PSN-L Email List Message

Subject: Re: Adjusting the period
From: tchannel1@............
Date: Tue, 27 May 2008 17:15:57 -0600


Hi Chris, =20

>Is it solid Al, 2.5" OD?   I meant to say 2.5 pounds, alum 3" x 3" =
cylinder.

Ted


  ----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Tuesday, May 27, 2008 2:40 PM
  Subject: Re: Adjusting the period


  In a message dated 2008/05/27, tchannel1@............ writes:


    >Is the seismometer right next to an outside wall?  (Yes)=20
    >Which direction does the wall face? (East/West)


  Hi Ted,

         A location close to a load bearing outside wall is likely to be =
effected by any movements of the wall, due to rainfall on the soil =
outside, the effect of the sun raising the wall temperature and also =
noise from winds.=20


    >What are the dimensions of the Lehman, what is the boom made of and =
what is the mass?  36" boom 1/2" alum rod.  Dim. 20"wide, 22" tall, 36" =
long.  The mass is 2.5# Alum cylinder


         Thanks. What is the weight of the Aluminum cylinder, please - =
or alternatively it's length? Is it solid Al, 2.5" OD?


    Chris, I think I found the drift issue?  I added a wing nut to lock =
the legs after adjustment.  I think they were moving over time?


         That could well be the problem. Threads are cut at about a 60 =
degree angle and are rarely a tight fit. If the thread and the nut / =
threaded insert are not of the same material, you get differential =
expansion which slides the threads radially. Similarly, unless the =
adjusting screw is under tension, any differential expansion of the =
frame relative to the Earth underneath will try to tilt the thread and =
cause it to slip diagonally. It is preferable to use a wavy spring =
washer between the wing nut / lock nut and the frame to keep a fairly =
constant tension. This will not be much effected by the small amounts of =
differential expansion of the base and the bolt.=20

         Let us know if this cures your drift problems?

         Regards,








Hi Chris, 
 
>Is it solid Al, 2.5" = OD?   I meant to=20 say 2.5 pounds, alum 3" x 3" cylinder.
 
Ted

----- Original Message -----
From:=20 ChrisAtUpw@.......
To: psn-l@..............
Sent: Tuesday, May 27, 2008 = 2:40 PM
Subject: Re: Adjusting the = period

In a=20 message dated 2008/05/27, tchannel1@............=20 writes:

>Is the seismometer right next to an outside = wall? =20 (Yes)

>Which direction does the wall face?=20 (East/West)

Hi=20 Ted,

       A location close to a = load=20 bearing outside wall is likely to be effected by any movements of the = wall,=20 due to rainfall on the soil outside, the effect of the sun raising the = wall=20 temperature and also noise from winds.


>What are the dimensions of the Lehman, what is the = boom made=20 of and what is the mass?  36" boom 1/2" alum rod.  Dim. = 20"wide,=20 22" tall, 36" long.  The mass is 2.5# Alum cylinder

       Thanks.=20 What is the weight of the Aluminum cylinder, please - or alternatively = it's=20 length? Is it solid Al, 2.5" OD?

Chris, I think I found the drift issue?  I added a = wing nut=20 to lock the legs after adjustment.  I think they were moving = over=20 time?

      =20 That could well be the problem. Threads are cut at about a 60 degree = angle and=20 are rarely a tight fit. If the thread and the nut / threaded insert = are not of=20 the same material, you get differential expansion which slides the = threads=20 radially. Similarly, unless the adjusting screw is under tension, any=20 differential expansion of the frame relative to the Earth underneath = will try=20 to tilt the thread and cause it to slip diagonally. It is preferable = to use a=20 wavy spring washer between the wing nut / lock nut and the frame to = keep a=20 fairly constant tension. This will not be much effected by the small = amounts=20 of differential expansion of the base and the bolt.=20

       Let us know if this cures = your=20 drift problems?

      =20 Regards,

       Chris = Chapman
=20

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