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Specialized Small Diameter Probe


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I have an application for a specialized metal detector coil. I want to use a Whites MXT to provide ferrous/non-ferrous discrimination information on targets which are deeper in quartz than common metal detector depths (2-30 feet). These targets were not found with found a metal detector.

I want to insert a metal detector probe encased in PVC pipe into a 2” dia hole bored into the quartz.

Reading Carl Moreland's “Coil Basics” tutorial at http://www.geotech1.com/cgi-bin/pages/common/index.pl?page=metdet&file=info.dat it seems to me one of the figure 8 configurations would work well in this application. I would insert the probe on adequate length sections of PVC into the drilled hole and connect to the MXT.

I’m armed with no metal detector probe building experience, a lot of RF experience, adequate test equipment.

Will this idea work in general?

Is one of the figure 8 configs the right topology?

Any other hints on making this probe before I start?

What distance from centerline of 2” dia drilled hole can I expect to discriminate on a US nickel sized object for ferrous/non-ferrous?

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Well, you found the Geotech website, and that is THE place for this question!

Will this idea work in general? - I think so.

Is one of the figure 8 configs the right topology? - Not necessarily. Could go DD or even concentric. Concentric generally give superior ferrous/non-ferrous indications. DD better handles ground mineralization, and figure 8 adds superb EMI resistance.

Any other hints on making this probe before I start? - yeah, get back to Geotech!

What distance from centerline of 2” dia drilled hole can I expect to discriminate on a US nickel sized object for ferrous/non-ferrous? - A few inches at best.

Problem is two inch hole with shell needed around coil. Take best case, a Hot Foot coil for MXT is 2" (actually a hair over that probably) by 9" long, DD configuration. Make a bit skinnier with cable out rear, bingo, you are done. I own Big Foot by same company, 3" x 18" figure 8, obviously could be made same size as Hot Foot.

A Sun Ray probe for MXT could easily run down the hole but minimal ability to look to side unfortunately.

The thing is coil depth is tied to the width of a coil, not the length. You will have what, maybe a 1-3/4" wide coil? Not going to get much reach.

One thing to look into is effect of extra long cable. You will have to compensate for that past a certain point.

Old page with Hot Foot and Big Foot info. Guy that made them passed away and they are highly sought after, sell for twice or more what they did new - because they are hard to make! But obviously it can be done. http://www.whiteselectronics.net/calif/accessories_loops_.htm

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A figure 8 would be the better route.  Figure 8 gets much better depth than the DD configuration when compared side by side.   For example,  The Bigfoot coil is capable of 12" on a quarter while the Cleansweep is only good to about 7" tops.  Same size coils, different configuration.  The Bigfoot is a 3x18 Figure 8, the Cleansweep is a 3x18 DD.

 

Figure 8 coils are like two box coils, only on a smaller scale.  The hard part about figure 8 coil design is the tuning.   

 

A two inch bore hole limits you to 1-1/2" or smaller coils sizes.

 

A 1.5" figure 8 would turn into a rectangular 1.5" x 9" figure 8, where the front 4.5" is the receive side and the rear 4.5" would be the transmit side.  

 

A 1.25 figure 8 would turn into a rectangular 1.25" x 7.5" figure 8, where the front 3.75" is the receive side and the rear 3.75" would be the transmit side.

 

A 1" figure 8 would turn into a rectangular 1" x 6" figure 8, where the front 3" is the receive side and the rear 3" is the transmit side.

 

You'd want to make all three and see which one works best.   That size is probably looking at 6 to 8" on a nickel size target on a MXT.

 

One thing to consider, 30 feet of cable connecting coil to detector is going to throw the whole thing off so after you get a coil built and tuned, you need to retune with a 30 foot cable in place.

 

Good luck

 

HH
Mike

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Thanks for the input, Mike.   Do you have any pointers on where to find figure 8 coil building tips other than the geotech site?  

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