PSN-L Email List Message

Subject: Re: Relevelling
From: ChrisAtUpw@.......
Date: Tue, 29 Jul 2008 16:08:55 EDT


In a message dated 2008/07/29, steinar@............. writes:

> I've had a SEP seismometer running for a month or so and everything
> has been fine.  However, a couple of days ago while we were away on
> vacation it began reporting no activity.  When I got back home I found
> that the mass had moved all the way to the side, so I had to change
> the tilt a little so it would move back to the centre again.
> 
> Is this normal behaviour for a Lehmann seismometer?  The mass moved
> away from the center at a time the house was empty, so I can't really
> think of any external disturbances which could have caused this.

Hi Steinar,

       Yes and no? 

       I fit a SS wavy spring washer and a SS lock nut on the levelling 
screws on top of the baseplate. This does seem to hold the balance a bit better and 
tends to reduce temperature effects. No screw is absolutely tight, so 
different contractions of the base and the ground with temperature can cause sideways 
forces making the thread move out of adjustment. Lehmans are very sensitive 
to tilt variations.

       Has there been any rain while you have been away? This is a common 
cause of real ground tilts. Did you switch off the heating while you were away? 

       Have you fitted metal ground plates under the adjustment screws? I use 
about 8 cm square x 2.5 mm thick SS. Putting adjustment screws directly onto 
concrete can cause problems as surface grit moves.

       Something has happened in your absense. The only question is what and 
can you avoid it happening again? 

       Regards,

       Chris Chapman   
In a me=
ssage dated 2008/07/29, steinar@............. writes:

I've had a SEP seismometer runn= ing for a month or so and everything
has been fine.  However, a couple of days ago while we were away on
vacation it began reporting no activity.  When I got back home I found<= BR> that the mass had moved all the way to the side, so I had to change
the tilt a little so it would move back to the centre again.

Is this normal behaviour for a Lehmann seismometer?  The mass moved
away from the center at a time the house was empty, so I can't really
think of any external disturbances which could have caused this.

Hi Steinar,

       Yes and no?

       I fit a SS wavy spring washer and a SS=20= lock nut on the levelling screws on top of the baseplate. This does seem to=20= hold the balance a bit better and tends to reduce temperature effects. No sc= rew is absolutely tight, so different contractions of the base and the groun= d with temperature can cause sideways forces making the thread move out of a= djustment. Lehmans are very sensitive to tilt variations.

       Has there been any rain while you have=20= been away? This is a common cause of real ground tilts. Did you switch off t= he heating while you were away?

       Have you fitted metal ground plates und= er the adjustment screws? I use about 8 cm square x 2.5 mm thick SS. Putting= adjustment screws directly onto concrete can cause problems as surface grit= moves.

       Something has happened in your absense.= The only question is what and can you avoid it happening again?

       Regards,

       Chris Chapman

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