BFO Coil Works For PI Detector?

[GB_Amateur]

George, that's some serious info I would have struggled mightily to figure out (best case).  It's sounding like the Garrett BFO coils just aren't going to make the grade.  What do you think about the Garrett Sea Hunter 8 inch mono that Rick mentioned worked on your Crossbow?  Does that have a shot on my TDI/SPP?  Just to be clear, I'm not really doing a science fair project as an existence proof but rather trying to get a cost effective solution that works about as well as could be expected from a factory coil.  Right now (AFAIK) the White's coils and the Razorbacks float, and I don't consider that satisfactory.  The Coilteks, even if they are neutral or negative buoyant, carry a heavy price tag.

Thanks again.

 

[Roughwater]

Don't know if this helps or not but I am pretty sure the Sadie and Joey coils are waterproof and I see them on line off and on for sale used, with prices much less than new.  I don't know about their buoyancy but being small they could be weighted easily and they should work fine on your TDI to the best of my knowledge without modification.   Just check the forum sites and ebay for used for the size coil that you feel you need.    

Here is list of GP coiltech coils and indicates which are submersible and other features of the coils:

 http://www.coiltekmanufacturing.com.au/files/6213/9926/8483/3363-3_Comparison_Chart-web-latest.pdf

 

[GB_Amateur]

Plan B (maybe I'm supposed to start a new thread...)

Thanks to the responses, particularly Geotech's, I'm abandoning the plan to modify the Garrett BFO coil for use on the TDI/SPP.  There are just too many required tweaks and, worst, when it works it doesn't work very well.

I have four coils as of now, maybe more soon .  12 inch White's stock (open structure, circular) "Aussie Mono", 6 inch (closed, circular) Super Pulse 150 (mono), 5in X 9in (closed, elliptical) Miner John folded mono, and 3.5 in X 6.5 in (closed elliptical) White's(?) mono.  The 12 in and 6 in rounds were sold by Jimmy Sierra with the original package.  As of now the latter two small coils are going to be my go-to dry land coils with the 12 in for quiet, flat ground.  That leaves the SP-150 as the weak sister, so....

I started this thread wanting a neutral (or even negative) buoyancy coil.  My current plan is to modify the SP-150 to become just that.  I started out by cutting it open -- actually easy since it is a two piece case, sealed around the belly with silicone -- so I just cut the silicone with an Exacto knife.  I did have to disconnect a ground wire soldered and taped to the bottom piece.  Pretty simple inside and lots of strong, tough epoxy holding most everything in place.

I don't really want to turn this into a bitch-fest, but I've got to wonder why this enclosure was used.  The lower piece is way too deep -- by 1/2 inch -- than it needs to be.  The first issue here is that it results in dead volume (leading to extra buoyancy).  But worse, it keeps the coil -- specifically the windings which are the real coil -- an extra 1/2 inch off the ground.  I can't count the number of pieces of advice I've read that say "make sure to keep the coil as close to the ground as possible.  Scrape the bottom across the ground for best signal penetration."  So they build a coil housing that wastes 1/2 inch of useful depth.  (scratching head...)

First goal is to find a replacement lower section -- effectively a 6.5 inch diameter low profile(!) coil scuff cover.  Then I need to find some small, non-conductive weights to put inside before sealing it up.  My measurements + calculation say I need about 240 g of mass (so about 1/2 lb weight).  Note that placing it inside the enclosure is the most efficient (least amt of weight) way to do this.  Putting anything on the outside of the coil (e.g. sandbag) adds to the overall volume and thus the buoyant force.

Now here's where the geologist's advice is requested.  Most dense materials are conductive.  Non-conductors tend to be low density.  Most rock has specific gravity in the ballpark of 2.7.  I have some marble (metamorphosed limestone?) right at 2.7 which I can probably break/cut and epoxy inside.  But is there something better, because the volume of marble required is going to make this a tight fit.  Any non-conductive, non-magnetic (oh, and affordable) materials out there that anyone here knows about with higher densities than typical rock?

 

 

[Geotech]

23 hours ago, GB_Amateur said:

George, that's some serious info I would have struggled mightily to figure out (best case).  It's sounding like the Garrett BFO coils just aren't going to make the grade.  What do you think about the Garrett Sea Hunter 8 inch mono that Rick mentioned worked on your Crossbow?

I'm the "Carl" half of Geotech... the Sea Hunter coil would probably work just fine. I use a Garrett XL500 PI coil on my Hammerhead design and it does nicely in the water.

[GB_Amateur]

I've kept quiet for a while now, but not do to lack of activity.  For starters I thank all the responders, especially Carl (Geotech), Rick (RickUK), and Roughwater for their technical recommendations.

I've moved on from the Garrett BFO coil mod idea and will start a new thread soon (week or so) with details and photos on two paths I've chosen, in short physically modding the White's 150 (mm) closed circular disc mono and electronically modding a recently acquired Garrett Infinium 10x14 mono.  I'm hoping I haven't bored everyone so much that they stop responding.  My view of this forum is that it is wide open to a variety of gold recovery and/or metal detecting interests, and anything which potentially leads to the improvement/enhancement/enjoyment of at least one reader without insulting too many  isn't a waste.