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 -----
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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