent in Iceland. It will give a much higher output=
=20
than the Alnico U magnets.

       Regards,

       Chris Chapman  =20
In a me=
ssage dated 2007/05/20, jonfr@......... writes:

Hi all
What is the best gain for Lehman type sensor? I am going to use coils
and magnet from Larry. The gain on the board is set on 1000.
J=F3n Fr=EDmann


Hi Jon,

       The 'best' gain is determined by the in= strument period, the strength of the magnet and the coil shape, area and tur= ns. In practice you set the Lehman to give a period of maybe 25 seconds and=20= select the gain to give counts of say 200. The signal will be mostly backgro= und microseisms, which you have to filter out, leaving the quake signals.        Can you not get any NdFeB square magnet= s? I use four off 1" square x 1/8" thick ones on two 6mm mild steel backing=20= plates and wind my own rectangular coils to suit, made from old glass circui= t board. See 'Sensor Coil / Magnet System' at the bottom of http://jclahr.co= m/science/psn/chapman/lehman/index.html
       This allows about +/-12 mm tilt drift t= o still give the same output sensitivity. You should be able to buy 0.125 mm= dia insulated copper wire ref. YN92A from your  Maplins agent in Icela= nd. It will give a much higher output than the Alnico U magnets.

       Regards,

       Chris Chapman

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