PSN-L Email List Message

Subject: Re: Gulf Of California on a seismic Detector
From: ChrisAtUpw@.......
Date: Wed, 4 Jan 2006 08:59:08 EST


In a message dated 04/01/2006, ian@........... writes:

Unfortunately I've just realised that my Lehman oscillates when it is  
cold. The temperature here declined yesterday afternoon and the thing  
started oscillating. It's -1C  at the moment and the graph looks like I 
spilled red ink all over it (  http://www.iasmith.com/realtime.htm ) .  

I don't rate my chances  very highly of finding out what is causing it to 
oscillate.

Ian  Smith


Hi Ian,
 
    The energy has to be coming from somewhere! If you  only see oscillations 
when the outside air is cold, this is likely to be the  driving source.
 
    Do you have any photos of the Lehman? Is it housed  in it's own sealed 
box? Does the box have an internal top heater? What  suspension system are you 
using? What damping system do you use?
 
    One of the ways you can get 'oscillations' from a  Lehman, is if the box 
and internal instrument is warmer than the  outside air, but there is no top 
heater inside the box to maintain a stable  vertical temperature gradient. 
Rolls of cooler air break off the inside top  / sides of the box when the thermal 
gradient gets big enough and then interact  with the boom. 
 
    You only need a few watts of power into Al housed  power resistors bolted 
to a 1/16" Al top plate inside the top of case.
 
    Is the top of the box completely sealed? How is it  'sealed' to the 
floor? Can drafts / air currents get in? One way to 'semi seal'  the housing is to 
surround the box with a 2" high wood rectangle on the  floor and fill the gap 
with dry sand.
 
    Worth trying?
 
    Regards,
 
    Chris Chapman





In a message dated 04/01/2006, ian@........... writes:
<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20 size=3D2>Unfortunately I've just realised that my Lehman oscillates when i= t is=20
cold. The temperature here declined yesterday afternoon and the t= hing=20
started oscillating. 
It's -1C=20 at the moment and the graph looks like I
spilled red ink all over it (= =20 http://www.iasmith.com/realtime.htm ) . 

I don't rate my chan= ces=20 very highly of finding out what is causing it to
oscillate.

Ian= =20 Smith
Hi Ian,
 
    The energy has to be coming from somewhere! If=20= you=20 only see oscillations when the outside air is cold, this is likely to be the= =20 driving source.
 
    Do you have any photos of the Lehman? Is it hou= sed=20 in it's own sealed box? Does the box have an internal top heater? What=20 suspension system are you using? What damping system do you use?
 
    One of the ways you can get 'oscillations' from= a=20 Lehman, is if the box and internal instrument is warmer than the=20 outside air, but there is no top heater inside the box to maintain a stable=20 vertical temperature gradient. Rolls of cooler air break off the inside= top=20 / sides of the box when the thermal gradient gets big enough and then intera= ct=20 with the boom. 
 
    You only need a few watts of power into Al hous= ed=20 power resistors bolted to a 1/16" Al top plate inside the top of case.
 
    Is the top of the box completely sealed? How is= it=20 'sealed' to the floor? Can drafts / air currents get in? One way to 'semi se= al'=20 the housing is to surround the box with a 2" high wood rectangle on the= =20 floor and fill the gap with dry sand.
 
    Worth trying?
 
    Regards,
 
    Chris Chapman

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