I've just introduced an angle-pois light with an energy saving 11 Watt 
bulb into the box as an experiment and temporary measure.  It may take 
an hour or two to have an effect...

Ian Smith

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




  
  


I've just introduced an angle-pois light with an energy saving 11 Watt
bulb into the box as an experiment and temporary measure.  It may take
an hour or two to have an effect...



Ian Smith



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