PSN-L Email List Message

Subject: Re: Gulf Of California on a seismic Detector
From: ian ian@...........
Date: Wed, 04 Jan 2006 14:53:43 +0000


Hi,

thanks for the reply, I'm still working through it!  In the meantime, I 
don't have any good pictures but here is a drawing of the housing that I 
just quickly sketched: http://www.iasmith.com/housing.jpg .  Let me know 
if the link doesn't work.

I have no heater.  I had hoped to avoid one but sounds like I need to 
consider it.

It's just the usual wire diagonal from the top of the boom.  The pivot 
consists of 2 ball bearings.  The boom has a flat metal plate on the end 
which pivots on them.  I have magnetic damping but it is not properly 
set up at present.  The system is currently under damped.  The weight on 
the end of the boom is copper and this doubles as the magnetic damping.

The internal may well be warmer than the outside (a thermometer would be 
nice addition too!).  In theory the box is sealed but nothing is perfect...

Perhaps a car battery to power the resistors would solve it.  I'll need 
to wait till I go back to work next week to get some of those heatsunk 
resistors.

Thanks

Ian

ChrisAtUpw@....... wrote:

> 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




  
  


Hi,

thanks for the reply, I'm still working through it!  In the meantime, I don't have any good pictures but here is a drawing of the housing that I just quickly sketched: http://www.iasmith.com/housing.jpg .  Let me know if the link doesn't work.

I have no heater.  I had hoped to avoid one but sounds like I need to consider it.

It's just the usual wire diagonal from the top of the boom.  The pivot consists of 2 ball bearings.  The boom has a flat metal plate on the end which pivots on them.  I have magnetic damping but it is not properly set up at present.  The system is currently under damped.  The weight on the end of the boom is copper and this doubles as the magnetic damping.

The internal may well be warmer than the outside (a thermometer would be nice addition too!).  In theory the box is sealed but nothing is perfect...

Perhaps a car battery to power the resistors would solve it.  I'll need to wait till I go back to work next week to get some of those heatsunk resistors.

Thanks

Ian

ChrisAtUpw@....... wrote:
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

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