PSN-L Email List Message

Subject: Re: Tiltmeter as Horizontal Seismometer
From: "Geoff" gmvoeth@...........
Date: Sun, 16 Sep 2007 05:51:49 -0700

What ever happened to the old fashioned oscillators like
colpits and heartly and stuff like that.
I understand if you take great effort to properly isolate the oscillators they are
quite stable.

----- Original Message ----- 
Sent: Saturday, September 15, 2007 11:20 AM
Subject: Re: Tiltmeter as Horizontal Seismometer

> In a message dated 2007/09/15, ptrowe@......... writes:
>> I left out one important piece of the circuit. Chris' reply to you jogged 
>> my memory. Since you drive the bridge with AC, you have to have a synchronous 
>> rectifier after the gain stages to turn the signal back to DC. This is done 
>> with an analog switch and an op amp. Easy.
>> As Chris mentioned, the homemade liquid filled tube may be your best bet for 
>> building the sensor yourself. I saw one of these at Cal Tech many years ago. 
>> It was
>> about 2 meters long and had a horizontal tube filled with Mercury. They 
>> sensed the level of the Mercury in a cup at each end capacitively. With today's 
>> capacitive sensor ICs, this should be easy to do.
> Hi Chuck,
>       There is a distinct advantage in using a sine wave drive if you want 
> really low noise. This can be generated with a crystal oscillator driving a 
> binary counter and some load resistors into a summing amplifier and a low pass 
> filter. It is described on Sean Morrissey's website.
>       You might be interested to read Gile, WW, Geophys. J. Roy. Astro. Soc. 
> vol 36 1974 pp 153-165 "A Mercury Pendulum Seismometer" Your local library 
> should be able to get you a photocopy. He describes a seismometer built by 
> Benioff, but he does not go into the technique of using large end cups to extend 
> the natural period,  or strong overdamping to flatten the natural pendulum dog 
> leg response below the resonant period.
>       Mercury is highly toxic, particularly the vapour, even at room 
> temperature, so your equipment needs to be 100% sealed. Always handle it over a deep 
> plastic tray to catch any spillage. It forms amalgams with many common metals 
> and it will 'rot' components made of brass, copper, aluminum, zinc, solder.... 
> SS is OK. It is also very expensive! 
>       Regards,
>       Chris Chapman   

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