PSN-L Email List Message

Subject: RE: HELP
From: steve hammond shammon1@.............
Date: Thu, 30 Nov 2000 23:52:33 -0800

You are at the fun part. I have two Lehman's and I use the older PSN amp 
design. I set my gain to 2000 and then adjust it up or down as needed.
The operation you describe sounds correct for the stage you are at. I have 
a 12-bit A/D and can induce a count of -/+ 40-50 by walking next to the 
device. The amount of gain is governed by two factors. The gain of the 
amplifiers and the sensitivity of the device.  I was looking at your 
pictures, nice work, so let me suggest the following, it should be doable 
when setting initial device sensitivity. Your boom looks like it is about 
36-inches. With a 36-inch boom and .25 degrees of forward tilt of the base 
plate, a level boom will have a natural period of 18.2 seconds.  If I were 
setting up the device, I would first set the gain to around 2000 and then 
set the undampened period of the device to 10-seconds making sure that the 
boom returns to center whenever after it is excited. Next, I would slowly 
lengthen the period to 20-seconds undampened and then adjust the damping 
magnets to stop the boom from swinging in 3 1/2 cycles. This will bring the 
dampened period to about 17-18 seconds. At that point you are ready to 
record and see what you get. Increase / decrease the amp gain based on 
urban noise thresholds. For the first 30 days or so the boom may drift off 
center overnight. This is due to the nature of the metal frame and the 
floor under it settling. At that point you are ready to wait and see... 
Another point,  I can't emphasize how important an airtight cover is. I use 
a full sheet of 1/2-inch plywood to make a box 2X2X4 and then put 2" 
insulation foam board inside it which thermally protects the device from 
sudden temperature changes and tape the seams in the plywood and floor. 
Look'en good-

Regards, Steve Hammond
PSN Aptos, California

-----Original Message-----
From:	Bryan & Regina Goss [SMTP:bgoss@...................
Sent:	Thursday, November 30, 2000 9:24 PM
To:	psn-l@..............
Cc:	Larry Cochrane
Subject:	HELP

 << File: ATT00001.html >> Ok I got my seismograph built and I used Larry's 
A/D card and filter/amp, My question is how do I know if I have to much 
It looked as if I did so I put 15k res at r29 this reduced noise but I 
don't know if it was to much. I can walk across my shop floor
30x 40x concrete slab and going to seismograph I see the trace go up about 
an inch then slowly go back to center when I walk away just the opposite, 
The trace seems to drift up and down a little guess this is normal??????
I really need help

this is before the damper and cover


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Larry Cochrane <cochrane@..............>