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Metal Detector

#1 User is offline   Papa-Bear-NYC 

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Posted 12 January 2006 - 11:36 AM

I am interested in getting a metal detector, primarily for benchmarking. I'm not interested in a super professional model but a good recreational model would do it. Size and weight is also a factor as I would want to bring it on some mountain climbing hikes.

Information, comments and potential retail outlets (including on the web) would be appreciated.

And a question: I understand these things work on magnetism. But since benchmark disks are generally brass or bronze and occasionally aluminum, how can a metal dectector find them since these metals are all non-magnetic?

Thanks
Pb

#2 User is offline   Photobuff 

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Posted 12 January 2006 - 12:31 PM

I'm no expert on these gadgets, but I do design electronics for a living. First, any metallic object that gets in the field surrounding the coils will have an effect. A decent detector will be able to give you a different indication for ferrous and non-ferrous, however it can also be fooled. I'm using a cheap detector from Harbor Freight that I got for the whopping sum of about $26. It's extremely effective, and gives a tone of increasing volume for non-ferrous stuff like bronze benchmarks, and a decreasing volume for iron. IMO, most of the features on more expensive detectors are for discriminating between sizes and types of targets- you want to find the gold coins, not the nails and aluminum pull tabs. I can also tell you that the cost of the parts and complexity of the design in most detectors is such that they should never cost more than $100 or so. I could never bring myself to buy a very expensive detector without proof of unique design and impossibly good performance.

Here's the rub. Most detectors can't detect much further than the diameter of their sensor, and that's reduced further for buried targets. My Harbor Freight bottom feeder model can see a benchmark buried 6-8", and no more. If you want to go deeper, you need a larger head. Right now I'm looking for a mark I know is a foot or so down, if I'm lucky. I just today dug a 1' trench and used the detector on the bottom, figuring that would give me a better chance. Man, what a lot of work. Didn't find it- still need to go another 10' to the north if it doesn't kill me.

There are also interference sources like "hot" rocks. Some rocks have a lot of iron, and look like a target. The density of the dirt is a factor. The moisture content is a factor. I'm not trying to scare you, just pointing out that you have to learn to interpret what these things tell you. It's not black and white. IMO, the model Harbor Freight sells for $26-39 seems like a super deal, and works well for benchmarking. It's light and collapsible. They have a higher priced model ($60?) that I have no experience with. After that, look mostly for ferrous/non-ferrous discrimination, and the size of the head. Be sure to get headphones or adapt the ones you may already have. You need 'em to keep out traffic noise and wind, lest you not hear the changes that suggest a benchmark is nearby!

#3 User is offline   Photobuff 

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Posted 12 January 2006 - 12:43 PM

Just a side note on the ferrous business that might help. Old radio men used to adjust the tuning of coils using a thing called a tuning wand. This was a wand with one brass end and one iron end. When you put the iron end near a coil, the inductance goes up. When you put the brass end near the coil, the inductance goes down. By probing the radio coils with each end of the wand, and listening to the signal, they were able to determine which way the coil had to be adjusted, and about how much. Metal detector circuits are usually just some form of oscillator, with the frequency determined by the inductance of the coil. If something causes the inductance of the coil to change, the frequency goes up or down. Better detectors have several sets of coils, and operate at a fairly high frequency, but the basic principle is that either inductance or coupling between the coils is altered by the target, so the output changes. The rest of the circuitry determines how that change is presented to you, and what types of change represent "junk" targets and should be suppressed. For my purposes, I use no suppression (discrimination) and a tuning setting that gives me maximum sensitivity. I use a big copper washer a dummy mark, and adjust for maximum detection distance.

#4 User is offline   mloser 

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Posted 12 January 2006 - 02:43 PM

I used to use a kid's detector by National Geographic that I got off eBay for about $25. It is small, yellow, and totally portable. The head is about 2 inches around, so it is no surprise that I have only found marks just underground with it. Still, it has gotten me a few.

A few months ago I sprang for a larger, but will $25 model no-name detector. So far it hasn't done me any good. For one thing, I think it is broken, as sometimes it "tunes" in reverse, so that it makes noise all the time except when it detects something. I have been unable to find the setting that makes it the most sensitive to anything in order to make it the most useful to me. There seems to be some secret, but the manual just tells you how to tune it to what you are looking for, not what the tuning settings do at all. There are Discrimination and Tune knobs and you have to fiddle with them while pressing the secret red button that does who knows what.

I think I will head to Harbor Freight and see what they have.

#5 User is offline   Bill93 

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Posted 12 January 2006 - 03:16 PM

There may be some detectors that work on the magnetic permeability variations (the iron/brass tuning wand). I think these are a minority.

Most metal detectors fall in two general classes. One detects changes in the earth magnetic field due to magnetic materials, like iron rods and pipes, and perhaps current-carrying wires. This is what the professional surveyor uses to find land corner pins. They can find iron at great depths, but aren't so useful for benchmarks. They sell on ebay (used) for $300-600. The major brand is Schonstedt and there are others.

The treasure finders almost all work on some principle that detects conductivity (low electrical resistance). There are variations on how the field is excited and received. Pulse versus CW excitation, and frequency of operation make some difference in discrimination capability. Some of the discrimination can come from using both phase and amplitude information from the return signal. Expensive units may use combinations of effects (conductivity and permeability) to get better discrimination.

This post has been edited by Bill93: 12 January 2006 - 03:16 PM


#6 User is offline   Photobuff 

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Posted 12 January 2006 - 03:27 PM

mloser- the one you describe is the Harbor Freight model. Tuning is easy. Turn the right hand knob (discriminator) fully CCW. The red button is the zero button. Hold the zero button down and adjust the tuning so the meter reads "1", quite low on the scale. Now, adjust the volume for a steady tone. Sweep the detector and periodically hit the red zero button. An increase in loudness of the tone indicates non-ferrous metal, fer instance a benchmark. A decrease indicates ferrous metal. Large pieces of ferrous metal will increase the volume, but have a different character as you start to go over it. Anyway, that's the maximum sensitivity adjustment. No discrimination, as that just reduces sensitivity, and tuning set for maximum Q (found by experimentation). The general conditions around the head need to be zeroed out, and that's why you should hit the zero button every ten seconds or so, or if you bump the head (changes its relationship to the shaft). The other settings rarely need to be changed. Were that the manual were clearer!

Place a bunch of copper pennies on the ground to simulate a non-ferrous benchmark and sweep over those to get a feel for what to expect. Do the same thing with some iron nails. "Better" detectors will have circuits to do what the zero button does automatically, but I actually prefer this arrangement.

Bill93- I was trying to keep my explanation as simple as possible, and in fact the DOT guy I met today had one of the units you described first. Great for re-bar, but appeared completely useless for benchmarks. Built like a tank though. As for the detectors we're interested in, you can pump in more power with pulsed operation, and do other tricks like phase and frequency, but IMO the basic detection always reduces down to some kind of resonant circuit and/or transformer coupling problem. The sellers make a lot of interesting claims, but I see a lot more marketing hype than science. What I'd love to know is how to do deep detection without a large head, and without resorting to that ground penetrating radar mentioned a while back, but I think natural laws just make it difficult to impossible. I'm actually considering winding a much larger replacement head for my cheapie unit- I think it's actually an unobtainable option from the factory, because they refer to it in the manual.

This post has been edited by Photobuff: 12 January 2006 - 05:53 PM


#7 User is offline   mloser 

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Posted 12 January 2006 - 06:29 PM

Thanks Photobuff. Those directions sound familiar but not exactly what the tiny manual says to do. I hope I managed to somehow reverse the ferrous/non-ferrous readings and if I play with it a bit I can get it to work right. Your suggestion to keep zeroing the unit is very good too--I noticed that it gets out of kilter.

Also, I am lucky enough to have an old benchmark in my backpack to use as a test object. Any suggestions for what to use for a brass bolt (PRR) or a monel metal rivet (Reading RR), which is most likely ferrous.

M

#8 User is offline   Photobuff 

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Posted 12 January 2006 - 08:35 PM

Brass should be similar to bronze and copper. I'd go out to the shop and just pick up a piece of brass rod :antenna: or get a piece at the hardware store. Maybe even use a stack of pennies. Won't be the same, but similar. No idea on monel. Maybe it's similar to stainless steel. Obviously the detector won't be useful if the bolts and rivets are set in a steel bridge! I didn't know exactly what a "copper bolt" would look like, but today someone who should know told me they don't necessarily have a head, and may just be a copper plug in a hole. It sounds like something that, with a little dirt, would be easily overlooked.

#9 User is offline   Black Dog Trackers 

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Posted 12 January 2006 - 09:08 PM

As an aside, brass is basically an alloy of copper and zinc, and bronze is basically an alloy of copper and tin. U.S. cents made in (part of) 1982 and after are actually zinc with a thin coating of copper. You can tell the difference in a pre- and post- 1982 U.S. cent if you drop it on a table and listen to its ring. The post-1982 ones have a comparatively dull sound.

#10 User is offline   mloser 

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Posted 13 January 2006 - 07:00 AM

The brass bolts I usually look for along the PRR are huge things--about 1 1/2 inches in diameter, with a round head. They are hard to miss if they are visible, but like all marks they can be pretty tough to find if covered by dirt or anything. I guess I should take the detector out and test on one I already found. This is a typical PRR bolt, a bit worse for wear and a shaky photo to boot: JU1549 PRR Brass Bolt.

As for the monel metal rivets, they are definitely steel--some sort of stainless steel. Most I have found are still shiny. The Reading Railroad loved these things more than the Pennsy loved their copper bolts! They are very small though--about the size of a dime: KW0607 Monel Metal Rivet

Usually either of these are pretty easy to find. The locations are pretty standard so I can walk up and see them, or shift some dirt and leaves and find them quickly. Some have stymied me though and I have dug and metal detected in vain, so far. KW0932 has been the worst so far. It appears to be an easy find, as nothing seems to have changed enough for the bolt to be gone. But the combination of three bridges at one location, all numbered the same, or nearly the same, plus some overgrowth, plus some communications pipes, make this one a tough find. Sooner or later I will either find it or convince myself it isn't there.

#11 User is offline   Papa-Bear-NYC 

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Posted 13 January 2006 - 07:21 AM

Quote

I didn't know exactly what a "copper bolt" would look like, but today someone who should know told me they don't necessarily have a head, and may just be a copper plug in a hole. It sounds like something that, with a little dirt, would be easily overlooked.


I've seen a lot of copper bolts in my territory, most of them set from 1909-1914 by the City of New York when a systematic leveling of the 5 boroughs was done. They are small, less than an inch in diameter, and have no heads, so you could call them plugs instead of bolts if you like.

Here's an illustration from the report which was published in 1914. It shows several type. They are intended to be set vertically or horizontally in massive structures, typically granite.

Posted Image

Here's one of the first type, set on a step of the N. Y. Historical Society building.

Posted Image

Here's one of the vertically set types, from the Custom House (which is now KV0579). For those interested, there is a note in my GC log for this mark giving some background.

Posted Image

And lastly, here is one of the smaller ones, set in the Anchoragre of the Williamsburg Bridge on the Brooklyn side.

Posted Image

I would guess this type of mark is less likeley to be the target of a metal detector because the typical setting doen's tend to get covered over, but one never knows.

Pb

This post has been edited by Papa-Bear-NYC: 13 January 2006 - 07:45 AM


#12 User is offline   Photobuff 

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Posted 13 January 2006 - 11:39 AM

Wow, that's just super info- thanks! The "bolt" I'm looking for is supposedly set in the top surface of a stone (not cement) rr drain culvert headwall. It's dirty, covered with roots, and somewhat eroded. I don't even know if the top layer of stone is the original top layer. The type of stone has a strong effect on the metal detector, thus it's not very useful. I need to hack away some roots (the trail people already took care of the tree, but didn't care about bottom half) and clean off the surface (not easy) to see what's there. Perfect weather today, but I have another target in mind...

#13 User is offline   mloser 

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Posted 13 January 2006 - 11:41 AM

If it is old PRR it may well be a raised bolt. The majority I have found along the ex-Pennsy here are that type. I wouldn't rule anything out though!

#14 User is offline   Bill93 

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Posted 13 January 2006 - 04:08 PM

I've found some marks described as 'BOLT' which turned out to be a machine bolt or carriage bolt set in the concrete with the head slightly projecting. Usually a common hardware store size, less than 1" head. These have been in culverts and bridges along highways. not RR.

#15 User is offline   Rotareneg 

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Posted 13 January 2006 - 07:55 PM

It was a good thing my uncle brought his metal detector with him when they drove to my moms house for Thanksgiving, without it looking for HF1282 (UDALL) would have been a rather futile effort. I wasn't entirely sure how to operate the thing, but after using reference mark one to experiment on I was able to find the actual BM pretty quickly.

This post has been edited by Rotareneg: 13 January 2006 - 07:55 PM


#16 User is offline   lost02 

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Posted 15 January 2006 - 08:56 PM

Papa-Bear-NYC – thanks for starting this thread.

So, there are a few marks that we could probably recover if we had a metal detector. Photobuff mentioned a couple inexpensive models from Harbor Freight - $40 with a 6.5” coil, and $60 with an 8” coil. How much deeper could you detect a mark with an 8” vs. 6.5” coil? Would a model with a 10” coil for $80-100 be about right?

#17 User is offline   Photobuff 

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Posted 15 January 2006 - 09:45 PM

These are really low end metal detectors we're talking about- you can spend many hundreds if you're really into it. The general rule is that metal detectors will detect at about the diameter of the coil. Maybe a bit better under good conditions, a bit worse for bad conditions. Hopefully the expensive ones do better! My experience, short though it is, is that a bronze disk is a fantastic target, and you'll "see" it at about the coil diameter or slightly more. Where the thing really shines is if you have a mark buried a few inches below the surface, and you're not hindered by heavy vegetation or , around here, heavy snow. :P

#18 User is offline   lost02 

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Posted 16 January 2006 - 12:57 PM

Photobuff - thanks for the info. We really don't want to spend a lot on the metal detector, but we do want to spend enough so that we have a useful tool. One mark we couldn't located is probably buried only a few inches under some hard pack soil. Another is under an unknown amount of vegetation/rocks/dirt/etc (no snow out here :) ). I'm sure we'll need it for others. We would like to just make one purchase and not have to go out and get another one later.

#19 User is offline   2oldfarts (the rockhounders) 

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Posted 17 January 2006 - 04:51 PM

We have an el cheapo from Wally World - a Bounty Hunter Pioneer with a 6" coil. I just tried burying a brass belt buckle (size is 2 1/2" X 4" X 1/8" thick) and measuring how deep the detector would still pick it up at.

I dug a hole 12" deep and laid the buckle flat (like a BM disk) and covered it over about 6" and swept the detector over it. No problem at that depth, so I added 2 more inches of sand (we're here in the desert, remember.) and swept again, no problem. So I added another inch and was about to try again when the low battery light came on so I will need to try again later with fresh batteries.

It has found a 2oz tack hammer at about 10" deep.

I found it helpful with this model to sweep an exposed benchmark to get a feel for the tone it makes with brass, before heading out for a buried benchmark.


John

#20 User is offline   lost02 

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Posted 17 January 2006 - 08:05 PM

2oldfarts – thanks for the test results. Guess you have the VLF/Pioneer – sounds like it does the job. We haven’t searched out of the valley yet, but I’m sure we’ll branch out eventually.

#21 User is offline   Bill93 

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Posted 17 January 2006 - 09:41 PM

Dry sand is probably the easiest environment. Wet clay would reduce the range somewhat.

#22 User is offline   Team_Talisman 

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Posted 15 February 2006 - 04:02 PM

Instead of carrying a metal detector with me, in my back pockets are two 16 inch long copper rods(bent into a sidewards L. stick short end loosly in hand hold about 6 inches in front of me and where they cross do a little scratching of the bround with rock hammer and bingo benchmarks are found , geo caches under rock piles and pine needles and twigs and letter boxes.....

Of course all people for some reason are unable to do this. its kinda like witching for water.......

#23 User is offline   BuckBrooke 

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Posted 16 February 2006 - 09:14 AM

I have a coupon for Radio Shack; anyone have experience with their models?

#24 User is offline   seventhings 

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Posted 16 February 2006 - 01:21 PM

I use a First Texas Manufacturing Co. "Treasure Tracker" (no model number) with an eight-inch head. My father bought it at Radio Shack about 20 years ago. It has controls for "Sensitivity", "Ground Cancel" and "Discriminate". I have no idea how they work. I center the "Discriminate" and "Ground Cancel" controls and increase the "Sensitivity" control until I just null-out the tone. It will find a survey disk down to about one foot, and the tone is very distinct depending on the metal. I can easily tell a RR spike from soda can, and a length of scrap wire from a brass disk. It is, unfortunately, extremely sensitive to the metal (iron?) content of certain types of gravel.

Best of all, it does not give a hoot about Poison Ivy.

W

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