PSN-L Email List Message

Subject: Re: Adjusting the period
From: tchannel1@............
Date: Tue, 27 May 2008 13:51:20 -0600


Hi Chris,   =20

>Which compass direction is the boom please?   East/West

> Where is the seismometer located?    (Basement)  What sort of floor is =
it?   (Concrete)  (ss plates for the legs)
>Is the seismometer right next to an outside wall?   (Yes)=20
>Which direction does the wall face? (East/West)

>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

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?

Thanks, Ted









  ----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Monday, May 26, 2008 8:05 PM
  Subject: Re: Adjusting the period


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


    The reason I know it is missing earthquakes is that I am running =
three channels, One vertical spring, one tiltmeter, and one =
Lehman..........The Lehman misses a lot of events the other two record.


  Hi Ted,

         OK. I accept that. Which compass direction is the boom please?


    Yes I think part of it is directional.

    Would you think this could account for it..............The Lehman is =
set for twenty seconds, but at twenty seconds, I think it is maxed out, =
that is to say,  I don't think I could get 21 seconds.
    Even 20 seconds, for this unit, may be too much.   I say that =
because it drifts, sometimes falling 1/2" per week to the left or to the =
right.  I adjust it back to  center and it stays only for a few days.

    Might it simply be this unit is not capable of working at 20 =
seconds. The sticking you mentioned.........could this be because it is =
overextended, perhaps this machine is only capable of a WORKING PERIOD =
of 15 seconds?


         No. If it works at 20 seconds, it should continue to do so. I =
am talking about stiction in the bearings. When we first tried out our =
school seismometer, they had not fitted a hard counterplate on the end =
of the Aluminum boom as I had specified, just a SS bearing on the =
vertical. The bearing made a tiny dent on the end of the boom and it did =
not respond properly. I then glued a SS scalpel blade to the boom and it =
was like a different instrument!

         What happens if you try to increase the period beyond 20 =
seconds? The original type Lehmans with a knife edge for the bottom =
bearing had difficulty getting beyond about 12 seconds. The knife edge =
either failed or dug into the counterface. A really dumb idea.


    When I set it to 20, and move the boom, it does go to center, but =
because of the drift issue, it may not be capable of HOLDING CENTER. and =
perhaps this is stopping its sensitivity to all but the larger =
earthquakes?


         The instability could have several causes. One of these is the =
suspensions. You have not reminded me what you are using for the top and =
bottom suspensions!

         The drift with time could be related to temperature changes, =
maybe direct movement of the adjustment screws or movement over the =
ground due to differential expansion of the frame. What metal are the =
adjustement screws? Is the threaded insert / nut made from the same =
metal? Do you have three SS metal plates glued to the floor to support =
the adustment screws? Or are the screws in direct contact with the =
floor? I fit a nut and a WAVY spring washer on TOP of the baseplate, so =
that the three adjustment screws are ALWAYS held in tension.=20

         I am not 'being fussy'.  The axis offset is maybe 0.32 degree - =
0.1" if the vertical is 18" high.

         It is much more likely to be direct tilt of the floor. Where is =
the seismometer located? What sort of floor is it? Is the seismometer =
right next to an outside wall? Which direction does the wall face?

         What are the dimensions of the Lehman, what is the boom made of =
and what is the mass?

         Regards,








Hi Chris,    =
 
>Which compass direction is the boom = please?   East/West
 
> Where is the seismometer located?   =20 (Basement)  What sort of floor is it?   (Concrete) =  (ss=20 plates for the legs)
>Is the=20 seismometer right next to an outside wall?   (Yes)=20
>Which=20 direction does the wall face? (East/West)
 
>What=20 are the dimensions of the Lehman, what is the boom made of and what is = the=20 mass?  36" boom 1/2" alum rod.   Dim. 20"wide, 22" tall, = 36"=20 long.  The mass is 2.5# Alum cylinder
 
Chris, I=20 think I found the drift issue?  I added a wing nut to lock the legs = after=20 adjustment.  I think they were moving over = time?
 
Thanks, Ted
 
 




----- Original Message -----
From:=20 ChrisAtUpw@.......
To: psn-l@..............
Sent: Monday, May 26, 2008 8:05 = PM
Subject: Re: Adjusting the = period

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

The reason I know it is missing earthquakes is that I am = running=20 three channels, One vertical spring, one tiltmeter, and one=20 Lehman..........The Lehman misses a lot of events the other two=20 record.


Hi=20 Ted,

       OK. I accept that. = Which=20 compass direction is the boom please?


Yes I think part of it is directional.

Would you think = this could=20 account for it..............The Lehman is set for twenty seconds, = but at=20 twenty seconds, I think it is maxed out, that is to say,  I = don't think=20 I could get 21 seconds.
Even 20 seconds, for this unit, may be too = much.  =20 I say that because it drifts, sometimes falling 1/2" per week to the = left or=20 to the right.  I adjust it back to  center and it stays = only for a=20 few days.

Might it simply be this unit is not capable of = working at 20=20 seconds. The sticking you mentioned.........could this be because it = is=20 overextended, perhaps this machine is only capable of a WORKING = PERIOD of 15=20 seconds?

       No. If=20 it works at 20 seconds, it should continue to do so. I am talking = about=20 stiction in the bearings. When we first tried out our school = seismometer, they=20 had not fitted a hard counterplate on the end of the Aluminum boom as = I had=20 specified, just a SS bearing on the vertical. The bearing made a tiny = dent on=20 the end of the boom and it did not respond properly. I then glued a SS = scalpel=20 blade to the boom and it was like a different=20 instrument!

       What happens = if you=20 try to increase the period beyond 20 seconds? The original type = Lehmans with a=20 knife edge for the bottom bearing had difficulty getting beyond about = 12=20 seconds. The knife edge either failed or dug into the counterface. A = really=20 dumb idea.


When I=20 set it to 20, and move the boom, it does go to center, but because = of the=20 drift issue, it may not be capable of HOLDING CENTER. and perhaps = this is=20 stopping its sensitivity to all but the larger=20 earthquakes?

       = The=20 instability could have several causes. One of these is the = suspensions. You=20 have not reminded me what you are using for the top and bottom=20 suspensions!

       The drift = with time=20 could be related to temperature changes, maybe direct movement of the=20 adjustment screws or movement over the ground due to differential = expansion of=20 the frame. What metal are the adjustement screws? Is the threaded = insert / nut=20 made from the same metal? Do you have three SS metal plates glued to = the floor=20 to support the adustment screws? Or are the screws in direct contact = with the=20 floor? I fit a nut and a WAVY spring washer on TOP of the baseplate, = so that=20 the three adjustment screws are ALWAYS held in tension.=20

       I am not 'being = fussy'.  The=20 axis offset is maybe 0.32 degree - 0.1" if the vertical is 18"=20 high.

       It is much more = likely to be=20 direct tilt of the floor. Where is the seismometer located? What sort = of floor=20 is it? Is the seismometer right next to an outside wall? Which = direction does=20 the wall face?

       What are = the=20 dimensions of the Lehman, what is the boom made of and what is the=20 mass?

      =20 Regards,

       Chris = Chapman
=20

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