PSN-L Email List Message

Subject: Re: Lehman construction
From: BOB BARNS roybar@........
Date: Tue, 11 Jan 2000 15:58:38 -0500

  I agree with John about using a ball against a flat surface (although
I use a disk with a small radius edge) for the pivot.  I think that a
1/8" ball or smaller would be better than a 1/4" ball.
  I disagree that a "12 sec. or even smaller" period would be OK.  As
Sean-Thomas has pointed out, the sensitivity of a Lehman varies as the
 of the period, i.e., a 20" period would be 4 times better than
a 10" period.
  I usually run my Lehman at about 20" and have gone as high as 24".  At
20", I have to adjust the level about twice a week with the Lehman
sitting on a concrete basement floor.  
Bob Barns

john c cole wrote:
> jim
>  i have been reading your mail with interest. i read the reply submitted
> by frank and agree with his reply as well as the others.
>   here is what i have to say. eleven to twelve seconds is about all that
> you can expect out of the detector  as it is constructed. i could not get
> any more out of mine and it was built the same way.
>   the time period is controlled by the upper and lower pivot point. even
> the smallest amount of friction  or resistance at these two points is
> critical . we are talking about extremely small amounts.
>   to lower the friction at the lower pivot point, cut the end off
> flat,drill a hole in the end of the pendulum ,drill a hole large enough
> for a 1/4" ball bearing to fit in. about half of the bearing protruding
> from the pendulum. the surface that the ball bearing rest against should
> be sanded with no.400 paper then with no600 paper. the smoother the
> better. absolutely no holes drilled to keep the pendulum from migrating.
> the weight of the coil and the lead weight will keep it in place. it will
> never ,never slide out while it is in operation. it will slide off
> sometime while it is being adjusted. put a drop of super glue in the hole
> before inserting the ball bearing. if you like, put a l;little more
> around the edge of the ball bearing after the first has hardened.
>   the top pivot point. i use .015 music wire for the lower section up to
> the turn buckle . the upper wire from the turnbuckle to the top pivot
> point is .010 thousandth. are you with me? the best source for the wire
> of the correct diameter is ,your local  music store. the e string of a
> guitar is used. that is the first small string. cost,50 cents to a dollar
> . the diameter of the strings aer written on the package .the smaller
> diameter on the string at the top the longer the period.
>    using this method i can get any time that i desire. i usually run mine
> at about sixteen seconds . twelve seconds or even less is ok. i think
> that most professional detectors run at fifteen seconds. any higher and
> you will be adjusting all the time .
>   ps i have four detectors up and running at this time.
>                                                       your friend john
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Larry Cochrane <cochrane@..............>